Root of Roof Problem May Lie Underneath

House Doctor | Gary Dymski

February 12, 2004

Q. With this winter being unusually cold, we've had a problem with ice dams. We have a ranch with a low-pitched roof and a front-porch overhang. The roof has soffit vents except for near the front-porch overhang, as well as insulation in the attic and ventilation fans at each end. We had a covered gutter system installed that keeps leaves and debris out but allows water in. Ever since, we have more and more problems with icicles and ice dams.

The icicles are coming out behind the fascia boards and from under the shingles against the house under the front-porch overhang. In the 30 years we've been living here, we've never seen anything like this. I am afraid of the damage this is causing. What can we do to prevent this?

-Madeline Morris, Setauket

A. I doubt your gutters are the problem. Ice forms on roofs because of inadequate attic ventilation, inadequate attic insulation or both. Heat rises through the interior of the house and attic, causing snow on the roof to melt. As the melted snow runs down the roof toward the gutters, it freezes. That's because the roof temperature near the eaves is colder than in the middle. As melting snow refreezes, a dam is created and water backs up and seeps through openings in the roof.

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To eliminate ice dams, you'll need to restrict the amount of heat that rises through the attic by adding another layer of insulation. The attic temperature (I assume you have unfinished attic space) should be as close to the outdoor temperature as possible. Check the R-value of your attic insulation. This is the rating that measures the insulation's resistance to heat flow. The higher the number, the better the resistance. On Long Island, the recommended R-value for attic insulation is R38, although in colder areas and upstate, it's R49. Perhaps adding another insulation layer is necessary. To do this, fiberglass batts should be placed between the ceiling joists over existing insulation. To determine the R-value, multiply 3.2 by the thickness of the fiberglass insulation. To achieve R49, you need fiberglass insulation about 16 inches thick.

Another step is to add soffit, fascia or roof vents or even ridge vents. Your current insulation may be covering soffit vents, preventing proper air flow. Painted vents or vinyl soffits installed against closed wood won't provide air flow in the attic. If the vinyl was installed against closed wood, remove the vinyl and drill 2-inch holes into the wood every foot or so. Then reinstall the vinyl soffit. Cooler air should enter the attic from the lower soffit vents, then force warm air through higher roof and gable vents.

Also, examine sources of heat loss within your home. Improperly vented bathroom and kitchen fans or even heat ducts can cause warm air to collect in the attic. These fans should be vented directly to the exterior of the home.

The venting situation might be difficult to evaluate and correct on your own. Contact a reputable roofing contractor. An ice-shield barrier on the roof might help. This involves removing shingles, placing a 3-foot-wide liner-type material near the edge of the roof, then nailing shingles over the barrier.