responds: Depending on the wood type,
moisture content, and exposure, deck boards can
cup, twist, or decay. Theoretically, you can ensure
that a board will cup downward to shed water, and
thereby reduce deterioration, by exposing one face
or the other. In fact, seldom does a week pass that
I’m not involved in a discussion about
whether to install deck boards either bark side up
or bark side down. But such recommendations usually
only account for one of a handful of factors that
content. The original moisture
content of a piece of lumber controls a
board’s final shape. Wood is stable when
its moisture content is above 30% (fiber saturation
point). As wood dries below 30%, it shrinks. Wood
shrinks and swells twice as much in the direction
parallel to the growth rings as it does
perpendicular to the growth rings. The combined
effect of these different rates of movement causes
lumber to deform.
Flat-sawn lumber, which is cut so its wide face
is parallel to the growth rings, cups as it gains
or loses moisture. A good way to visualize the
typical distortion is to imagine that the growth
rings straighten as wood dries. Therefore, wet
lumber will tend to cup toward the bark side as it
dries. Kiln-dried lumber, on the other hand, is
usually surfaced after the lumber has been dried,
and will tend to cup away from the bark side as it
Heartwood is often more resistant to decay than
sapwood of the same species. But heartwood is
difficult to impregnate with wood-preserving
chemicals, while sapwood is easy to treat. As a
result, the most resistant face of treated wood is
often the bark side.
Shelling. Growth rings
have two parts — earlywood and latewood.
The inner layers of each growth ring (closest to
the center of each ring) are formed during the
early part of the growing season. The outer layer
grows later in the season. Repeated cycles of
wetting and drying sometimes cause the layers of
earlywood to separate from the layers of latewood.
This separation, called shelling, occurs
infrequently. When it does, it is associated with
flat-sawn softwoods like Southern Yellow Pine and
Douglas fir, that is laid barkside down.
In-use conditions. No
matter which face is up, the ultimate shape of a
board will be influenced by its moisture content.
If one side is wetter than the other, that side
will expand and cup the board. Since the underside
of a deck built close to the ground experiences
higher relative humidity than the upper surface,
which is exposed to wind and sun, the boards often
Pick the best face.
When all is said and done, you needn’t
remember all these factors when trying to decide
which face to install up. In truth, the answer is
very simple: Pick the best looking face, selecting
out the faces with the most knots and wane, and
install your decking best face up. Secure fastening
and an annual coating of water repellent will do
the most for keeping the boards flat.
— Paul Fisette is a wood technologist
and director of the Building Materials Technology
and Management program at the University of
Massachussets in Amherst, Mass.