Buckling is an example of a serious moisture issue that often could have been avoided with acclimation before installation.

An old professor of mine once said, “95 percent of all problems that you will experience in wood in your career will be moisture-related in one way or another.” Thus far, he is correct. Regardless of your level of experience, there is one critical aspect that you must take into account in order to produce a claim-free wood flooring installation: control of the moisture content (MC).

This is the most critical aspect to properly manufacturing, installing or maintaining any wood-based product because wood is “hygroscopic.” Hygroscopic means “water loving,” and hygroscopic materials gain and lose moisture depending on the environment. When wood gains or loses moisture, it changes dimension. Dimensional change in solid and (to a lesser extent) engineered wood flooring can wreak havoc, and depending on the amount of dimensional change, defects such as buckling, gapping, checking, splitting, cupping and crowning can occur. It is important to understand a bit about the manufacturing process and how the ambient atmospheric conditions can affect wood flooring’s performance.

RELATED: Understand the Science of Water and Wood Floors

For example, most manufacturers produce solid and engineered wood flooring products going into the box with an MC of 6–9%. MC is defined as the weight of the water contained in the wood, expressed as a percentage of the weight of the dry wood. Wood products produced for interior applications in the U.S. and Canada are generally manufactured to hold the 5–9% MC range.

Dimensional change in solid and (to a lesser extent) engineered wood flooring can wreak havoc.

Generally speaking, those interior environments hold approximately that same atmospheric MC, also known as equilibrium moisture content (EMC), year-round. EMC is defined as the MC at which wood neither gains nor loses moisture when surrounded by air at a given relative humidity and temperature. In essence, when wood at a specific MC is placed into an environment that does not match its MC, the wood will gain or lose moisture until equilibrium with the environment is met. The result of this gain or loss in MC is dimensional change.

Most manufacturers require that wood flooring products be “acclimated” to the environment in which they are going to be installed. Improper acclimation typically results in gapping between boards (in drier conditions) and buckling or crushing (in wetter conditions). For example, if an installer fastens 3⁄4-inch-thick, 31⁄4-inch-wide Northern red oak (predominantly flatsawn) flooring with an average out-of-the-box MC of 6% in an interior atmosphere that has an 11% EMC without proper acclimation, he or she can expect approximately 4.4 inches of expansion in an installation 20 feet in width.

Nearly all of the dimensional change in wood flooring is across its width and through its thickness, with virtually no change along its length. Time exposure is a critical element that is rarely discussed when using this calculation. The calculated value of dimensional change does not take into account varying interior EMCs; therefore, the 4.4 inch total change in dimension is a worst-case scenario. Due to the short annual seasons in many temperate areas, only a percentage of the total calculated change is experienced with an installed hardwood floor.

No matter where you are or which product you’re installing, controlling moisture on every job site is critical to avoiding those 95 percent of wood floor problems.

RELATED: How to Prevent Cupping and Worse in Summer Months

See more on this topic: Moisture & Wood Floors

Brian Beakler, PhD., is manager of innovation at Lancaster, Pa.-based AFP (Armstrong Floor Products).

Dear Brian,

I always enjoy articles about moisture related issues. And, most issues are moisture relation. I suggest you change a word in your article...... Cupping, curling, buckling, gapping, checking, splitting, crowning ARE NOT DEFECTS. They are issues.

Norm Kaswell
Flatsawn red oak = 8.6% tangential shrinkage from green (30%) to oven dry (from USDA Forest Service Handbook). So a MC change from 6 to 11% is prorated to 1.41% (8.6*(11-6)/(30+0.01*8.6*6). 0.0141 x 3.25 = 0.046" swell (board will swell in width to 3.296). 20 ft = 240". 240/3.25 = 73.85 boards. 73.85 x 0.046 = 3.38 total change in dimension
The methodology used in the scientific realm to estimate dimensional change (from shrinkage and swelling) between 6% and 14% can be found in section 12-15 of the USDA Wood Handbook and it utilizes defined coefficients for dimensional change. The equation used: Change in dimension = Initial Dimension [Tangential Coefficient of change for N red oak (MC (final)-MC (initial)]. So using what was provided in the example we see that Change in dimension = 240" [0.00369 (11%-6%)] resulting in the change in dimension being about 4.4" over the 20' installation (also assuming no gaps between the installed boards).
The disadvantages of using the dimensional change coefficient (found in table 13-5 in the 2010 Wood Handbook) are (1) it's less accurate, (2) it's only valid for 6-14% MC, and (3) there are fewer species listed compared to the shrinkage values in Table 4-3. I use a formula taught by NC State Univ that's very close to the one referenced on page 13-17 of the 2010 Handbook or p. 120 (bottom left corner) of Understanding Wood by Hoadley. Here is a simpler (but less accurate) version of the formula (see p. 119 of Understanding Wood):

8.69% = Green to OD tangential shrinkage value for Northern Red Oak.
(11-6)/30 = 1/6 or 0.167
8.69% x 0.167 = 1.45% (prorated shrinkage value)
240" x 0.0145 = 3.48"

There's an online calculator at


It calculates the value as 3.6857
Thanks for the article Brian. Very insightful. I believe though, if my mind serves me correctly, and I am sure it depends on the species, that you can expect around 0.1% change along the length. With some of the newer, wider and much longer plank flooring, some reaching length of up to 10-12 feet that 0.1% can become larger than you would think. This makes proper acclimation and maintaining a consistent RH even more important. I have heard of issues with excessive gapping in the colder climates where the flooring can be severely dried out during the heating season. Thoughts?
I absolutely agree with your comment regarding the small longitudinal change creating gapping over longer length boards. This does make acclimation extremely critical!! The reality of longintudinal change comes down to the micro-anatomy of the wood's cellular structure and its orientation along the length of a board. Most of the bonding sites on cellulose (which makes up a large % of the wood cell) for water adsorbtion or desorption are lateral in relation to the length of the length of a board (or a tree as it stands on the stump). Because of this fact, most dimensional change occurs in the tangential or radial directions. Sometimes, depending on species or other site/natural influences, cellulose that make up the wood's cellular structure will not be oriented in a parallel fashion to the length of board (i.e. spiral grain). Hence, some of those lateral sites for water gain/loss now have some influence on the overall longitudinal change of a board. Hope this helps!
My understanding is that 6-14% range for Dimensional Change Coefficient is chosen for 2 reasons; it roughly corresponds to normal relative humidity conditions in use (30% to 70% RH), and it is close to linear (shrinkage happens faster earlier in the drying from fiber saturation and slower below 6%). Also , my copy of Hoadley states 28% as fiber saturation point for red oak; i like to fiddle & argue the math, but the general principal is clear!
Hardwood Industry Professional Wednesday, 04 February 2015
If you depend on Change Coefficient to determine what's actually happening on a real world installation then you don't understand wood flooring. Find me a "predominatly flat sawn" wood floor as Beakler suggests in this article. 4.4 inches of growth across 20 feet based on an increase of 6% to 11% is not real world
Hey "professional." That probably wasn't the point of the article....

You live in PA so you can follow this. If I acclimate my flooring in January in a house with forced hot air heat for 10 days-2 weeks. I have my Armstrong "American" haha Scraped hickory flooring delivered at 7%. With the forced hot air the house is at 30% rh. Now my wood looses mc to 5 and I install it. Don't you think in the summer when the rh even with AC is at 65% that floor will cup as the flooring mc goes up?? You have to know your zones and the rh & mc swings that will occur with the change of seasons and plan for that IMHO. This is why I never acclimate more than 2-3 days in the winter..
The HW Professional is right. An installed floor can't move 4". First of all it is fastened. That's why it cups with a moisture gain. It could gain the 4" if it were lying there unfastened if you left a 2" expansion space on each side. Otherwise it would hit he wall and buckle. Of course the end user would probably be complaining about squeaking long before that much swelling could occur.
[color=navy][/color]Clients need to be advised of the importance of maintaining a fairly stable relative humidity in their homes. A hygrometer costs approximately $25 dollars. It will give you a reading on the relative humidity in your home. Acclimation is crucial, but so is indoor climate control. If you are investing thousands of dollars on new flooring, you need humidity control as well to protect your flooring investment. Check out Aprilaire.
Interesting , I am starting a small cheese board business in South Africa. I want to make our boards mostly from white oak which I buy in various lengths at a thickness of approximately 1inch. So my question is when cutting these boards down the middle in its thickness..how will the moisture on the outside of the board differ from the side the cut has been made? Will this cause cupping of some sorts because of moisture levels being different on rough and cut side? Thanks for a good article!