Estimating moisture content changes

Q: We have a floor that has developed some cracks between the pieces. We know that the moisture content right now is 6.2 percent MC. Is there any way to calculate the moisture content at the time of installation prior to the development of cracks?

A. I have included this flooring question in my column as it illustrates how we can make a very good estimate of historic MC values for wood panels, table tops, and so on.

On April I visited the Rooney residence in Chippewa Falls, Wis. I was asked to examine the sapele flooring which was cracked. In this situation the floor was made of 4-inch wide pieces of quartersawn sapele. We assumed that the floor was without cracks when installed, so the cracks indicate that the floor has shrunk after installation. Such shrinkage was caused by a loss in moisture.

To ascertain the extent of the shrinkage, I choose a typical appearing floor section that spanned 56 inches (14 pieces of flooring). I measured the present width that these 14 pieces spanned and it was 55-25/32 inches. Dividing this by 14, the average initial width of the flooring was 3.98 inches. This width is close enough to 4 inches as to be acceptable (unless the specification states otherwise).

I then measured the size of the gaps in this 55-3/4-inch span using a rules marked in 1/100 of an inch and a magnifying glass. There were four gaps and they were a total of 0.18 inch. The total size of the gaps (0.18 inch) divided by the width of the floor (55-25/32 inch) multiplied by 100 gives the percent of the distance that was gaps, or the amount of shrinkage. This value was 0.32 percent.

The textbook shrinkage for quartersawn sapele going from green (30 percent MC) to oven-dry (0 percent MC) is 4.6 percent. (Such data is in Chapter 4 of the 2010 edition of the U.S. Forest Products Lab’s Wood Handbook.) This means that for each 1 percent moisture content loss, q-sawn sapele will shrink (4.6 percent divided by 30) 0.15 percent. I can calculate the estimated moisture loss after installation by dividing the measured shrinkage of 0.32 percent by this 1.5 percent value. So, 0.32 divided by 0.15 equals 2.1 percent moisture content moisture loss.

Conclusion: After installation, the floor lost 2.1 percent moisture content and developed the observable cracks. Because the measured moisture value is now 6.2 percent MC, this means that the floor was installed at 8.3 percent MC.

Additional note: It is unlikely that these cracks will close tightly if only 2.1 percent is added back to the wood over a month’s time. For several reasons, including the hysteresis effect, it may take an extra 1 or 2 percent moisture. To achieve this high MC of about 10 percent MC would require more than 54 percent RH, which is not an acceptable level of humidity for many homes or offices. Further, if they do close after a long, high humidity exposure, they will reopen when the humidity drops.


Have something to say? Share your thoughts with us in the comments below.

Profile picture for user genewengert
About the author
Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.