Q: 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.
A: 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.