Roof with ridges
To get to the bottom of this wrinkle, look below the surface
The roof on my five-year-old cottage is plagued with little ridges. The
house is a timber-frame structure covered with 6-inch-thick structural
panels filled with urethane insulation. It was built in wet-weather
conditions and the timbers were green when assembled. Do you think that caused the problem? — Scott, Portsmouth, N.H
Tom Silva replies: What's happening is that moisture vapor inside the house is getting between the ends of the roof panels because they weren't sealed properly with foam. When the vapor hits the roofing felt, the felt expands, which in turn pushes up the shingles. Problem is, the felt doesn't flatten out even when it dries, so the ridges don't go
away. Heavier shingles may help to keep felt from pushing up — but then again, maybe they won't. And even if they do, the moisture may
eventually cause the panels' OSB sheathing beneath the felt to rot.
The solution is to strip off the shingles and felt and inject spray
foam into the joints through small holes drilled every 8 inches. But
first hire a company with thermographic imaging equipment (your local utility can probably recommend one) and have them take pictures of your
roof so you know the location of all the heat leaks; moisture vapor
follows the same paths. Then, after the gaps are filled, have them take
pictures again, to make sure nothing was missed.
One more thing: Before
you reroof, add some ventilation. Nail lengths of 1x3 strapping facedown
to your bare roof. Run them from peak to eaves with their centers spaced 16 inches apart, cover with ½-inch sheathing and builder's felt, then
shingle. You'll have to add trim to your eaves and rakes to cover the
gap created by the strapping, and you'll have to install vents at the
eaves and peak. When you're done, this roof-on-a-roof will keep the
underside of the shingles cooler and the top side of the panels dry.
I know this isn't the news you wanted to hear, but I'm afraid there isn't
a cheap fix. If you haven't already, get in touch with the builder and
the panel manufacturer. Perhaps the manufacturer has a warranty you can
take advantage of. Or maybe the builder will want to maintain a good
reputation (or avoid a lawsuit) and will cover or defray the cost of
putting things right. Just be glad you caught the problem early. I once
saw a house where all of the panels had to be removed due to a lack of
joint sealant; the sheathing was just rotting away.