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)
 

Carolina(s) On My Mind

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

HowardBrickman_SurvivorMan.jpg
I hope that my loyal fans will forgive the temporary lapse in blogging. Hurricane Irene blew through town, taking with her our electricity for almost a week. On the plus side, I got to break out my “Survivorman” skills and lived to tell the tale. Wood has many other uses besides wood flooring. With my handy new Rocket Stove I was able to cut up the fallen branches and heat water for coffee and tea and also cook some one-pot meals.

By the time my power was restored, I was off to Charleston, S.C., to attend a wood floor class organized by Selva Lee Tucker featuring faculty from North Carolina State University. I must say: The class was yet another home run by Lee. With 10 years of instigating wood floor technical training under his belt, Lee has a knack for seamlessly combining hard-core practical and scientific knowledge.

The four-day class was instructed by two North Carolina State wood science professors and a wood floor guy from the Boston area. (Guess who the latter was?) The fee was $495, and those in the know can appreciate what an incredible value that is. It was a sell-out crowd, with over 50 attendees flying in from all over the U.S.

This was the sixth collaboration between Lee and NC State Wood Science faculty, several of which I have attended. Two of NC State’s finest taught fundamentals regarding the effects of moisture on the building envelope. An all-encompassing body of information was presented regarding the effect of temperature on relative humidity, how moisture moves through building components, and how fungi develop when exposed to moisture.

Dr. Phil Mitchell, wood products extension specialist and associate professor, has extensive experience working at major universities (Mississippi State, North Carolina State), and international wood products manufacturers (Weyerhaeuser). Dr. Phil is an acknowledged authority on the wood-moisture relationship with regard to academic as well as practical applications.

Dr. David C. Tilotta, associate professor and housing extension specialist, has a great deal of experience with contamination of buildings from chemicals and water, with extensive research and teaching background at the University of North Dakota and North Carolina State University. Dr. Dave introduced a software program from Oak Ridge National Laboratories that models temperature and moisture movement in exterior wall, floor, and roof systems.

On a personal note, do you remember how intimidated you were by your professors in college? This couldn’t have been further from the case with these two fun-guys who taught about fungi (pun intended). Tilotta and Mitchell are two of the best-natured and engaging college professors, and they took some fairly complex material and made the concepts much easier to digest (another fungi pun). The presentations were excellent, and the ease with which they answered questions worked to support true understanding. I left wishing that they had been my professors when I was an undergraduate (back when dinosaurs roamed the earth).

It has been exciting to see a major university put some effort into supporting the wood floor industry. Those of you out there who need studies, research, or laboratory testing should have the folks at the NC State Wood Products Extension Department on speed-dial. Dr. Phil’s email is phil_mitchell@ncsu.edu.

Last year, I had the opportunity to attend a three-day Wood I.D. class at the NC State Wood Anatomy Lab led by two of the leading wood anatomists in North America. The class was my personal favorite, and I’m hoping that it will become an annual event or at least every other year. I could see myself making a pilgrimage to attend. Even after 33 years in the wood flooring industry, I’m always pleasantly surprised to take away new insights about my specialty.

Stay tuned for my next blog: a step-by-step checklist to minimize installer liability when there are problems on the job.

The Concrete Jungle

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astmlogobWeek before last, I attended the Resilient Flooring Committee (called F-06) meeting at ASTM* Headquarters in Philadelphia. This committee is on the cutting edge when it comes to outlining methods and standards for testing moisture in concrete. I must say, it is impossible to not be impressed by such a smart group of hard working professionals striving to create good documents based on the best available science. The F-06 committee is the driving force behind developing the internal Relative Humidity testing of moisture in concrete (ASTM F-2170).  

Having studied the science behind drying and curing, I am always surprised by the amount of controversy surrounding the moisture testing of concrete. There are many millions of dollars in failures of coatings and floor coverings due to excessive moisture in concrete subfloors. Large commercial job failures are financially devastating for flooring contractors. How does the song go?  “Send lawyers, guns, and money.” It seems to me that it is easier to test the concrete for moisture than to figure out how to weasel out on a failed floor.

The fascinating part of this meeting was the discussion and balloting concerning a proposed new guideline for using electronic concrete moisture meters. Anyone knowledgeable about these meters realizes that while they are useful for finding building leaks (high readings for wet areas versus lower readings for dry areas), they do not actually measure the quantity of moisture in the concrete. In fact, the proposed guideline stated:

This meter is not intended to provide quantitative results as a basis for acceptance of a floor for installation of moisture sensitive floor finishes.

It was a spirited discussion and while the vote was close, the document was sent back for further revisions.  In case you were curious, I voted for further revision.  It seems that there is no pressing need for a prestigious organization like ASTM to produce a document which might be misinterpreted, or worse yet, mis-used by unscrupulous individuals.

All in all, it was two days well spent furthering my understanding of the concrete industry, and highly recommend becoming involved with ASTM to anyone interested in doing the same.

*ASTM = The American Society of Testing and Materials is an organization that helps groups of interested volunteers work to create documents that standardize test methods and procedures for concrete.  It is a transparent and open process for anyone who wants to participate.