Wood Movement – You Can’t Stop it

Most flooring contractors and 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. With a moisture meter and an understanding of wood movement, most wood movement problems can be avoided.

The Nature of Wood

Wood is hygroscopic, which means its moisture content (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

HUMIDITY MOISTURE CONTENT VERSUS RELATIVE HUMIDITY
RELATIVE HUMIDITY WOOD MOISTURE CONTENT
0% 0%
25% 5%
50% 9%
75% 14%
99% 22-30%

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 flooring 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 acclimate 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 a low 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 floor workers and carpenters, the EMC is more helpful than RH.

Complete EMC levels for wood stored in unheated structures in your area of the country can be found from the National Climatic Center of the National Oceanic and Atmospheric Administration.

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 flooring contractors and carpenters should be aware of when it comes to wood movement.

  1. Width of material

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

  1. 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.

  1. 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.

  1. 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 flooring installations. It’s worth noting that panel material (plywood) moves 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.

  1. 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) 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.

 

Board Width in Inches

Species 4 8 10 12 14 16 24
Most Cedars & Redwoods -1/16” 1/16” 1/8” 1/8” 1/8” 3/16” ¼’
Most Pines, Alder, Ash, Walnut, Fir 1/16” 1/8” 1/8” 3/16” 3/16” ¼” 3/8”
Red Oak, Hickory/Pecan, Beech, Maple, White Oak 1/8” 1/8” 3/16” ¼” ¼” 3/8” ½”

If you want to know exactly how much the material you’re using is going to shrink or expand, you can use “Wood Web – Online Shrinkage Calculator” to easily calculate these values. Simply enter the high and low MC values and the width and species of the board.

  1. 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 is important that you document the delivered MC, just in case wood movement becomes an issue. But responsible wood installations 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 flooring material is 6-8%. In the real world, your material may arrive around 9-10%. 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.

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 will 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 dry walling, tremendous amounts of moisture are often added to the air, causing humidity spikes as high as 80-85% RH. If you are storing flooring 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.

Wood flooring should not be installed until the temporary construction humidity has subsided. Use an accurate digital hygrometer to measure RH (under $40). Generally speaking, wood flooring 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 engineered flooring construction for wide plank application instead of solid wood.

In-service high humidity issues

Typically, high humidity (constant levels above 60%) can be 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—85% RH means an 18% EMC. A 7”. wide piece of White Oak installed at 8% MC in such a room will swell in width over 1/4 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 flooring material will experience severe shrinkage. A carefully-worded disclaimer regarding wood movement would seem to be in order.

Understanding Accumulated Wood Movement

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.

If the material (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.

Common Myths

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, use engineered material, and specify vertical grain.

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

References:
Wood Handbook: Wood as an Engineering Material (Agricultural Handbook 72), Forest Products Laboratory, Forest Service, U.S. Dept. of Agriculture, revised 1987.
NWFA Technical Manual – Publication No. A 100, National Wood Flooring Association, Chesterfield
Wood Web- http://www.woodweb.com/cgi-bin/calculators/calc.pl