Key Takeaways
- Ice dams form when attic heat melts roof snow, and that meltwater refreezes at the cold eaves.
- The water backs up under shingles and can silently rot sheathing, soak insulation, and stain ceilings.
- The real fix is air-sealing the attic floor — not just adding more insulation.
- Never chip at ice dams with a pick or axe; a professional with a steam unit is the right call.
Ice dams are deceptively quiet. They form without any obvious signs, grow gradually through the winter, and by the time you notice water staining on the ceiling or peeling paint on the wall, they've often been working for weeks. Atlantic Canada winters are exactly the conditions they need — freeze-thaw cycles, periodic snow accumulation, and older homes with imperfect insulation and ventilation.
How ice dams form
The process starts with heat escaping through the roof deck. In most homes, the living space below the attic is warmer than it should be — either because the attic isn't well insulated, or the insulation has gaps, or air is leaking from the living space into the attic around pot lights, plumbing stacks, or the attic hatch.
This heat warms the roof deck above it. Snow on the upper portion of the roof melts and runs down toward the eaves. At the eaves, the roof surface is over the unheated overhang (no warm living space below), so it's much colder. The meltwater refreezes there, forming a ridge of ice — the dam.
As the cycle continues, more meltwater runs down and backs up behind this ice ridge. Water under hydrostatic pressure is remarkably good at finding paths through materials that are perfectly watertight in a downpour. It gets under shingles, through the roof deck, and into the wall cavity or ceiling below.
What the damage looks like
Inside the home, ice dam damage typically shows up as water staining on ceilings near exterior walls, peeling paint on exterior walls just below the roofline, wet or compressed insulation in the attic near the eaves, and in severe cases, mould growth in wall cavities. The damage can be significant and expensive — sheathing, insulation, drywall, and framing can all be affected.
On the exterior, look for thick ice buildup at the eave line, icicles (which are a symptom of the same process), and staining on the fascia and soffit.
Prevention: the right way
The permanent solution to ice dams is addressing the underlying heat loss. This means:
- Air sealing the attic floor — blocking all the pathways through which warm air rises from the living space into the attic. This is the highest-impact step.
- Adequate attic insulation — keeping the attic cold by preventing heat transfer through the ceiling.
- Proper attic ventilation — allowing cold outside air to circulate through the attic space, keeping the roof deck uniformly cold.
Ice-and-water shield membrane installed under the shingles at the eave is also standard practice and provides a second line of defence — but it's a barrier to damage, not a fix for the underlying problem. For a thorough technical treatment of the subject, the Ice on Roofs publication from the National Research Council of Canada is an excellent resource.
Short-term measures
Heating cables along the eave are a common short-term solution. They work by creating a channel through the ice dam for water to drain — they don't prevent the dam from forming, but they prevent the backup. They need to be properly installed and maintained, and they use electricity continuously through the winter.
Raking snow off the lower few feet of the roof after heavy snowfalls reduces the amount of meltwater available to back up. It's labour-intensive but effective.
If you see water staining near exterior walls in late winter or early spring: Don't assume it's a roof leak from rain. Ice dam damage is common in Atlantic Canada and is worth having a professional assess before assuming the worst (or the cheapest) explanation.