How do you prevent ice dams on your roof?
Short answer: Ice dams form when attic heat melts roof snow and the water refreezes at the cold eaves. The lasting fix is a cold roof: seal attic air leaks, bring ceiling insulation up to current local code with no gaps, and ventilate the attic from soffit intake to ridge exhaust. Clear snow with a roof rake from the ground; never climb onto an icy roof.
A note on sources: insulation figures here come from Building Science Corporation’s BSD-135 digest (2006). Older figures from a 1979 University of Minnesota Extension sheet are labeled; check numbers against local code.
How do ice dams form?
Heat escaping into the attic warms the roof deck. Snow on the upper roof melts, even on cold days. The meltwater runs down under the snow to the eaves, which hang past the heated house and stay below freezing. The water refreezes there and builds a ridge of ice.
More meltwater pools behind it, backs up under the shingles, and leaks into walls and ceilings.
What causes ice dams?
The root cause is a warm roof. Three problems usually drive it:
- Attic air leaks. Warm indoor air rises through gaps around light fixtures, pipes, chimneys, and attic hatches.
- Thin or patchy insulation. The 1979 Extension sheet stresses that a cold roof “cannot tolerate” voids, thin spots, or gaps in ceiling insulation.
- Poor ventilation. Without airflow under the roof deck, leaked heat stays trapped against the shingles.
How do you prevent ice dams for good?
The goal is to keep the roof deck close to outdoor temperature. Work in this order.
1. Seal attic air leaks
Bath fans and dryer vents should exhaust outdoors, not into the attic. Do not pack insulation tightly around chimneys or furnace flues; these need fire clearances, so check with your local building inspector. See how to find and seal air leaks.
2. Add insulation
Insulation must be continuous, with no thin spots over walls or at the eaves. Building Science Corporation recommends at least R-30 below a ventilated attic, R-35 below a ventilated cathedral ceiling, and R-40 below an unventilated one, with more in very cold climates. Ask your local building department or an energy auditor what your area’s code requires.
3. Ventilate the attic
Continuous soffit intake plus a ridge vent works well, with air chutes (baffles) at the eaves so insulation does not block airflow. The 1979 Extension sheet used a rule of thumb of 1 square inch of vent area per square foot of ceiling, split evenly between intake and exhaust. Building codes today set their own ratios, so confirm the right amount with your inspector or roofer. Building Science Corporation treats ventilation as a supplement to good insulation and air sealing, not a way to vent away large air leaks.
4. Add an eave membrane when reroofing
As a backup, Building Science Corporation recommends a self-sealing waterproof membrane under the shingles at the eaves, so any water that backs up cannot leak inside.
What can you do about an ice dam right now?
These are temporary. They do not fix the warm roof.
- Rake the snow off from the ground. A long-handled roof rake removes snow before it melts. Repeat after each storm.
- Try calcium chloride in a stocking. Roofers quoted by Yahoo News suggest filling a nylon stocking with calcium chloride ice melt and laying it across the dam, at a right angle to the gutter, to melt a drain channel. Only do this if you can place it safely without climbing onto the roof.
- Call a pro. If water is already leaking inside, hire an experienced, insured roofer or ice-dam removal service.
Safety first
- Do not climb onto an icy or snowy roof. Falls are the biggest risk. Work from the ground with a roof rake.
- Look up before you rake. Keep the rake and any ladder well away from overhead power lines.
- Watch for falling ice and snow. Stand clear of where the load will drop, and keep kids and pets away.
- Be wary of ladders on ice. If you must use one, have a helper hold it.
What should you not do?
- Do not chop or chip the ice. The 1979 Extension sheet warns against chopping through ice to the shingles. Metal tools can break shingles and worsen leaks.
- Do not use a torch. Roofers quoted by Yahoo News caution against open flames; they are a fire hazard on a roof.
Do heat cables prevent ice dams?
Heat cables (zigzag de-icing cables along the eaves) can help in spots, but they have limits.
| Pros | Cons |
|---|---|
| Melt snow and ice near the cable | Melting reaches only a few inches |
| Can keep a drain path open | Uneven melt can cause new dams above the cable |
| No roof climbing once installed | Use a lot of electricity all winter |
Treat cables as a stopgap, not a cure. Use only cables rated for roof use, plug into a GFCI outlet, and follow the maker’s instructions.
For more cold-weather prep, see the winterize your house checklist.