ask This old House
The cedar-shingle siding above our garage roof has turned green and has minor rotting. What can be done to repair and prevent this?
—KEVIN TAI, NORWELL, MA
TOH GENERAL CONTRACTOR TOM SILVA: Moisture-damaged siding is a common problem where siding butts up too tightly against a sloped roof, as can be seen at right. There should be a gap of at least an inch between the roof and the siding’s bottom edge to minimize prolonged exposure to moisture, though I prefer to leave a wider, 2-inch gap. When the gap is less than an inch, wood siding wicks up moisture and takes longer to dry out, creating an ideal environment for mold, algae, and rot. (Siding made of fiber cement or engineered wood can also deteriorate if installed too close to the roofing.)
To eliminate this wicking, the shingles’ bottom edges will have to be cut back. You can make the cuts yourself, if you don’t mind heights and your roof has a low slope—less than a 4/12 pitch. For steeper roofs, either wear a fall-protection harness or hire a siding or roofing pro.
You’ll need an oscillating tool with a wood-cutting blade, and a guide strip that’s 2 inches wide and about 4 feet long. Starting at the eaves, place
