ice dams: what to do?

​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 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. 
 
If you're experiencing excessive ice build-up over your eves or in valleys between roof faces, here's a few helpful hints: 

1. Ensure adequate attic ventilation (approx. 1sq ft/200-300sq ft of ceiling area); (no less than 50% at the gables or soffits, and no more than 50% through the roof)
2. If loose insulation tends to settle in the eaves, use 1/4inch plywood, carboard or paneling to create to create barriers between the rafters. Prefab soffit dams are available at hardware and building stores.
3. Ensure adequate insulation over your upper-most ceiling (the attic floor) and that the insulation does not contact the roof or block the soffit vents. A minimum insulating value of R20 (approx. 8 inches) is recommended. Most modern homes have 12 inches or more of insulation.
4. Carefully seal all ceiling penetrations (light fixtures etc.) to keep warm, moist air out of the attic. Apply insulation and weather stripping to attic hatches.
5. Heater cables can help prevent ice damming, but if you're applying the cables after the snow and ice have already accumulated, place the cables only at the gutter, or lower-most portion of the eves. Higher placement may cause meltwater to accumulate behind an ice dam, worsening the situation. 
6. Heater cables should be relied upon only as an interim measure, until you improve insulation and ventilation, or it's time for new roofing materials. When re-roofing don't forget to point out the problem areas to your roofing contractor, so he can install improved protection in the most vulnerable areas. 
7. In new homes, rain and ice shield 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.
8. If you decide to shovel some excess snow off your roof, exercise extreme caution - a fall off a roof can be fatal!  Also, ensure that you are not scraping the surface of your shingles. Asphalt shingles are especially prone to damage by heavy foot-wear, and by shoveling or sweeping. 
9. It is important that the rain gutters and downspouts are freely discharging meltwater. Gutters improperly sloped or not cleared of leaves and debris, can cause formation of ice dams rather quickly. Ensure downspout extensions are not frozen into the grass or soil.

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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!
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