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ICE DAMMING
By Gil Strachan , Number 23, 342 words
Ice dams are the accumulation of ice at the eaves and valleys of
roofs, and in some winters create major problems. Insufficient attic insulation and/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 (light fixtures etc.);
to keep warm, moist air out of the attic.
2. Ensure adequate attic ventilation (approx. 1sq.ft./200-300
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 loose
fill/blown-in insulation tends to settle in the eaves, use 1/4inch
plywood/panelling/aspenite to create barriers between the rafters/trusses. Prefab 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.
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!
See Also: Ice Dams: What to do?
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