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Ice Dams: Why Your Roof Grows Icicles, How to Stop It for Good, and What Not to Do This Winter
Warranty and maintenance Roofs and attics From a builder
Ice Dams: Why Your Roof Grows Icicles, How to Stop It for Good, and What Not to Do This Winter
Icicles are pretty in a photograph and expensive on a soffit. An ice dam is not a roofing problem and it is not really a weather problem; it is a heat-leak problem that shows up on the roof. Fix the leak and the dam stops growing.
Short version: An ice dam forms when heat escaping into the attic melts the underside of the snow, the meltwater runs down to the cold eave and refreezes, and the ridge of ice backs water up under the shingles. The permanent fix is in the attic, not on the roof: seal the air leaks (pot lights, plumbing stacks, top plates, the hatch), bring the insulation to full depth right out over the exterior wall, and keep the soffit and ridge ventilation clear so the roof deck stays cold. For water coming in today, have a roofer steam the dam off or rake the snow back; do not chip it, salt it or hang heat cables as a permanent cure.
How an ice dam forms
Snow on a cold roof stays snow. Snow on a roof with a warm attic under it melts from the bottom, and the water runs downhill under the snowpack until it reaches the eave, which hangs out past the warm house and stays at outdoor temperature. There it freezes. Every day the cycle adds a layer, the ridge grows, and the meltwater behind it has nowhere to go but up, under the shingles. Shingles shed water running down; they do not hold water standing still. It finds a nail hole, then the ceiling.
The heat gets into the attic three ways: air leaks (warm house air blowing up through every hole in the ceiling plane), conduction through thin insulation, and ducts or chimneys running through the attic. Air leaks are the big one. A house with R-60 in the attic and a dozen unsealed pot lights will grow dams; a house with R-40 and a tight ceiling will not.
The permanent fix, in order
- Air seal the attic floor: foam or caulk around every plumbing stack, wire, duct, pot light (use sealed, IC-rated fixtures), the top of every interior wall, the chimney chase with sheet metal and high-temperature sealant, and the attic hatch with weatherstripping and insulation on the lid. Air sealing an Ontario home explains why this comes first.
- Fix the insulation at the eaves. The spot where the roof meets the wall is where insulation gets thin because the truss is low there. Raised-heel trusses solve it in new construction; in an existing house, dense-pack or spray foam at the eave, with baffles to keep the soffit vent open.
- Ventilate: continuous soffit intake and ridge or high vents so cold outdoor air washes the underside of the deck. Ventilation is third for a reason: it cannot fix a leaky ceiling, and adding powered vents to a leaky attic pulls more warm air up from the house.
- Ice and water membrane at the eaves when you re-roof: the Code requires it from the edge to a line past the inside face of the wall in most of Ontario. It is a backup, not a cure.
- Get the ducts out of the attic, or insulate and seal them properly, and insulate the chimney chase.
What to do when water is coming in now
- Rake the snow off the lower three to six feet of roof from the ground with a roof rake. No snow, no meltwater.
- If the dam is already there and leaking, call a roofer who uses a steamer. Steam melts a channel through the dam without damaging shingles.
- In a pinch, a channel through the dam lets the pooled water out; it will refreeze in days. Treat it as a bandage.
- Put a bucket under the drip and pull back wet insulation so the ceiling can dry; mould starts in a few days.
- Then book the attic work for spring.
What not to do
- Chipping, hacking or shovelling the ice off the roof: shingles crack at minus 15, and so do people who fall off ladders.
- Salt or de-icer: it eats aluminum eavestroughs and stains everything below, and it does not fix the cause.
- Heat cables as the permanent solution: they cost money to run, shorten shingle life, and treat the symptom. They have a place on a problem valley or a north eave that cannot be fixed otherwise.
- More attic vents without air sealing first: a stronger stack effect pulls more warm air out of the house and into the attic.
- Blaming the roofer. A new roof over an old leaky attic grows the same dam.
Why some houses never get them
A house with a tight ceiling plane, full-depth insulation to the eave and a cold, vented attic does not grow ice dams, however much it snows. A house with an unvented conditioned roof (spray foam on the underside of the deck) does not either, because the deck stays at room temperature and the snow melts evenly or not at all. ICF walls help at the top plate: there is no stud wall for warm air to rise through, and the foam carries right up to the roof framing. ICF R-value covers the wall; insulation R-value covers the attic numbers. If you are building new, raised-heel trusses, sealed pot lights and a blower door test are cheap insurance against ever reading this page again.
Related guides on this site
Sources and further reading
Questions people ask
What causes ice dams?
Heat leaking from the house into the attic melts the underside of the snow; the water runs to the cold eave and refreezes, building a ridge that backs water up under the shingles. Air leaks through the ceiling are the usual source of the heat.
How do you permanently stop ice dams?
Air seal the attic floor, bring the insulation to full depth out over the exterior walls (raised-heel trusses in new construction, foam at the eaves in an existing house), and keep soffit and ridge ventilation clear so the roof deck stays cold.
Should I remove an ice dam myself?
Rake snow off the lower roof from the ground. Do not chip or hack at the ice; it breaks shingles and people fall. If water is coming in, a roofer with a steamer can melt the dam without damage.
Do heat cables stop ice dams?
They melt channels so water can drain, which treats the symptom at an operating cost and some wear on the shingles. They are a last resort for a spot that cannot be fixed from inside, not a replacement for air sealing and insulation.
Size it from a heat loss, not a guess
Every heating choice starts with the heat loss of your actual house. OntarioHeatLoss.ca produces the CSA F280 calculation and the mechanical design your permit needs, anywhere in Ontario.
Reviewed and updated October 2026. Costs are Ontario ranges, not quotes; code references are to the Ontario Building Code in force at that date.
