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Q.We recently encountered crumbling waferboard sheathing, which had been installed right down to grade. This sheathing wicked water about two feet up the wall of an enclosed porch. The clapboards and sills were so rotten they fell apart in our hands, but the rest of the wall appears sound. If the edges are protected from water, can we leave the old sheathing in place? Also, the framing and rim joist behind the rotted sheathing appear solid, though the surface of the lumber is black. Is this discoloration a type of rot?

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 construction problems.