Brickman Consulting

Wood Flooring Solutions.

Calculating Shrink/Swell and Why It Matters

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Cross posted from Howard Brickman's article on Hardwood Floors Magazine: Inspector Blog.

Wood shrinks and swells when it loses and gains moisture content (MC). For example, if a piece of 2¼" wide plainsawn red oak flooring were to decrease in MC from 8% to 5%, the net change in MC would be 3%. Using standard values from the Wood Handbook Table 4-3 Shrinkage Values of Domestic Woods, the net change in dimension would be .021" (2.25" x .086 x .03 / .28 = .021"). Expressed as a fraction, .021" would be between 1/64" (.015625") and 1/32" (.03125").

How is this information useful? Let’s take a real-world scenario and show how a quantitative understanding of dimensional change helps us perform an analysis.

SCENARIO:
We look at a wood floor that we installed last year where the customer has called to complain about gaps between boards. As part of our normal procedure, we look at the surface of the floor to see if individual boards are flat, cupped (concave), or crowned (convex). In this case, the boards are still very flat. Then we determine the size, frequency and distribution of the gaps. We note the minimum and maximum gaps, then we eliminate the smallest and largest gaps to describe the range, which characterizes the majority of the gaps (80-90%). In statistics this is referred as “eliminating the outliers." Now we choose several locations where the gapping is the most severe and begin a series of accurate board-width measurements, along with MC of the individual boards. Our results are:

MC of all of the boards is less than 6%. We estimate the MC at 5% based on interior RH for the last three weeks using Wood Handbook Table 4-2 Moisture Content of Wood in Equilibrium With Stated Temperature and Relative Humidity. The widths of individual boards range from 1/64" to 1/32" less than the manufactured width of 2¼". The gaps are located between every board and range in size from 1/64" to 1/32".

Danger! FORMULA ALERT: IF YOU BECOME SHORT OF BREATH, BREAK OUT IN A COLD SWEAT, AND HAVE DILATED PUPILS WHEN YOU READ FORMULAS, please skip this section of the blog. For you brave souls, let’s proceed.

FIRST FORMULA (Change in Dimension)
Δ D (change in dimension) = Manufactured Width x St (Shrinkage factor from Wood Handbook) x Δ MC / .28

SECOND FORMULA (Change in Moisture Content)
Δ MC = [Δ D x .28] / [ Width x St ]

With these two formulas we can:

1 – Predict the amount that a board will swell or shrink (Δ D) and
2 – Estimate the magnitude of change in MC (Δ MC) based on the current width of the board.

IT’S SAFE TO START READING AGAIN. Danger over.

Now it’s time for some SHRINKAGE RULES:

Rule Numero Uno: If a board is less than its manufactured width, it has lost MC since it was manufactured.

Rule Numero Dos: If a board is exactly its manufactured width, it is at the same MC as at the time of manufacture.

Rule Numero Tres: If a board is greater than its manufactured width, it has gained MC since it was manufactured.

Applying Rule Numero Uno, we know that our boards that are smaller than the manufactured width have lost MC. Using the SECOND FORMULA for Δ MC, we can pretty accurately quantify the change in MC.

For boards that are 1/64" narrower than 2¼", the Δ MC is 2.26%:

Δ MC= [Δ D x .28 ] / [ Width x St ] Δ MC= [ 1/64" x .28 ] / [ 2.25" x .086 ] Δ MC= [ .015625" x .28 ] / [ 2.25" x .086 ] Δ MC= [ .004375 ] / [ .1935 ] Δ MC= .0226 = 2.26%

For boards that are 1/32" narrower than 2¼", the MC is 4.52%:

Δ MC= [Δ D x .28 ] / [ Width x St ] Δ MC= [ 1/32" x .28 ] / [ 2.25" x .086 ] Δ MC= [ .03125" x .28 ] / [ 2.25" x .086 ] Δ MC= [ .00875 ] / [ .1935 ] Δ MC= .0452 = 4.52%

If we add the Δ MC to our current 5%, the boards that are 1/64" narrow were originally at 7.26% (5% + 2.26% = 7.26%). The boards that are 1/32" narrow were originally at 9.42% (5% + 4.52% = 9.53%). This allows us to estimate MC at time of manufacture between 7.26% and 9.52%.

I find these quantitative methods to be useful tools when working through the analysis of a wood floor that has evidence of a change (or changes) in MC. In new construction there are frequently several MC changes, starting with the adsorption of excessive moisture from the subfloor, then the eventual drying during the following winter heating season.

Let’s explore how doing all this rigmarole calculating helps with analysis. Let’s change our scenario by a single factor: instead of gaps that range from 1/64" to 1/32", how about gaps that range from 1/32" to 3/64" with individual board shrinkage that ranges from 1/64" to 1/32"? We have already done the calculations on the board shrinkage, but that doesn’t account for the additional size of the gaps. SO… something besides seasonal low interior RH would have to be the cause of the increased size of the gaps. Maybe the flooring was left on the job to “acclimate" and picked up some excessive moisture before it was it was installed? Or… (to be continued)
 

Try Ignorance

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Cross posted from Howard Brickman's article on Hardwood Floors Magazine: Inspector Blog.

If you think education is expensive, what do you figure the going rate for ignorance is? Or, do you think no one will notice when you are faking it? The “good” news is that most of your customers are very poorly informed and won’t know if your information is not correct. So if everyone uses the same incorrect information, who will notice? My intent in writing this blog is to challenge some of the mythology regarding wood flooring that has been incorporated into the collective consciousness of the wood flooring industry and to share some excellent sources of basic knowledge. But I will start with the warning that you should not believe anything unless you truly understand its basis in knowledge. With technical and scientific knowledge, this means that you need to understand the physical world described by physics, chemistry, biology, botany, and math.

An important part of understanding the knowledge is the words that comprise the vocabulary of every profession, skill, or specialty. Where it is feasible, it is best to avoid reinventing the wheel with regard to terminology. It is especially useful to bring terms from the basic fields of science into our specialty to build a bridge into the basic knowledge and establish a direct link with our specialty and what we can prove. Does this mean that everyone needs to become a physicist? No, but we should respect the terminology that describes the physical world and not invent our own mock science to describe what goes on around us.

So, here we go. Wood “shrinks” and “swells” when it loses and gains moisture content… like that fancy wool sweater that you should not have put in the dryer at the high heat setting. The words “expand” and “contract” generally refer to dimensional changes that occur due to changes in temperature. Although very similar in meaning to swell and shrink, it is not technically correct. The Wood Handbook is the one of the best sources for information about wood that is technically accurate. It is published by the Forest Products Laboratory in Madison, Wis. Better yet, it’s free if you have a computer with Internet access. It is a technical book that requires some understanding of basic science and math. It also contains almost all of the fundamental knowledge about wood and lumber that professionals would need for their entire career. If you are only going to have one book in your professional library, make it The Wood Handbook.

The wood floor bidness has developed some less-than-accurate habits over the years with the words we use to describe wood and how it works. We are not disciplined about using technical words. And using the correct words is a great habit to develop, because it greatly limits misunderstanding. Misunderstanding can easily transition into differences of opinion, which can easily transition into disagreements, which can easily transition into disputes and problems. Disputes and problems are bad for business, which wastes a great deal of our mental energy and material resources. Distractions that you enjoy are called hobbies, while distractions that are not enjoyable are called problems. It makes no sense to cultivate any hobbies that are not enjoyable . So when you refer to dimensional changes related to changes in moisture content, please use the magical words “shrink” and “swell”.