responds: Wood tends to shrink and swell about
twice as much along its growth rings as it
does across those rings. To see how this
effects deck boards, let me illustrate two
In the first, think of a regular flat-sawn board
with the rings forming long arcs along the end of
each board. As the board dries, the growth rings
contract along their length. The longer rings will
shrink from the shorter rings. Effectively, the
rings are trying to straighten out, which cups the
board towards the bark side of the tree. If the
board is picking up moisture, the reverse happens.
The individual growth rings expand along their
length, with the longer rings expanding more than
the shorter rings. This causes the board to cup
towards the inside, or pith-side, of the tree.
In the second scenario, consider a board with
one face toward open air and sunlight, while the
other face is toward moisture coming up from the
ground with poor air circulation under the deck.
When one side of the wood is dry, and one side is
wet, the dry side will tend to shrink and the wet
side will expand. As a result, the board cups,
regardless of the orientation of the growth
If the problem is related to the first scenario,
try using drier material, such as
dried-after-treatment pressure-treated lumber (DAT)
or kiln-dried material (KDAT). Or, choose
vertical-grain material. I would not recommend
laying out the boards to dry before installation,
unless those boards are well constrained. The
boards will still cup, leaving you with less usable
If the problem is related more to the second
scenario, take steps to equalize the moisture
differences on each face of the deck. Shading the
deck, designing a deck with lots of underside
ventilation, or even using a moisture barrier and
gravel beneath the deck will all help.
Regardless of which scenario causes the problem,
it’s critical to finish and maintain the
wood. Proper finishing helps slow the gain and loss
of moisture, thereby minimizing cupping. Apply a
water-repellent or penetrating stain while the wood
surface is dry, or choose pressure-treated wood
with a preapplied water-repellent. In either case,
apply a new coat regularly each year.
Chris Donnelly is a wood technologist and
consultant in Northford, Conn.