Humidity for wood rotting

If I put a piece of wood in a humid environment (basically humid air), it will start to rot. Does the humidity influence the rotting speed, and if so, what is the relation between the rotting speed and humidity?

Rotting is a biochemical reaction: basically catabolism (breaking down of complex compounds to simpler compounds). Cellulose or lignin or any other structural polysaccharide will not undergo spontaneous hydrolysis. This reaction is catalyzed by enzymes produced by the microorganisms like fungi and bacteria (the term for these decomposers is saprophyte). Saprophytes prefer to grow in humid and warm environments.

So the primary role of humidity is to promote the growth of decomposers. Simultaneously it provides the water required for the hydrolysis.

I'm afraid this is not a topic of chemistry, the main degradation process in wood decay is a matter of biological attack, of course life needs water so if the ambient is very humid the wood decompose very well if the ambient is dry the wood can be conserved easier. However wood is not a substance but a complex material, you have to investigate the degradation process for all the components. Chemically holocellulose is the most degradable part of wood, his microcrystalline structure improve the resistance but cycle of imbibitions can blow up the structure leading to a larger surface more suitable for degradation. The hydrolysis of cellulose however doesn't occur at normal temperature only peeling off can occur under 100 °C. Lignin is more and more durable if a little bunch of micro and macro organism wouldn't decompose it we would be sub-merged by a large quantity of lignin.

Study on the effect of relative humidity on moisture content and deterioration of members of timber structures

The state of temperature and humidity is said to be an environmental factor that induces degradation such as activation of rot fungi and dew condensation for timber structure. This study focuses on the temperature and humidity as environmental factors to induce deterioration such as activation of rot fungi and dew condensation. It is investigated the relationship between temperature and humidity and wood moisture content and deterioration.

Original languageEnglish
Publication status Published - 1 Jan 2018
Event 2018 World Conference on Timber Engineering, WCTE 2018 - Seoul, Korea, Republic of
Duration: 20 Aug 2018 → 23 Aug 2018


Conference2018 World Conference on Timber Engineering, WCTE 2018
CountryKorea, Republic of
Period 20/08/18 → 23/08/18

About Wood Rot

There are two types of wood rot. One of them is dry, and one is wet. Normally, wet rot sounds more dangerous because water damage can create a lot of problems. However, in this case, the dry rot is even more dangerous as it eats out the wood slowly, yet completely.

Wet Rot

Wet rot is created by moisture — it’s a fungus attracted by wetness, and it doesn’t attack just wood but also other materials. It doesn’t destroy wood as much as dry rot does, but it does need to be fixed as soon as it’s noticed. After a while, wet rot can also destroy the structure of a building.

Just spilling something on wood probably won’t cause this fungus to appear. What will, on the other hand, is moisture present in the wood for a longer amount of time. Here are some of the most common reasons wet rot appears:

Even more, problems cause wet rot — as long as there’s wetness in the wood. So, keep an eye out for that. Here are some of the signs that you will notice:

  • If the wood is darker than usual or darker than other wood
  • If the wood is soft or generally feels wet
  • If it looks cracked and easily breaks
  • You can actually spot the fungus
  • It became smaller
  • Smelly wood

The easiest test to perform is to stick something into the wood, and if it doesn’t meet resistance, you probably have wet rot. Keep in mind that you’re more likely to notice a smell with wet rot than you are with dry rot. It will smell musty and like the ground.

Some of the most common areas affected by this type of rot is a basement or a roof.

You probably know that wet rot is dangerous because it softens the wood, and it makes it less structurally sound. Over time, it can cause a lot of damage, but if repaired on time, you can avoid seeing the effects entirely.

To treat wet rot, you have to find where the wetness comes from and then dry it out — fix pipes, remove leaks, dry walls, etc. Then, you can do proper filling and repairing . In some cases, you may need to replace the wood. Treat the wood with a fungicide. You should also spray it on areas that are not quite yet affected by wet rot, but they have been damp for a while.

There are hardeners too that can help you restore some of the wood’s strength.

Dry Rot

Dry rot is a fungus that attacks wood. It causes decay of wood, especially in the parts of it that make it strong. This rot spreads easily, and it doesn’t need any water to do it. It actually creates more moisture by eating wood. Dry rot is far more dangerous because it can attack and reach so many places.

It destroys the structure of your building, and it could bring everyone’s life in danger because of this. To identify it, you should invite a professional. They will be able to confirm the presence of the fungus much faster and with precision.

If you don’t see it right away, you will probably need to replace a lot of wood. It appears in buildings and then just waits for the right place to attack. But they need good conditions for this, like slightly wet wood and moisture in the air. However, dry rot doesn’t just appear in the air. Here are some reasons why it’s there:

These are the most common reasons, but there could be more as well.

Dry rot spreads in stages. In the first stage, the rot is just a spore that travels through the air. Once the conditions for their activation are met, the spores land on wood and grow into hyphae, which will make the wood inflated and start deteriorating it.

Next, there will be mycelium, which looks like cotton and can spread over more wood. After that, the fungus is at the top of its strength as it takes the shape of a mushroom and starts birthing new spores that will spread around.

So, dry rot actually spreads in two ways — one as mycelium and one as a big mushroom by creating spores. However, dry rot is hard to find in the earliest stages. For example, you won’t see the spores. But that’s when it’s the easiest to stop it. Once you notice it, it probably already did a lot of damage.

If you want to, you can look for a mycelium if you believe you have dry rot or for a fruiting mushroom since you’ll be able to clearly see those. Then, you might spot a sort of skin over your wood in strange colors. Look for red dust as well — these are the new spores.

You might notice dry rot by seeing wood shrink slightly, get drier and more crumbly, actually see it cracking and breaking or becoming darker. If the rot is under your floor, you will likely not spot it, but the floor will seem a bit spongy, almost like a trampoline with a creaking sound.

If this rot gets into the building walls — the masonry in particular — you might be in serious trouble as it will destroy many parts, but you won’t be able to see it while the fungus grows. ]

Identifying a Dry Rot Outbreak

The first sign of the existence of dry rot is often the sudden appearance of a fruiting body (sporophore), or the shrinking/distortion of timbers.

Unlike many other wood destroying fungi dry rot can readily grow over and through porous masonry provided that there is a nutritional source (wood) from which it can spread this ability allows the spread of the fungus from one area to another. This generally only occurs to any significant level where the masonry is damp either via severe sources of water ingress or rising damp, etc.. Dry rot will not spread over or through masonry that is ‘dry’, ie, that does not contain free (capillary) moisture.

A further feature of the fungus is the formation of ‘strands’, thick walled structures which develop in the fungal growth. These are resistant to desiccation and carry nutrients from the food source to the growing tips of the fungus when the organism is growing through or over nutritionally inert materials, eg, masonry, soil. They support the spread of the growth. However, without a source of food growth is terminated.

When the decay is advanced the shrinkage causes the wood to split in a cuboidal manner (this is typical of a number of wood destroying fungi), and it is therefore not always easy to distinguish between dry rot and other fungi (wet rot — brown rots) in the absence of fungal growth. Click the following link for more information on what is wet rot.

Should infected timbers dry out or the food source be removed the growth will cease but, depending on conditions, it can take considerably longer for the fungus to die.

For example, in timbers that have dried down to below 20% moisture content, the fungus can remain dormant for up to about a year at ambient temperatures before dying. However, this period may be prolonged at lower temperatures.

If infected wood is removed then the growth is very limited and quickly terminated, but the growth can remain viable in damp masonry at low temperature (eg, 7 o C) for up to 9 years, and up to 1 year at ambient temperatures.

Should new, untreated or inadequately treated wood be put back into direct contact with damp infected masonry, even though the original decayed wood had been removed, then fungal growth may start and spread into the new wood so initiating further decay.

In most cases, the optimum conditions for growth and decay of timber have been derived under carefully controlled laboratory conditions. In practice, however, it is highly unlikely that such optimum conditions would be present, or at least for any length of time.

Thus, for growth and decay, the use of ‘optimum data’ within a field situation to evaluate the growth or decay activity by dry rot is unlikely to prove valid. Furthermore, it is possible for dry rot to remain ‘dormant’ under unsuitable conditions for a while, only to become ‘active’ when such conditions become more favourable.

Products to Stop Rot

While I’m a big believer in intelligent exterior design to shed water efficiently, there are times when rot has begun and you need to stop it.

The main product that I use to treat rotten wood is also the same product that works great for preventing termites and other insects that destroy wood.

BoraCare is a borate treatment that is easily applied to wood and over time it migrates throughout the entire mass of the wood. It doesn’t just sit on the surface, but treats the whole thing making the wood unappetizing for insects and uninhabitable for fungi.

If you have rot, this is the way to treat it to make sure the spores are killed and won’t come back.

Understanding how and why rot works is a big part of the battle that old house owners fight. Hopefully, this bit of knowledge will better arm you in your battle. Now get out there and fight!

Founder & Senior Editor

I love old houses, working with my hands, and teaching others the excitment of doing it yourself! Everything is teachable if you only give it the chance.

7 Most Rot-Resistant Woods

Here is a list of the most rot-resistant woods that you can find. They are dense, hard, and have tightly woven fibers that make the wood resistant to water. The result is a rot-resistant wood that you can use outdoors without fear that your wood will deteriorate fast. Although these varieties of wood are more expensive, they turn out to be cost-effective in the long run thanks to their long life.

Teak Wood Table (Image:

An all-time favorite, teak sapwood has a rich honey-gold color and tight fibers that keep water out, thereby resisting rot. Another reason for teak’s high resistance to rot is the natural oils it secretes that keep insects at bay. Teak can be rough on tool blades, and therefore difficult to work with. The high price of teak also serves as a deterrent from using it freely.


This wood is reddish-brown and it darkens with age. A special feature of mahogany is that it exhibits “chatoyancy,” a three-dimensional, iridescent appearance when cut in a particular way. The degree of rot resistance of mahogany varies according to the growing conditions of the trees. You will find this wood to be very durable and versatile for all of your woodworking projects. However, this variety of wood is also quite costly.

Spanish Cedar

Spanish cedar has the appearance of mahogany with a reddish-brown color. The wood secretes natural oils through pockets that keep insects at bay and make it water-resistant. Spanish cedar has a low density and is a soft wood, which reduces its strength.

White Oak

The light brown color makes this wood a popular choice if you need good-looking wood. The tyloses that white oak contains create a closed-cell structure that resists moisture from seeping into the wood. The water-resistance of white oak makes it preferred by shipbuilders and homeowners who require a durable wooden finish to their interiors.


The flaming red color of redwood gives it a good reputation. The level of rot resistance of this wood depends on the age of the tree. Older trees tend to be more durable than the younger ones. But a drawback of the older trees is that there is a tendency for cracking, and that can allow moisture to enter, thus causing rot. The solution here is to apply a suitable sealant or apply oil regularly to the wood.


Cypress comes in an extremely light shade of brown that is almost white. It may show patches of darker brown. Like redwood, cypress also varies in its level of rot resistance according to the age of the trees, the older ones being the most durable. Applying a water-repellent sealer can add to the water-resistance of this wood.

Rot-Proof Composites

Composites are a class of material that combines wood and synthetic materials and the resultant material looks like wood. Wood/plastic composites (WPC), for instance, contain wood fiber, sawdust, and thermoplastic resins. Some of these composites contain recycled material, but whatever the case, they are all rotproof.

WPCs are devoid of defects and are not prone to compression. However, the low density and high expansion and contraction rates can cause problems. Also, they lack a high mechanical strength. But, the main advantage of WPCs is their complete resistance to rot. WPCs are much cheaper than natural wood, so they are fast gaining popularity today as a durable and cost-effective option to regular wood.

Because wood shrinks and warps as it dries, woodworkers want it to be pre-shrunk before they use it. Furniture maker Lonnie Bird weighs in,

“I don’t want the wood to shrink after I use it because the wood will warp or split.”

Bird, who runs the School of Fine Woodworking outside Knoxville, Tennessee, says he knows wood shrinks seasonally but wants to minimize the shrinkage and expansion by drying the wood to a moisture content of about 8%.

To make sure the wood is properly dried, he always uses a moisture meter before working with it.

The moisture content of freshly cut wood is typically somewhere between 40-200%. If you’re wondering how wood can have a 200% moisture content, here’s how that works…

Because the moisture content of wood is equal to the weight of the water in the wood divided by the weight of the wood without the water, it’s possible for the moisture content to exceed 100%. In other words, the water weighs more than the wood fibers.

How Dry Should Wood Be for Woodworking?

The acceptable wood moisture content normally ranges from 6% to 8% for woodworkers who build cabinets, fine furniture, musical instruments, dishes, toys, decorative art, boat restoration, or various other wood products.

However, this range will vary slightly according to the geographic region because of varying RH levels.

The normal moisture content of wood (or EMC) varies from 7%-19% depending on the RH in the air.

If an interior location has an average RH of 40-52%, wood placed there will have an average EMC of 8-9%. This is based on a chart in Wood Handbook: Wood as an Engineering Material.

Therefore, in order to avoid post-construction problems, a woodworker building a cabinet for this particular interior environment would need to dry his wood to a moisture content of 8- 9% beforehand and then keep it that dry during the construction process.

The best way to do this is to use an accurate moisture meter.

Acceptable wood moisture readings on a meter normally range from 6% to 8% for woodworking. Normal moisture content of wood ranges from 7% to 19%. Be sure to acclimate the wood to desired EMC of the interior environment before you use it.

How Does The Wood Rot?

If the conditions for life of the Fungi or bacteria are correct, air and water, then the wood rot will begin.

Fungal spores are ever present, and conditions are normally correct for them to start eating the wood first.

As the fungi eat away those cell walls, they open up the spaces between those tubes, and as the fungi dissolve the doors between one wood cell and the next, the wood porosity is opened up more and more. This allows more dew or rainwater to be more rapidly absorbed in the wood, thus providing more humid wood which is more favourable to rapid fungal growth, thus accelerating the wood rot. As the wood becomes more porous it holds enough water to favour growth of not only fungi but bacteria, and between them they eat first the porous summer growth rings and then the harder winter growth rings, and finally there is nothing left.

Why Does Dried Wood Absorb Moisture?

The hardwood lumber that you buy at the lumberyard began as a living tree. As a living tree, the trunk of the tree provided a way to transport water from the roots to the leaves. When the tree is cut down that water moving system is still present in that tree and much of that water is still in the lumber when it is first sawn.

Hardwood lumber just cut, referred to as green lumber, can have a moisture content of 40% or more! After hardwood lumber is sawn, it is typically air-dried to around 15% moisture content. Hardwood lumber will be continued to be dried in a kiln to a moisture content of 6-9%.

Wood is hygroscopic, which means just like a sponge the moisture content will change depending on the relative humidity of the surrounding air. When humidity increases, the wood absorbs moisture from the air causing the wood to expand. When the humidity decreases, the wood releases water into the air and the wood shrinks.

What Direction Does Wood Expand In?

When wood expands and contracts because of changes in moisture content, hardwood will move in a predictable way. Wood shrinks most in the direction of the annual growth rings (tangentially), and half as much across the rings (radially), and only slightly along the grain (longitudinally). or from the center of the tree to the outer edge).

Hardwood shrinks most in the direction of the annual growth rings (tangentially), about half as much across the growth rings (radially), and only slightly along the grain (longitudinally).

Flat-sawn boards will cup away from the heart of the tree. The shrinking will occur mostly in its width.
Rift-sawn boards will warp and shrink into a diamond or trapezoidal shape.
Quarter-sawn boards will shrink slightly in both length and width.

What Do I Do About Moisture and Wood Movement?

The solution to this problem is quite simple: don’t stop the wood from moving, but rather account for its movement. With a little bit of knowledge, you can predict the degree of wood movement, and take action to accommodate the movement.

Expected movement can be accurately predicted, which means avoiding potential problems down the road.

In this article, we’ll explain the importance of understanding wood movement, how to use a moisture meter to measure the moisture content (MC) of trim, how to decide when a load of trim should be rejected, and how to accurately estimate how much trim will move after it’s installed.

Most finish carpenters are aware that seasonal changes in humidity cause trim and flooring to shrink in the winter and expand in the summer. But few realize that the expected movement can be accurately predicted and potential problems avoided. It’s our premise that with a moisture meter and an understanding of wood movement, most wood movement problems can be avoided. Plus, with this data, finish carpenters can accurately predict how trim and flooring will behave after it’s installed.

Wood Movement — You Can’t Stop It

Wood is hygroscopic, which means its MC will fluctuate based on the relative humidity (RH) of the surrounding air. As humidity increases, the MC increases, and the wood expands, and as the humidity decreases, MC decreases, and the wood shrinks. This relationship is referred to as Equilibrium Moisture Content (EMC), and can be accurately predicted.

Understanding Equilibrium Moisture Content

The moisture content of wood is tied directly to the relative humidity of the surrounding air. The higher the relative humidity, the higher the MC of the wood. Period. If you’re installing wood that’s recently been transported, or installed on a job, it might take a little while for the material to reach its equilibrium moisture content (EMC) with the air—in other words, for the wood to accommodate to the humidity level for the climate around the wood: the wood may take on more moisture or it may dry out. For example, if wood at 10% MC is exposed to 25% RH, the wood will dry to 5% MC (and shrink as it dries).

The EMC helps us understand the response wood will have to relative humidity, whether it will shrink or expand. For woodworkers and carpenters, the EMC is more helpful than RH. The simplified chart to the right shows the EMC values of wood when stored at the humidity and temperatures indicated.

Complete EMC levels for wood stored in unheated structures in your area of the country can be found HERE.

How Wood Moves

If the MC of the wood you install is too high, excessive shrinkage may occur, along with the risk of problems of unacceptable gaps and cracks in the wood itself. When the MC is too low, the wood may expand, and may buckle, bow, and distort surrounding material.

There are six key areas finish carpenters should be aware of when it comes to wood movement.

1. Width of material

The wider the board, the more movement will occur (the term “board” technically refers to wood 1 1/2 in. thick or less, but for this article its use will refer to wood typically used by finish carpenters). It’s a direct proportion: an 8-in. board will move twice as much as a 4-in. board, and a 12-in. board will move 3 times the amount as a 4-in. board. And it’s important to keep in mind that a glued-up panel behaves basically as one wide piece of lumber.

2. Grain orientation matters

Boards are characterized as being either “flat sawn” or “quarter sawn.” Quarter sawn lumber (also referred to as “rift sawn” or “vertical grain”) shrinks and expands roughly half as much as flat sawn. Most over-the-counter finish material is flat sawn, and you should assume flat sawn values unless you’re sure your material is quarter sawn. Quarter sawn lumber has annular rings that are oriented between 45 and 90 degrees to the board’s face. Flat sawn grain orientation falls between 0 and 45 degrees to the board’s face.

Wood Grain (Note: Click any image to enlarge. Hit “back” button to return to article”)

3. Moisture content of the wood at delivery

The only way to accurately predict wood movement is to know the MC of the material when you receive it. Moisture content is measured using a moisture meter. Failure to check your delivered material means you have no chance of anticipating movement problems. Furthermore, material that measures outside of the acceptable MC level should be rejected.

4. Humidity inside and outside the structure

Homes in most of the U.S. that lack humidity control typically experience interior levels of humidity from 25% RH to 65% RH. This range of humidity will cause a 6% change in the MC of the wood. This change in MC will cause a 12-in. wide maple board to change 1/4 in.

When material is installed that was delivered at an unacceptable MC, or the humidity range in the structure exceeds typical values, the amount of wood movement increases—and can cause problems even in well-designed trim details. It’s worth noting that panel material (plywood, MDF, composite materials) move at about 1/10th the rate of solid wood.

In most of North America, exterior humidity levels range from 60% RH to 70% RH in summer and winter, but are lower in the Southwest, and higher near large bodies of water. If the material is delivered at 6 to 8% MC, it can experience more than a 2% change in size as it adjusts to the EMC.

5. Species affects the amount of movement

Wood movement depends in part on the species. A 12-in. wide western red cedar board will fluctuate 1/8 in. while the same size maple board will fluctuate 1/4 in. The formula for calculating wood movement is complex and extremely accurate, but tedious.

One simple rule of thumb serves as an approximate guide to predicting wood movement: “Most species of flat grain material will change size 1% for every 4% change in MC.” Applying this formula to a situation where the seasonal EMC ranges from 6% to 10%, a 12-in. wide board will change dimension 1/8 in.

I’ve put together a rough chart (see below, click to enlarge) that offers approximate movement values for various widths and commonly used species of wood. These values are based on flat sawn lumber, and offer a general idea of anticipated annual in-service movement. The movement values for quarter sawn lumber are approximately 1/2 the flat sawn values.

If you want to know exactly how much the material you’re using is going to shrink or expand, use this online shrinkage calculator. Simply enter the high and low MC values and the width and species of the board.

6. Applied finish does not stop movement

While it’s true a high quality finish will slow the rate of moisture exchange, it will not stop it. Material finished on all surfaces will expand or contract at a slower rate than raw wood, but make no mistake—finished wood will eventually acclimate to EMC levels.

Events That Increase Movement Risks

There are many events that can contribute to excessive wood movement issues. Nearly all of them can be prevented before they cause a problem if—and only if—you measure the MC of the wood as soon as it’s delivered, and avoid using wood that is too wet or too dry for the expected in-use EMC. The moment the wood is delivered, it begins to acclimate to the surrounding environment. At the very least, it’s important that you document the delivered MC, just in case wood movement becomes an issue. But responsible carpentry can’t be accomplished without reading the delivered moisture content of the wood and planning for wood movement during and after acclimation.

Excessive MC in delivered material

Optimum MC for interior millwork is 6-8%. In the real world, your material may arrive around 9-10%. For installations in unheated areas, the preferred readings are in the 12-14% range, assuming an area is protected from the weather. In most cases, you can deal with material that’s a couple of points high, but keep in mind that the wider the stock, the greater the movement. Ideally, the moisture content of wood should not change more than 2% when put into use.

Think through your trim details and consider how they will react when the wider assemblies shrink. With wide glued-up material, slightly higher MC levels may not be acceptable. If you’ll be installing wide material, it’s a good idea to be upfront with your supplier and let them know that the material’s MC must be within the range you specify. As a last resort, you may choose to dry the wood in your shop if the shop’s EMC is low, and have any shrinkage problems show up before the wood is installed.

Delivered material that’s too dry

This is seldom an issue for interior trim, but can be a real issue for exterior trim. Material delivered at 6% MC, and installed outside, will acclimate at 12% in the more humid months, resulting in a 6 point MC change. This swelling of the material can cause significant problems in situations where installation creates accumulated movement (more on this below).

Long-term storage of trim material

If you plan on storing trim material for any length of time in an unheated area, keep in mind that, in most parts of the US, the material will acclimate to roughly 11-12% MC. (See the humidity moisture content chart at the beginning of this article.)

If MC is too high, lower readings can be achieved by moving the material into a heated area. The amount will depend on the temperature and humidity of the storage area. The change in MC won’t happen immediately, and the material in the center of a pile will change at a slower rate than the material at the edges. Spacing the material so all surfaces are exposed to the air helps, as does good air circulation throughout the pile. You’ll need to take sample readings with your moisture meter to determine when the material reaches your intended MC.

Higher temperatures result in a more rapid change in MC when the humidity remains constant (roughly speaking, moisture moves twice as quickly for every increase in temperature of 20 degrees). And despite what you may think, moisture gain or loss does not stop when temperatures fall below freezing. The moisture in wood is chemically bound in the walls of the wood cells and cannot freeze.

Typical on-site humidity

At certain points during construction, such as when pouring concrete, plastering or drywalling, tremendous amounts of moisture are often added to the air, causing humidity spikes as high as 80-85% RH. If you are storing finish material on-site during these periods, be sure to keep them wrapped in a vapor impermeable material (like plastic) with as few gaps as possible. Wood stored in this manner will not pick up any appreciable moisture.

Interior trim should not be installed until the temporary construction humidity has subsided. Use an accurate digital hygrometer to measure RH (under $40). Generally speaking, interior trim should not be installed when the humidity is above 60%, or the material may climb above acceptable MC levels.

Humidity in un-heated areas fluctuates about 10% therefore dry material (6% to 8% MC) installed in un-heated areas will swell significantly. It’s important that the MC of exterior trim be within 2-3 points of the EMC values for the area before it is installed.

In-service low humidity issues

In heating climates, older, drafty homes may see humidity drop, measuring 20% RH in the winter. The EMC in this environment will vary nearly 8% wintertime to summertime. Homes with wood stoves and no humidity control can see EMC swings of up to 11%. In extreme environments, consider using cabinet grade plywood for wide panel application instead of solid wood.

In-service high humidity issues

Typically, high humidity (constant levels above 60%) is not an issue. But if you find yourself working on a project that includes a room with a spa, heated pool, or damp crawl space, proceed with serious caution󈠥% RH means an 18% EMC. A 12-in. wide piece of birch installed at 8% MC in such a room will swell in width over 3/8 in. Letting your material acclimate to the high MC levels before installing is one approach, but keep in mind that if there is ever a period where the pool is drained for a significant time, and the humidity drops to typical levels, the trim material will experience severe shrinkage. A carefully-worded disclaimer regarding wood movement would seem to be in order.

Understanding Accumulated Wood Movement

Glued-up solid wood panels behave as though they were one wide board—a 24-in. wide panel will shrink and swell four times as much as a 6-in. board. But what about a series of boards installed side by side (T&G flooring, for example)? While it’s true that each board can move independently, accumulated movement can cause significant problems, typically when the newly installed material gains moisture. (See photo, right)

If the material in non-glued assemblies (flooring, for example) is installed “tight”, and there’s no gap to absorb expansion as the material gains moisture, the increase in width of each floor board becomes cumulative, and causes the entire floor to “grow” buy the sum of each piece’s individual movement. In cases of excessive shrinkage, unacceptable gaps can result between each floorboard.

For example, random width oak flooring is delivered at 8% MC. The width of the room is 12 feet, and the floor acclimates to a high level of 11% MC, the cumulative movement is about 1 3/8 in. In the real world, a lot of this expansion is “lost” as the fit tightens up, but in some cases the wood fibers compress, and fiber compression can cause grain ridges. By using a moisture meter, and predicting the movement, you can decide whether you should install the material “tight” or “loose” to absorb what you know will be an increase in material width.

Moisture content on exterior trim can range from 12% to 16% depending on the region, time of year, and location of the material. (Click images to enlarge)

Common Movement Issues

Paneled Passage Doors

Experienced door hangers know that a paneled passage door with a tight reveal will shrink in the winter and possibly stick in the summer. (Remember, if you live in California, the winters may be more humid than the summers!). But basing your door gap on the time of year you hang the door can be a mistake if you don’t know the MC of the door.

The seasonal width changes of a door are controlled by the MC change in the door’s stiles.

If that fir door you’re getting ready to hang in the winter has been stored for six months in an unheated building, the moisture of the 5-in. stiles may easily measure 12-13% MC. After that door is hung, the MC of those stiles will drop to 6%, and the door can easily shrink 3/16 in. Knowing the MC at the time of installation provides the needed guidance.

And keep in mind that the door panels in this example will shrink significantly after installation. This won’t affect the fit of the door, but if the door finish is applied at the MC noted, there will likely be unfinished wood exposed as the door panels shrink to their in-service width. (See photo, left) This is particularly noticeable when a light wood is stained dark.

By measuring the MC of the door stiles, you can base your door gap on established movement values, not guesswork, and avoid callbacks when the fit becomes a problem.

Doors with horizontal battens

Unless you’re setup to build these doors properly, avoid them. The typical horizontal batten door is built using T&G material for the door face, and then battens are fastened to the back of the door to hold things in place. As the seasonal MC of the T&G material rises and falls, the boards expand and contract, but the battens—with their grain running in the opposite direction—resist that movement, forcing the door to cup inward or outward depending on the direction of the movement.

The detail below is one method used for cabinet batten doors that successfully allows for seasonal wood movement.

Captive panels

Resist the temptation to “picture frame” a solid wood panel—the way some woodworkers new to the craft miter a nosing or a frame around a tabletop. The miter joint will always fail when the panel expands and contracts. Instead, use a breadboard nosing design so that the wide panel can shrink or swell without destroying the surrounding joinery. (See below)

Inside corner trim

When installing trim that covers an inside corner, fasten the trim through the corner and into the substrate so the adjoining finish material can move independently as its MC changes. A typical example is base shoe molding. The best practice is to nail base shoe to the plate, with a long nail that doesn’t penetrate the baseboard or the flooring. But that’s not practical on most jobs.

The second choice is to fasten the baseshoe to the baseboard. Yes, the baseboard will lift off the floor in the heating season, but rarely more than 1/16 in. A wide floor, on the other hand, moves more than a 6-in. piece of baseboard if you nail the base shoe to the floor, the base shoe may separate significantly from the baseboard.

Common Myths

Wood is stable at below freezing temperatures.

The moisture in wood is chemically bound in the walls of the wood cells and cannot freeze, and expansion and contraction continues at below freezing temperatures. Wood does acclimate more slowly at lower temperatures.

Wood will expand on warmer days and contract on colder days.

For all practical purposes, thermal expansion and contraction of wood is not an issue. That said, warmer temperatures speed the exchange of moisture within the wood. Moisture exchange will happen more rapidly at warmer temperatures, but there is no thermal movement of wood worth measuring.

It doesn’t matter if lumber is kiln-dried.

Kiln-dried hardwood lumber typically leaves the kiln near 6% MC (softwoods at 10-12%). But all kiln-dried material will acclimate to the surrounding EMC levels. The significant advantages of kiln-dried material is that it is typically heated to at least 130 degrees in the kiln, which will stop any insect activity, and also “set” the sap in resinous softwoods (sap in resinous air dried material can bleed from the board after it’s installed, especially when interior temperatures rise in the summer).

They don’t make wood like they used to.

It’s true that most of the old growth timber is gone, but properly dried vertical grain material has highly desirable movement characteristics. If you’re seeking material that will move the least, choose one of the more stable species, and specify vertical grain (and be sure to check your wallet before ordering!).

But most importantly, owning and using a moisture meter and knowing the in-use EMCs is an inexpensive way for carpenters to predict and avoid wood movement problems that could require costly repairs.


Carl Hagstrom is a partner in Woodweb, the leading online resource for professional woodworking. He has an extensive background in residential construction and architectural woodworking. He is also a contributing editor at the Journal of Light Construction, and a Certified Professional Building Designer.

Gene Wengert—having taken an interest in woodworking since 7th grade shop classes—was employed by the US Forest Products Lab as a college student starting in 1961. He worked in solar lumber drying, as well as discoloration of wood due to UV light. He then worked on the weathering of wood with the Lab and received a BS degree in meteorology from the University of Wisconsin. He continued to work on moisture related issues and developed expertise in processing northern and Rocky Mountain aspen, going from environmental benefits of the species through sawing, drying, and marketing. (Aspen is splinterless, did you know?) He worked at Virginia Tech as a wood specialist for the extension service, consulting with the wood industry daily. He also managed Tech’s sawmill and dry kiln.

For fun, Gene has taken up long-distance bicycle riding (at age 55) and has done two trips from the Pacific to Atlantic Ocean and three from the Gulf to Minneapolis.

Dr. Gene Wengert is Professor Emeritus in Wood Processing, Department of Forestry, at the University of Wisconsin (Madison). He is also a technical advisor at Woodwebs’s Sawing and Drying Forum, and Commercial Kiln Drying Forum. He frequently contributes to trade journals serving the primary lumber processing industry, and is president of The Wood Doctor’s Rx, LLC, through which he provides educational and consulting services to lumber processing firms.


58 Responses to “Understanding Moisture Content and Wood Movement”

Good article, I have not seen the panel kerfing detail before, interesting. Looks a bit fragile to handle during assembly, but interesting none the less.

On the freezing topic: When you say “water” in wood does not freeze I assume you are referring to bound water not free water. If that is the case, are all species the same across the board when comparing their capacities to hold bound water?

For example, if you were to dry three different species of wood from “green” to say 15%MC, would each of them be free of “free” water when they got to say 17%, then as the drying process proceeded we would be talking about reducing water bound in the cellular structure of the board.

Or, would it occur at different levels depending on the capacity of each specie to hold free water?

This is a great article. One issue I have seen is that when a house is built very fast the concrete will put a lot of moisture in the air. This will cause the wood work to swell after it is installed. It takes concrete a month to release all the water out of it.

Good article Carl!
A couple of comments:
What you call edgegrain or VG (vertical grain) lumber is usually called and sold as quartersawn, at least in hardwood. There is no question that it is much more stable than plainsawn (what you cal flatgrain or tangential.)

In my area, for at least 150 years in high end work, flooring is installed against baseboard without shoe molding. The base is installed first, and the flooring is installed scribed tight to it. In spite of what your lazy or ignorant flooring guy says, there is no gap, even after years of fluctuating humidity, and repeated sanding or scraping. This is because where the end grain of the flooring hits the wall, movement of the flooring is essentially zero with the grain, and where the edges of the flooring hit the wall there 1/16″ or less expansion (in 2-1/4″ oak t+g), because the flooring is fastened to the subfloor, and each piece expands and contacts by itself- not the whole floor as a unit. I’ve only seen unacceptable gaps between the boards when the flooring has been very poorly dried, or allowed to get wet before installation, and the only time I’ve seen a floor buckle from expansion is when there has been a major leak or flood, or when the flooring was installed over really green concrete.

You are right about each piece expanding as a unit and not being cumulative across an entire surface. If it was cumulative the boards near the walls would break loose from their fasteners.

Nice Sketchup drawings too! Did Carl do them?

We are located in the Valley of the sun. we have two furniture factories here and often ship to moist environments such as the ocean. There fore we are shipping furniture from an arrid environment to a moist one. The particular job I have a question about will be shipped from our factory in phoenix to Sedona, AZ. We are building a number of pieces out of solid woods and wondering if maple moves less than poplar or alder wood. Which do you recommend at 5″ board widths by 2.25″ thick?

Good article about shrinkage on newly stained and installed
panel doors and what happens when you don’t check the MC.
As a home inspector on sometimes new properties some clients ask why the staining wasn’t completed. Now we know.

I am currently drying out my house due to water damage. Is it possible that wood can be over dried? If wood is already installed (no worry about it expanding), can the MC go down to 0% without any risks? Will a MC of 0% put the wood at more risk for fire?

was wanting to know what you recommend for a moisture content of a solid alder wood cabinet door should be when installed in Colorado Rockies at 11,000ft

I am assuming 8-9 percent will this be ok?

Without looking at the map, I would imagine your
area is a bit drier overall then the average US.

8-9 percent, in my opinion, is a bit high …. given
that you’re in a potentially drier part of the county,
I’d be looking at trying to achieve 6 to 7% for the target MC.

Keep in mind that the environment the wood will be
placed in is paramount. If it’s a newer house with
solid climate control (humidifying in the winter), then
you’ve got more wiggle room. If it’s a house that has
a wood stove for the sole source of heat in the winter,
then you better work on crafting an iron-clad
disclaimer …. the joke for those kind of houses
is that in the winter, the MC “goes negative”.

So – generally speaking, I think your target MC is a little
high – everything I’ve read indicates you get in way
more trouble if your lumber is on the too wet side
of the ideal, then if it’s on the too dry side of ideal.

As an aside, I’ve lived in my house for over 30 years, and
the only heat we have is a wood stove (6500 degrees
days in the winter – I burn 6 cords of wood).

I built my original cabinets out of tulip poplar
way back – when I didn’t have a clue about how
MC worked. By February each year, the panels
in the wider doors would shrink so much, you could
see light between the panel and the stiles.

When we remodeled the kitchen a few years back,
I asked the guy who did my cabinets to prefinish
the door panels before gluing the doors up. I know
things are moving more then typical (I have to tighten
the door/drawer pulls around Feb every year)

Hope the long winded reply is of some help

I’m currently having issue with some 3/8 poplar nickle joint that was installed in a house in south Florida. It was installed in a variety of areas, interior walls, exterior walls, on plywood and on drywall. While there are a few boards that are cupped slightly the overall product looks great. Except one room that on two exterior walls the boards are cupping considerably more. The wood was installed over the drywall glued and nailed as a wainscot. There is considerable movement in this room unlike any other in the home. Any thoughts?

I am a furniture installer who has been having trouble when fitting high end joinery in house here in the uk and abroad. The MC is fine and all checked prior to manufacture so I know this is fine, so my question is about the RH in buildings. What is the ideal percentage that I should be fitting in? And what is a too high and too low percentage to be installing in?

As an HVAC Contractor this is a reference piece that I will keep near and dear to my heart. This provides a starting point for discussions for all projects with a framework of numbers which in my mind is so much better than the qualitative discussions that lack empirical evidence.

I just bought a MC Device and coupled with my humidistat we are now documenting all the entering wood in any new project that we take part in to ensure client success and assist the trim team.

My belief is if we all point our fingers at the problem rather than each other we are poised for greatness.

I have 3/4 inch solid oak flooring. What is the acceptable moisture reading for the wood once installed. I have a cupping problem which I am told is due to moisture, however, the flooring reading are 14-16. The man who installed did not allow for the floor to acclimate to my home, He purchased and installed the same day. Please advise.
Thank you,
[email protected]

You should consult a wood flooring specialist in you area.

This is such a helpful article! This past summer I had to rip out and do a major overhaul on the deck on a recently purchased house. It is certainly amazing to think of the science and weather factors behind building and maintaining outdoor structures. The previous owners of the house set up a horrible mist system that mixed with our Texas summer and made for one giant project. Also, there was another great post that helped me understand how to work through having water features on our wooden decks safely . Saved us the hassle!

We are proposing to glue 6mm toughened backpinted glass to MDF or plywood panels to ceate a Glass wall. We are worried about which material will expand and contract more. The wall will be internal but will be in a large open Office reception. What do you recomend?

But for material recommendations you should consult a specialist in your area.

I was wondering if you could advise… I installed some 3 1/4 prefinished maple floors on my main floor. I did it during the humid months of the year and the AC wasn’t running. We have had a long dry winter in Ottawa, and the gaps in my floor are huge. Some at 1/8 of an inch and collect all kinds of stuff in there! If the humidity goes up, will they expand? or is this possibly permanent? Also I was considering having them sanded with oak dust laquer filler and sealed with a water based sealant. Rught now with the gaps it looks so rough. Below is the basement with unfinished ceilings and fresh cold air for the furnace.
Thanks! so disappointed…

Thank you for your article. I have property in the northwoods of Wisconsin and over the past five years have worked with a wonderful builder. His work is consistently accurate, solid and tight. My projects in the past have been small – an interior remodel of a two bedroom one bath guesthouse, and the building of an enclosed garage. My latest project – building a 1200 sq. ft. family clubhouse – is my most adventurous and costly endeavor. The clubhouse is complete except for the interior wood. I originally chose t/g alder with a pre-finish stain but quickly realized that the $26,000 bill for 4000 sq. ft of material (for the ceiling, walls and floor) was beyond my budget comfort zone. So I am rethinking my choice of material. Part of my confusion – and the reason I am writing you – is that in the guesthouse project (completed 5 years ago) I used t / g cedar without any finish and have never seen any gapping or buckling.
Question 1: Given your information about moisture content, was I just lucky in the guest house project or is my builder so good that he chooses only material that will not gap or buckle?
Question 2: If I use a soft wood (kiln-dried cedar, pine or tight knot fir) in the interior of the clubhouse do I have to stain or seal it? My experience with the cedar tells me “no.” Your article says it will slow the movement but not necessarily stop it. If I purchase my lumber from a reliable company, and the material is kiln-dried is that enough of an insurance that I won’t have any problems with gaps or buckling?
Question 3: Am I naïve in thinking that $26,000 is too much to pay? As this will be my final “big” building adventure I’d like to do it right.

Thanks for your help. I look forward to reading your response.

All wood should be sealed. Sealer doesn’t stop moisture content from changing, but it helps. The reason you had no problem with the cedar is probably because it was installed at the correct moisture content.

We can’t make any comments about pricing. Pricing varies considerably from one region of the country to another.

Good Afternoon
Would be most grateful if somebody could advise me on a water moisture issue. We had a sewage water leak from a pipe in the stack system unknown to us for 11 months , the leak was repaired but we didn’t realise the impact of the leak into the void space above the kitchen ceiling and under the shower floor which the timbers and joist was treated but brown stains kept appearing which we thought was perhaps the stuff drying out.. five months after the leak a water moisture was placed on the kitchen ceiling about 4 foot from the area which gave a reading of 17% without piecing. My concern is that l have a disabled person in the house who developed serious respiratory problems and took about 13 months before we realised that it was from the leak, in the next 4 weeks this person will be having major surgery so l am ensuring that l have done all what is needed, would 17% be what would have been when we had the leak or would this be a reduced reading, we also have horrid orange stuff in grout on the bathroom tiles appearing could this be connected Thanks for any advise you can give

This article was intended to help carpenters and contractors understand wood and wood movement due to natural moisture content changes. There was NOTHING in the article about correcting damage from leaks. You’ll need to contact a contractor or mold remediation specialist in your area.
Best regards,

Amazingly helpful article.

Question: I live in SoCal, we bought a home with water damage. Our water damage specialist has dried the house slab in our lower floor to 11.2 and believes we’ve removed enough water. If I want to install oak flooring – A. Am I crazy and should consider a different material and B. I’d like to install an 8″ wide engineered Siberian oak which is higher end in price.

Any advice would be greatly appreciated.

I wouldn’t ask your water damage specialist. I’d ask the hardwood flooring installers! Will they warranty the floor if you have them install it on an 11% slab. I don’t think so :)

Any idea on a door problem my client is having ? Client has two exterior doors that are 1&3/4″ thick standard height and width made of alder wood. Shaker type. The room is water heater and storage room. Sits back under a very wide and deep overhang, so no direct sun or weather. Doors keep moving up to 1/2 and inch. My company stained and finished the doors. We in fact sealed tops and bottoms of doors, prior to carpenter installing hinges and hardware. All parties say doors were never cut or shaved. Any idea how they can be moving this much ? Sometimes they’re fine other times when its moist out, they swell up.

I suspect it’s not just the doors that are moving. Could be the jamb isn’t secured really well, could be the framing that’s taking on moisture, the whole wall, etc. I’d read the moisture content of everything in the area and see how much it’s all changing.

This is driving me nuts. In my (own) home, 2300 sq ft ranch, 3 years old.
I have arch ways in my home, in the winter I get lots of cracking, most of my tile has cracked. I have up to 1/4 in gaps between my base boards and floors… it even affects my interior doors.
I am at my wits end, me nor none of my builder buddies can figure this out….
any thoughts or ideas would be great..

Fantastic information.
I have been asked to float a 13 by 16 foot depression in a polished concrete floor up to an exact depth to accommodate some 3/4″ solid red oak flooring. Once installed the oak will be routed in places to make up map of this area. Once done, the oak floor will be coated with 1/4″ of resin. This is for a museum entry area. The building is climate controlled, the concrete slab has a vapor barrier, and has been enclosed and under cover for well over a year.
The plan is to apply a two part moisture barrier in this depression, then add a self leveling concrete underlayment to bring it up to the ideal depth, then adhere the 3/4″T&G oak with Bostiks Best wood floor adhesive, then cap the wood with 1/4″ of resin to achieve the same height as the surrounding polished concrete floor.
The oak will be butted up to the concrete which it is surrounded by, meaning absolutely no room for expansion…… I suppose we can always hope for shrinkage instead of expansion?
Any thoughts or advice are very much appreciated.

Hey Randy, I am curious how your described project turned out. Did you leave room for expansion? Have you had any issues with either expansion or contraction?

We want to glue 14 pieces(12 in. by 6 in.) of veneer .5 mm thick together. Does someone have a specific technic ( ex. hot press, specific glue , points to consider… )

Excellent information. I finally have an answer for the cracked cabinet doors for a custom kitchen I did…and had to replace them all (30 doors).

So if anyone reads this far, my two cents. I live in Albuquerque, NM. We have the same inverted (depends on your point of view, really) humidity cycle as a few other commentators on this thread. Generally high (depends on your point of view, marginally) levels of moisture in the atmosphere in the winter, and low(er) levels in the summer. So, I googled average humidity in albuquerque. BTW- you should all do the same for your town. and I found out some interesting things. Not only are humidity levels generally highest just before daybreak (this is throughout the year), but our humidity levels (on a daily/monthly/yearly) are right in the manufacturers sweet spot of 30 -50%. OK… aside from gloating why is this important? Because it led me to ask some questions. Since, flooring expands with atmospheric conditions, yet Albq.’s atmospheric
conditions are SOO VERY unlike the vast majority of the users experiences, how could we be having similar situations. And, what is the rate of absorption of atmospheric moisture, vis-a-vis, humidity, in various species (and brands) of wood flooring, specifically. And is said rate a ratio, i.e., as the moisture levels seek “balance” with one another, does the rate at which wood absorbs (or weeps) moisture slow down or remain constant. And how does “straight up” heat energy (regardless of its’s relationship w/ humidity) enter into the mix. And then up here in Albq. not only is there less 02 around, due to the higher altitude having a lower atmospheric pressure, but insolation happens-errr.. more Think of insolation as a term for describing the amount of the sun’s solar radiation at a given space, and for a given period of time, expressed in watts. You know how it’s-28F on Everest and every body’s got 3’rd degree sunburns inside their parkas, that’s insolation – sort of. But, and this is where it gets really cool, the suns energy comes into the earth as highly energetic waves, which crash into the atmosphere, and the water vapor contained therein, carom off the H2O molecules and either bounce off, are absorbed by, or pass thru our flooring. Thereby warming it up. Making our strips of oak slightly larger, and, in the process of drying out, they begin to shrink. The meteorological universe, being an open system, has something to say about this and perhaps the earlier “plasmified” water vapor molecules complete the hydrologic process, and begin to coalesce back into water vapor raising the humidity. So one begets the other and we are left where we started, only more confused. But we now know there are other natural phenomenon at play. It is not a zero some game, this humidity thing. What we’ll call the first rule of flooring: a floor at rest tends to get up in the middle of the night to take a pee, or, get a drink of water, or both. A floor is never at rest because there is ALWAYS an odd number of molecules of humidity(?) to be shared by the atmosphere and said floor, creating the old game, “I’ve got more than you do”. And when they are both too tired to play ring around the rosy, and finally dazedly stumble off into their respective places (with atmosphere bear-hugging floor) for a few, fleeting moments rest, the sun looks down upon the exhausted trembling duo, ponders the universe and the creator who could waste such energy/passion/idiocy/stupidity on the young, looks up toward the horizon, seeing his wife, Moon, staring back at him w/ her shrewd all knowing moonness, shrugs, blinks, yawns,stretches, peers once more at the youths, scratches his penumbra, lets forth a perfect beam that shones the two in heavenly, glistening light, Their shivering stops, peace is theirs, if only in their dreams, a moment of forever to walk out in the morning dew…
raising his head from contemplative reminiscence, he spies Moon, his wife, who catching his eye, nods her approval, winks lasciviously, and high tails it for the night. Sun groaned giving chase muttering not again and day sprang forth across our small, yet totally significant, planet. And then Sun remembered that Moon wouldn’t be hiding from him today, it was a special day, after all. Yep, today I get to tap that ass. God, I love a good lunar eclipse.

Hey bored dude, Sorry for the run on thing, I actually had a place to go w/ that, but I got poetic at 4 am. Anyway, I hope you don’t ban me

Can you comment on the effects of cryo-treating wood? There are a number of services popping up that propose (and expensively offer) this as a method of giving “new” wood the desirable properties of old growth timber.

ok, i’ve been googling buckling floors etc. i have a brand new house, less than one year old, the ceiling planks are falling off and the floors are buckling bad! i’ll see if i can upload a few pics. this wood is poplar. i’m sure my ex builders will try to blame all this on me.

buckled, then started falling off. crazy!!

I’ve been a flooring flooring contractor in the same city now for 31 years , I always provide the best service possible for every client ! I’ve always appreciated my clients and the work that they’ve provided me through the years .
Last year a contractor that was working with on a complete renovation witch involved my services.
I was asked to instal over 3800 sq feet of laminat flooring , please note my contractors client is in a wheelchair .
I installed the flooring over 1/4 ” mohagany subflloring that was over 5/8 spruce tg , there where no doors or casings at the time of my laminate instal .
The product in question was from Home Depot and by no means a high quality product ! I’ve installed laminate the same way for all these years leaving approxametly the thickness of the floor as a gauge for my expansion at the walls .. Approx 1/2 inch , note the drywall was tight to the floor , after my complete instal the contractor moved forward and installed all the casing and jambs and baseboards. The job looked perfect ! Everyone was happy , I got a big hand shake and paid in full for my services .
NOW : here’s why it get really tricky !
7 Months go by , it’s late August , the house humidity levels are through the roof , I could simply feel it , the flooring had a huge buckle in the hallway , I asked the client that was using his wheel i chair if he knew what the humidity was at ? He said that he’d unplugged the hrv system and didn’t believe that this was a humidity problem and rather the flooring was installed to tight ! I tried to remedy the hallway but to no avail ! The material was to cheap and it could handle the reinstall . He had also did a lot of damage by rolling back and forth stressing out the integrity of the click system ! .. I immediately contacted the contractor in font of my client and pointed out a few areas where I could see that the flooring couldn’t expand properly.. It was an area at the end of a approx 25′ long hallway leading to an outside deck , the sill wall a ramp type for the client and the flooring couldn’t slid properly with the overly high amount of humidity ! The contractor said he didn’t have time to repair the problem at this point because he was on another project .
I then went to Home Depot and purchased , by luck ! The exact same product and dy lot, 8 boxes or so . I had the job book for the Monday and I’d hired a laminate flooring specialist that had trick tools to re instal the laminate flooring .. Note we where doing a section and doing that becomes difficult because of the click systems and all the painted door jams exc… I needed the go ahead from the contractor and get his assistance in fixing the sill and the fact that all the baseboards and casings had to be removed in order to do the fix correctly ! I could contact him and didn’t feel comfortable going foreword and didn’t want to have this repair turn into an expensive nightmare ! By the Wednesday the client sends me a text and says ” don’t bother showing up , you installed it ….. To tight and the contractor is fixing it , this didn’t allow me back to aleaveat the problem ! the next day the contractor called and said that they were going to fix the floor and realizing that the humidity was way to high and the client wasn’t doing his part in maintaining the proper acceptable rang , he said that they where going to cut all the drywall to allow for even more expansion .. I also recommended raising all door jams and casings the thickness of a credit card to also aid it proper expansion allowances .. He said there was no problems doing this and that they would also instal expansion mouldings in every bedroom doorway that would also help and that there was no costs to me . The client sent me yet another text on the Wednesday showing me pictures of the contractor and his men doing the repair , he was still very upset and his mesages where not very kind ! ONE YEAR LATER ! at the exact same time of year, there total repair expanded again , buckling the floor the same way ! The other problem is that there repair installer didn’t have the know how or the proper tool that I talked about to get those last few boards in and as a result damage many of the ends .
Now the client in a wheel chair is livid ! He’s wanting a complet replacement , the contractor is denying that they did the repair and is trying to put this damage on me . Here’s my problem ! At the end of the day the humidity in the clients home was way to high! No problems in the winter, only when he closes his doors and has his air conditioner on in the hot August months . They didn’t allow me the chance to remedy the problem and they went again themselves and attempted it . I truly believe that if proper humidity levels where maintained and that the contractor had allowed for a better flooring for wheelchair traffic this all would have never happened! Thanks for listening , please comment

I am a 12 year old 7th grader in Chicago and am doing a science fair project on the effects of humidity on different types of wood. Do you know of any place that I can go to meet with someone to talk about this… help

I’ll email you directly.

I am trying to get an answer on what I think should be a fairly simple question. I am putting down kiln dried alaskan yellow cedar 5/4࡬ deckboards. The boards sat on dunnage under a tarp for about 5 weeks on site. Location: Tacoma, WA. Rain.. Rain…..and more rain. Ok, so assuming average moisture content for kiln dried on delivery, this lumber should not shrink, but may actually swell as it absorbs moisture. I can find the data on from green to kiln dried change in radial and tangial %, but not the reverse. If I am understanding all this correctly.. My board gap is going to decrease through the winter, and then come back to install width (providing tbey have not absorbed all moisture in the last few weeks). I don’t want too large of a gap, and don’t want them to swell tight against each other. I think 3/16 should be pretty safe. Could someone please take a moment to lend their opinion.

I’m in the process of boxing in the pressure treated 4ࡪ’s columns on a front porch I added to my house with Western White Oak. I ran short of the oak and ended up having to buy two more boards appox.3/4″x 6″x 96″ the problem that I now have is that my original boards are in the 11% to 12% range and the two new boards are both about 8% after two weeks in my garage (unheated) which is at about 65% r.h. (rainy season in the PNW).
I’m wondering if it might be possible to add moisture by actually dampening boards with a damp cloth or if I should just bring them into the house (normally 65/ 66 degrees& 35% humidity)and wait them to reach the 11 to 12%
that I need them to be?

I realize after rereading this article, that bring the two pieces of wood inside is not the answer the trim inside my house is all at 6%
when I checked yesterday after posting my question.
I’m still wondering if my thought of damping the wood might work or if my only alternatives are to wait till I get the same reading on all the wood or might steaming the wood ( I’d have to build a steaming box) work?

Our company is purchasing custom wood parts for light fixtures we have custom finished. The parts are coming from Pennsylvania and sent to Colorado. The first several hundred pieces, purchased over a period of 1-1/2 years had no problems. Now all of a sudden we are having problems with the wood cracking after it is in Colorado for several days. I am placing the parts inside of a 10″x10″ plastic zippered bag after arrival, but it doesn’t seem to help much. I’m thinking they need to do this before shipping. Is there anything else we can do to keep the moisture inside the parts until they are finished (during shipping and until finishing starts)? Any information you have would be greatly appreciated.

We’re on Long Island and just experienced the extended extreme cold (for us) with temperatures in the single digits and the Relative Humidity below 25%.
The heat was off in the house and we returned to find the front door open. Could there have been enough shrinkage for the door to become unlatched?
I know this is not a fair question with many variables not “nailed down”, but is it a possibility?
PS. The last person out swears that the door was closed and locked, and was not me!
Thanks, George

We are having trouble with expansion of our fitted kitchen cupboard doors. The doors are doors are shaker style and are made in Italy( although the manufacturer ‘s brochure state their kitchens are made by themselves in British factories).
The frame structure is solid ash and centre panels in veneered ash on MDF.
Within 3-4 weeks of installation (May2017), we noticed movement of the doors and assumed hinge adjustment would rectify the problems. As time went on ,it became apparent timber expansion was the issue.
The kitchen was installed by an independent qualified tradesman in our holiday home (advised by ourselves at the time of purchase).
The large national company have refused to inspect the kitchen as they did not do the installation.
They say the cause of the expansion ( and very slight contraction in the winter) is due to local humidity and dampness conditions.
The majority of our doors are either catching when closing or will not close at all. Our previous oak kitchen in this house gave no movement problems.
We have been arguing our case for several months but the manufacturer -retailer (same company)refuse to address the problem. Anecdotally, we believe there have been other similar complaints.
Our opinion is that the materials used are not fit for purpose.They state the timbers used are kiln dried and stored in the correct conditions and then manufactured in Italy. The doors were pre fitted to the units on delivery- the doors originally fitted perfectly.
If the information they have given us is correct, should the doors move to the extent of not being able to close them?A gap allowance of 3mm between the doors was originally allowed.
As we are at a stalemate situation, we would be grateful for any information you could give us.
Many thanks
John & Ann Hearnshaw

Birch plywood is widely used in LNG industry. Usually 9mm or 12mm plywood consisting of 5 plies, or 7 plies respectively is used. From the content of this site, I understood that wood products interact with ambient and the equilibrium moisture content of wood products are determined by the ambient air temperature and relative humidity. But I know that plywoods consists of many plies, each ply 1.2

1.5 mm, and chemical adhesive is used between plies to bond each plies. My questions are
1. Plywood products with many plies interact with ambient vapor ?
2. The equilibrium moisture contents of 9mm and 12mm plywoods are very similar ?
3. Chemical bond layer beween plies affect the interaction between the plywood product and the ambient vapor ?

I appreciate your answer.
Best regards.

I have a transition between Pergo Outlast+ laminate and tile. I really hate T molding. Is it possible to leave a 1/4 gap between the laminate and the tile, filling in with silicone caulk (same color as tile grout) and sprinkle grout at the top?

The silicone caulk will allow for expansion while the grout will make it look like the the tile grout. Since it’s a floating laminate, would glue work for the last piece of laminate next to the tile?

Hello, thanks for this educating read! What do you know about any relationship between elevation and moisture content? We’re in Montana at 5000ft. We regularly receive material from the west coast. It will arrive at 8-10% and seems to remain there during milling. It then goes to get painted or stained. It seems to pick up some MC during this time, but some product is water borne so it likely picks up some MC from that and from sitting in a room where other material is drying. Then, it is shipped up to the mountains where we install it at 7500 to 8500 ft. We have horrible problems with shrinkage that seem to be worse than the MC and RH difference would indicate. It seems possible that the lower air pressure associated with higher elevation might drive additional moisture out of the wood. We have experienced pretty severe shrinkage of 1࡬ fir material that registered as low as 5%. Any thoughts?

I’ve heard that wood which has been around for a long time -decades – centuries – is not as affected by humidity changes and I would like to know what happens as the wood ages for long periods that makes it less susceptible to moisture change.

Not true. Wood is wood. Yes, old growth heartwood, especially vertical grain, isn’t affected nearly as much as fresh growth, sapwood, face grain wood. But all wood is hygroscopic: takes on moisture and releases moisture depending on the humidity. So all wood moves, some a lot, some a little. Just like everything else we use for building homes.

I am building stair treads out of maple 2 1/4”
Wood flooring and gluing them together to make it 11 1/4”
Then repeat the process and glue 2 layers to make it 1 1/2” thick by 11 1/4” do you see any future problems by doing it this way? And is wood glue sufficient or should I use a premium wood flooring adhesive? Also I am building them in my garage at a temperature between 50 and 60 degrees is this okay?
David Nelson

It’s not the temperature that matters but the humidity in your garage compared to the in-use humidity inside your home.

Also detailed information on moisture such as RH and EMC lets you make more educated woodworking, flooring installation, and renovation decisions so that you can prevent costly mistakes.

This is great post understanding moisture content and wood movement. Thanks for sharing.

Very detailed. I am glad to read this post.

Thanks for the tip to think of a glued-up panel as one piece of lumber. I don’t know much about lumber honestly. But even the smallest information about it helps me out.