A.Henri de Marne
responds: The black discoloration is probably
a mold. This is a surface mold; it does not have
roots that penetrate into the wood, but it is a
precursor to true rot. If the wood is still solid,
you should be okay if you treat the lumber with
liberal doses of a copper-based wood preservative,
such as Cuprinol Green (Cuprinol Products, a
division of Sherwin-Williams; 800/424-5837).
As for the sheathing, if it is truly waferboard
and not OSB, it is not a structural panel. However,
it might be all right for a porch wall. Cut the
damaged sheathing back to good wood, and replace it
with pressure-treated plywood after you have
treated the framing with the wood preservative.
You can also install 15-lb. felt over the
pressure-treated sheathing before replacing the
clapboards. I have gone back to installing 15-lb.
felt for this type of application, rather than
housewrap, after I inspected several condominiums
with rot problems caused by rain splashback under
the drip line. The sheathing on these units was
covered with a housewrap, which was black, and the
underlying sheathing and framing were completely
rotted out at the base of walls.
I was told by DuPont (the maker of Tyvek) that
housewraps are "water repellent" — that
is, they will shed water due to occasional wetting.
But if soaked for a long time, the housewrap will
become porous and allow water to seep through. I
have yet to see an instance where 15-lb. felt
absorbed enough water to become porous.
Before replacing the clapboards, treat them with
the same wood preservative used to treat the
framing. Apply it on all surfaces, and dip the cut
ends in the bucket of preservative for about 10
seconds, if the clapboards will be painted. If
there is a skirtboard, consider using
pressure-treated wood, and flash over the top of it
with aluminum Z-flashing.
Henri de Marne, is a consultant in
Waitsfield, Vt., specializing in moisturerelated