How the ice forms
An ice dam needs snow on the roof, cold outside, and a roof that is warmer at the top than at the bottom.
- Heat lost through the ceiling warms the decking above the living space.
- The snow melts from below, even when it is freezing outside. The water runs under the snow cover.
- The water reaches the roof overhang, which sticks out past the walls and is warmed by nothing.
- There it refreezes. A band of ice forms, then thickens with each cycle.
- The water that keeps coming builds up in a pool behind that dam.
The fix lies in insulation and attic ventilation.
Why water gets under the shingles
A shingle roof is not a sealed surface. It works like fish scales. Water runs from one course onto the next and never finds an opening, as long as it flows downward.
Behind a dam, water stops flowing. It sits, rises, and ends up passing under the edge of the shingles, the wrong way. It then finds the nail holes and the joints in the decking.
The eave membrane is protection, not a cure
On a recent roof, a self-adhering membrane covers the bottom of the slopes under the shingles. This membrane seals around the nails and stops water that backs up.
The eave membrane limits the damage, but it does not get rid of the ice. If the dam rises higher than the membrane, water still gets through.
Fixing the cause
A roof that stays cold over its whole surface does not make ice dams. To get there, work in this order.
- Seal the ceiling. Warm air leaks around the hatch, light fixtures and pipes heat the attic more than anything else.
- Top up the insulation, especially above the exterior walls, where it is often thin.
- Restore air flow, so cold air from the soffits sweeps the underside of the decking.
If water is already getting in
Protect the inside first. Move the furniture and catch the water. Ask us whether we can come and remove the ice from the roof, then notify your insurer. If water is coming in and you cannot wait for our answer, also look for a company that answers the phone.
What to avoid
- Breaking the ice with an axe or a hammer. The frozen shingles break with it.
- Going on the roof. An icy sloped roof is dangerous. Leave ice removal to a properly equipped company.
- Counting on heating cables. They open a path for the water, but they fix nothing and use electricity.
Frequently asked questions
Do icicles on the roof edge mean an ice dam?
Icicles on the roof edge do not necessarily mean an ice dam. A few thin icicles after a sunny day are normal. Large icicles that come back all winter, with a thick band of ice above the gutter, show that the roof is losing heat.
Does home insurance cover ice dam damage?
Coverage for ice dam damage depends on your insurance policy. Check with your insurer what is covered and under what conditions. Take photos of the ice and the damage inside.
Will a new roof solve the problem?
No, a new roof alone does not solve an ice dam problem. With its eave membrane, a new roof protects better against water that backs up. It does not keep ice from forming if heat keeps escaping. A roof replacement is the right time to fix the ventilation too.