by Gene Wengert
email@example.com WOOD DOCTOR’S Rx
Symptom. When a piece of lumber is
initially flat and straight, but after
drying the lumber is no longer straight
but bends or crooks to the left or right,
this defect is called side-bend, sweep or
Causes. Wood does not usually shrink
in the lengthwise direction, but there
are three instances where we see sub-
stantial lengthwise shrinkage:
a) Juvenile Wood, which is the wood
within about 15 growth rings around
the pith (center) of a log.
b) Tension Wood, found only in
hardwoods, is caused when the tree is
stressed while growing. Tension wood
is very weak and characterized by fuzzy
or stringy fibers when sawing, but can
be hard to identify.
c) Compression Wood, found only
in softwoods, is caused when the tree
is under stress and is mostly in the
juvenile portion of the log. It is denser
wood that is very weak and makes the
stem oval shaped.
Cures. There is no cure or changes in
drying practices that control this warp.
However, if the sawyer can position
the growth rings in a piece of flatsawn
lumber so that the rings are centered,
with the right edge (looking at the
end grain of the lumber) a mirror
image of the left edge, side bend can
be minimized. Quartersawn often will
have juvenile wood on one edge and
not the other, so quartersawn and rift
sawn lumber are prone to side bend
warp. Tension wood is spread through-
out a log, so it cannot be eliminated or
controlled when sawing. Compression
wood can only be minimized by elimi-
nating piece with this wood.
Symptom. Cup is warp in drying that
occurs from edge to edge.
Causes. Cupping is caused when one
face of the lumber shrinks more than
the other face. Because the bark side
of lumber shrinks more than the heart
wise, this defect is found in flatsawn
lumber. The natural difference be-
tween two faces is larger when the lum-
ber is closer to the pith, which usually
is low grade lumber due to the many
knots near the pith; lumber far away
from the pith, upper grade usually, has
little difference so it is not likely to cup.
Cup is accentuated by allowing partly
dried lumber to regain moisture (rain,
snow, too humid in the kiln). Cup
amount is also increased as the lumber
Cures. Cupping is a natural occurrence,
so we cannot eliminate it. To minimize
cupping, avoid rewetting partly dry
lumber and avoid over-drying.
Symptom. Bow is warp in drying along
the length that makes the lumber look
like a ski; that is, the opposite direction
of side-bend warp.
Causes. If the lumber is not stacked flat,
then it will be bent and assume this
bent shape. Also, bow can occur when
one face shrinks more than the other
face. Juvenile wood, compression wood
or tension wood that is on or closer to
See more at the Wood
Dr. Knowledge Center
See more columns at
Sponsored by Northwest Hardwoods.
Rx for Better Drying, Part I
A look at symptoms, causes and cures for a variety of problems.
; Want more? To search a full list of
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.