An ice dam is a ridge of ice that forms at the eaves of a sloped roof and traps melting snow behind it. The trapped water backs up under shingles and leaks into walls and ceilings. Ice dams are almost always caused by heat escaping into the attic, so the lasting fix is usually inside the attic, not on the roof.
How ice dams form
Three conditions come together:
- Snow on the roof.
- A warm upper roof. Heat leaking from the house into the attic warms the roof deck above the freezing point, so snow melts from underneath even when the outdoor air is below freezing.
- A cold roof edge. The eaves overhang the outside wall and stay at outdoor temperature. Meltwater running down refreezes when it reaches them.
Over days and weeks, the ice builds into a dam. Meltwater pools behind it, and because shingles shed water rather than hold it, the pooled water works its way under them and into the building.
Where the heat comes from
Most heat that reaches an attic comes through air leakage rather than through insulation. Warm, moist house air rises and finds its way through gaps in the ceiling:
- Attic hatches without weatherstripping
- Gaps around plumbing stacks, wiring and chimneys
- Recessed pot lights that aren't airtight
- Top plates of interior walls
- Bathroom and kitchen fans that vent into the attic
- Ducts and furnace equipment in the attic
Thin or uneven insulation adds to the problem, as does a lack of ventilation to carry heat out. Complex roofs with valleys, dormers and low-slope sections are more prone to ice dams because snow collects and drains slowly.
The damage ice dams cause
- Stained and damaged ceilings and walls
- Wet insulation, which loses much of its value and can stay wet for a long time
- Rot in roof sheathing, fascia and wall framing
- Mould in attics and wall cavities
- Gutters torn off by the weight of ice
- Large icicles that are a hazard to people below
The same warm, moist air that causes ice dams also often causes frost and mould on the underside of the roof deck. See moisture, mould and indoor air.
Prevention that works
1. Air seal the ceiling
Sealing the gaps between the house and the attic is usually the most effective step. It cuts the heat and moisture reaching the attic. It is best done by a contractor experienced in attic air sealing, with care around chimneys and other hot surfaces, where only suitable materials can be used.
2. Insulate to the right level
Once air sealing is done, add insulation to bring the attic to a current recommended level, keeping it even across the whole ceiling. Use baffles at the eaves so insulation doesn't block the soffit vents.
3. Ventilate the attic
Good ventilation, with air entering at the soffits and leaving near the ridge, keeps the roof deck closer to outdoor temperature. Ventilation helps, but it can't make up for large air leaks. Our guide to attic ventilation and insulation goes into more detail.
4. Vent fans and dryers outside
Bathroom fans, kitchen hoods and dryers must exhaust to the outdoors through sealed, insulated ducts, not into the attic or soffit.
5. Protect the eaves
Building codes generally require eave protection, a waterproof membrane under the shingles along the lower edge of the roof, in areas where ice dams are likely. It reduces leaks but does not stop ice dams forming. Check the requirements in your province when the roof is replaced.
What about heat cables and raking snow?
Heat cables can keep a channel open for meltwater, but they use energy, need regular checks and don't fix the cause. Removing snow from the lower roof with a roof rake used from the ground can reduce ice dams in a heavy winter.
Don't chip ice from the roof. Chopping or prying ice damages shingles, and working on an icy roof or ladder is dangerous. If an ice dam is causing a leak, call a qualified contractor who has the right equipment, such as steam, and fall protection.
Flat and commercial roofs
Low-slope roofs don't usually get classic ice dams, but they have their own winter problems: frozen drains, ice in scuppers, snow drifting against parapets and equipment, and water backing up when drains are blocked. See roof drains and ponding water.
How ehZee can help
If you've had ice dams, we can inspect the roof, eaves and attic to find where heat and moisture are getting in, and give you a prioritized list of fixes. See our roof inspections, or call 1-833-888-0091.
Common questions
Do big icicles mean I have an ice dam?
Not always, but large icicles along the eaves are a common sign that snow is melting from heat loss and refreezing at the edge. Check for ice build-up behind the gutters and stains inside.
Will a new roof stop ice dams?
A new roof with proper eave protection may stop the leaks, but the ice dams will keep forming if heat is still escaping into the attic. Fix the attic at the same time.
When is the best time to fix the cause?
Attic air sealing and insulation are easiest in warmer, dry weather. A winter inspection is still useful, because frost and ice patterns show where the heat is escaping.