Ice Damming

By Gil Strachan

Ice dams are the accumulation of ice at the eaves and valleys of roofs, and in some winters create major problems. Insufficient attic insulation or attic ventilation can cause roof snow to melt and slide down to the eaves, where the roof surface is cooler and the melting snow freezes to the roof. Subsequent melting causes water to pool behind this dam and sometimes the water seeps between the shingle layers, wetting the roof deck and possibly the walls and ceilings below.

Interior damage caused by ice dam. What To Do?

1. Carefully seal all ceiling penetrations such as light fixtures to keep warm, moist air out of the attic.

2. Ensure adequate attic ventilation of approximately 1 sq.ft. per 300 sq.ft. of ceiling area (no less than 50% at the gables or soffits, and no more than 50% through the roof)

3. Ensure that soffit vents are not blocked by insulation. If blown-in insulation tends to drift into the eaves, use 1/4inch plywood or paneling to create barriers between the rafters. Pre-fabricated barriers are also available. A minimum insulating value of R20 (approx. 8 inches) is recommended.

4. In new homes, roofing felt is applied from the edge of the roof to a point not less than 12" inside the inner face of the exterior wall. In some localities this eaves protection must go 10 or 12 feet up the roof (especially on low-sloped roofs) and the full length of valleys.

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5. Try roof heater cables as an interim measure, until it's time for new roofing materials. Don't forget to point out the problem areas to your roofer, so he can provide adequate protection materials.

A word to the wise: Roofing is an apparently "easy enough" job for anyone, but there are many fine points that may escape the do-it-yourselfer. A professional roofer can be your best friend!

About the Author: Gil Strachan is a professional home inspector, representing Electrospec Home Inspection Services in east-central Ontario, Canada since 1994. www.electrospec.ca is one of the web's largest, most authoritative home inspection information sites.

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Related Attic and Ice Damming Information

Attic Insulation Tips
Installing Gable Vents
Frost Forming in Attic
Insulating and Ventilating Attics
How to Prevent Ice Dams - Start in Your Attic

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