Unsealed area may be cause of leaky new roof

Q We put a new roof on our 14-year-old townhouse late last fall and it leaked on and off all winter. The association had several contractors out here and they all had different opinions and none of their fixes worked. Insulation was added, and the roof and shingles were inspected and found sound. We had bigger ice dams this winter than we've had for years. Why is this happening, and where do we go from here?

A It's hard to know for sure what was installed and how. But winter leakage of new roofs is not unheard of, even when the job was done "right," and it has nothing to do with the shingles.

Rather, it's likely that the area under the roof is not sealed from the rest of the house. That can be the case in old homes or new. The space may appear sealed as you stand in your home and look around. But rooms with light fixtures, recessed lights, electrical outlets, switches, inset speakers, even a chimney, may not be. Air can leak in and around these, and when that happens, you have trouble:

Ice dams

Warm, moist air leaking from the living space into the space beneath the roof warms the roof enough to cause the snow on it to melt and then refreeze, creating an ice dam at the roof's edge. Subsequent melting will back up behind the dam and under shingles, leaking into your house.

If you had a problem with ice dams, a new roof alone won't necessarily solve the problem, nor will additional insulation. Indeed, a new roof can exacerbate ice dam problems if extra ventilation was added (more or better roof vents). These exhaust "holes" can pull more warm air into the space under your roof, if that space isn't sealed off from the living space. More warm air means more ice dam problems.

Building codes typically require that a strip of water-resistant material (usually a rubber-like membrane) be placed under the shingles along the roof's edge to minimize ice dam leakage, but that doesn't always work. The membrane provides protection 4 to 6 feet up a roof. If ice dams get large enough, water can reach higher than that and leak into the house. That's particularly true in valleys (areas where two roof sections meet).

During frigid weather, rafters and roof underlayment are so cold that any warm air from the living space that gets into the attic will condense, freeze and create an accumulation of frost. When weather moderates, the ice and frost thaw and the resulting moisture drips down through insulation and into the house. Again, additional or improved ventilation might worsen the problem if the attic isn't sealed from the living space.

Since your roofing job was done correctly and the attic insulation meets standards, you'll need to seal the attic. It's a labor-intensive job and special equipment can ensure the effectiveness of the seal. Consider hiring a weatherproofing or insulation contractor. But you'll want to be very specific. Not every contractor knows the latest techniques for preventing ice dams and attic frost. Read the free publications on attic air leaks from the State Energy Office at www.commerce.state.mn.us. Type "Attic" or "ice dams" into the search window. Or call 651-296-5175 or 1-800-657-3710 for a free copy by mail or to speak with an energy specialist for more information.