by Gene Wengert
firstname.lastname@example.org WOOD DOCTOR’S Rx
QWe buy lumber from a reputable dealer, store it and then make various
Virtually all complaints are within the
solid wood panel products
and ship them to the customers, all
over the United States. All too often
for us, the customer reports in the
wintertime some end cracks and
some upward cupping, which we have
to have repaired. Expensive! What
is causing this and what can we do?
first six months.
AGood question. We are 100 percent certain that the defect of splitting you men- tion is the result of drying
after the product is made. (Oftentimes
the splits at a glue joint are from a weak
glue joint due to a manufacturing error.
Splits within the wood are actually pre-existing, but undetectable, cracks in the
lumber that reopen and worsen after
manufacturing. Dry wood is too strong
to split from a small moisture change.
Better joints and better defect detection
when cutting the lumber can indeed
help, but still the issue is drying of the
Likewise, we are about 99 percent
certain that cupping is because the
wood is not dry enough, so it dries
after installation. We might argue that
the customer’s home is too dry for the
wood’s moisture content (MC), but this
is so rare, that we know the wood is too
wet for the customer’s home.
To solve this problem, let’s go over
some basic knowledge about wood.
1. A little swelling (which is a gain
in moisture) creates minor problems
in most cases. A little (or even more
than a little) shrinking (which is a loss
in moisture) creates big problems with
cracking and cupping. Further, if the
moisture change is rapid, the problems
are worse than a moisture change that
takes six months (winter to summer or
summer to winter).
2. Air at 30 percent RH will create
6 percent MC in wood, so we call the
air as having 6 percent EMC conditions
(equilibrium moisture content). A 6
percent increase in RH to 36 percent
RH means that the EMC increases by 1
percent to 7 percent EMC; a 6 percent
drop in RH means the EMC drops by 1
percent to 5 percent EMC.
3a. When outside air is brought
into a manufacturing plant and the
air is heated to comfortable levels for
workers, its relative humidity drops to
very low levels (under 6 percent MC in
wood), even when the outside is near
100 percent RH or foggy. Oftentimes,
the outside humidity is already quite
dry. Check your own humidity outside
by watching the local weather at noontime on TV. (Heated air around the
finishing booth is extremely dry.)
3b. Note that this same effect occurs
in homes that are heated. So, the EMC
in homes in the wintertime runs about
30 percent RH average, or 6 percent
EMC. If the lumber is not 6 percent
MC, then it will change its MC when it
reaches the plant or home.
See more at the Wood
Dr. Knowledge Center
See more columns at
Sponsored by Northwest Hardwoods.
End cracking and cupping reported
The defect of splitting is the result of drying after the product is made.
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Wood Doctor’s Rx question and answers, go to
Gene Wengert, “The Wood Doctor,” has been training
people in efficient use of wood for 35 years. He is
extension specialist emeritus at the University of Wisconsin-Madison.