Jan 24, 2015 3 min read

Q. Our roof was replaced after a hail storm in 2010 by an out-of-state contractor who did several homes in our neighborhood. After a few years, I noticed black mold growing on the underside of the plywood but only on the back side of roof. The front side has no mold. The sun rises in the front of home and sets in the back. I believe the home faces southeast.

A few contractors have said it may be from ventilation, but that hasn’t changed in the 23 years we owned the home. I notice water drips on the nails in winter that have come through the plywood and that the plywood is damp on the back side of house. I had a mold remediation company give me an estimate to rid the mold, but am concerned that it will grow back unless the root of the problem is discovered. Could it be no tar paper used on the back side?

A. It is entirely due to lack of ventilation. The antiquated vents are probably at each peak of the gables, not enough to release water vapor that builds in the attic. This vapor condenses on cool spots, on the shaded parts of the roof. Sunny sides of the roof are not affected because of the heat generated by the sun. The lack of tar paper may be allowing release of a bit of water vapor through the roof, but not enough to change things.

What you must do: Install a ridge vent, one that will allow moist air to escape through entire length of the roof’s ridge. Then install vents on each side of the roof in the soffit, which is the under part of the roof overhang, on each side of the roof. The best soffit vent is a 2-inch-wide screened strip going the full length of each soffit.

Q. My wife and I are removing wallpaper from horsehair plaster walls in our late 1800s vintage house. The plaster is in good shape. Is it OK to apply a super thin layer of ready mixed joint compound once we have all paper and glue removed?

A. A skim coat is not going to do much good, because it will follow the imperfections, but there are two tricks you can try. One is to keep the old paper intact, then paint it. If that doesn’t work, take off the paper, and buy a canvas-type paper called Cover-All, which will cover imperfections and make the finish paintable.