WOOD DOCTOR’S Rx
by Gene Wengert
ewengert@wisc.edu
Estimating moisture
content changes
Testing for and avoiding
longitudinal stress
Q:We have a floor that has devel- oped 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 ques- tion 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
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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.
Q:We are using four pieces of 4/4 hard maple to form a thicker
piece of wood. When we look at these
pieces after they are glued up, we notice that they are warped lengthwise.
We call it bow. Oftentimes they have
bowed so much we cannot use them.
What are we doing wrong?
A:I would have to do some testing to be 100 percent, but my initial
answer is that the lumber has drying
stresses in it. These stresses are lengthwise stresses in your case and would not
show up in the common “clothes pin”
stress test. (Drying stress is also called
casehardening, but nothing is harder.)
The water in the adhesive you are using
releases these stresses on the glued
edge, but not on the other edge. As a
result, you will see lengthwise warp. The
cure is to get these stresses removed
in drying by properly using a process
called conditioning. Conditioning is a
brief steaming treatment at the end of
the drying cycle.
The best way to test for lengthwise
(also called longitudinal) casehardening is to take a piece of lumber about
8 inches wide and 24 inches long. Rip
this into two pieces each about 4 inches
wide. Put the two pieces back together.
If there is too much stress, there will be
a gap between the pieces when you put
them back together.
Note that some species of wood, such
as yellow poplar, and many non-native spe-
