Up on the rooftop, there's a leak

Q My roof is leaking in several places and I need an inexpensive quick fix for the winter. I believe the source of the leaks is the ridge vent. We have a plywood gambrel roof with one layer of asphalt shingles.

- D.G., Newcastle, Maine

A"It is unusual to have a serious leak at the ridge of a roof," says Howard Clark, a construction supervisor in Hopkinton, Massachusetts. "But winter ice dams and wind-driven rain bring occasional problems on flatter peaks."

He suggests checking the width of the cut in the sheathing under the vent and cap. This gap should extend only about 1 inch beyond either side of the ridge board, or be no more than 3 inches wide, if the framing doesn't have a center-ridge member. It's very important that the ridge vent and edges of the shingle cap reach at least 2 inches beyond this slot on each side, well covering it. If the gap is too great and coverage too small, wind will blow in the rain.

Gambrel roofs are tricky to ventilate, especially if there is no attic under them. The break in the roof pitch midway up means framing that often interrupts the flow of ventilating air from the soffit vents at the roof's eaves to the continuous vent at the roof's peak.

That's why on gambrel roofs there are usually vents installed in the fascia at the midroof break. This way, the upper, flat section of roof draws air to keep enclosed rafter cavities and insulation free of condensation.

Not properly vented, a gambrel may literally wilt and sag because of trapped moisture. This increases the likelihood of ice dams occurring on the less-steep portion of the roof. And with inadequate ventilation, air may be drawn in instead of expelled at portions of the ridge, bringing water with it.

Readers: Pose your questions and we'll seek out experts on home repairs, gardens, food, and family legal issues. Send queries to the Homefront Editor, The Christian Science Monitor, One Norway Street, Boston, MA 02115 or e-mail home@csps.com

Compiled by Judy Lowe

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