Ice dams and Ontario winters: prevention, ventilation and insulation

1. 01

   Heat escapes

   Warm indoor air leaks into the attic through pot lights, plumbing stacks, hatch openings and top plates.

2. 02

   The upper roof warms

   That heat warms the deck above the heated part of the house, while the eave stays at outdoor temperature.

3. 03

   Snow melts and runs down

   Meltwater flows under the snowpack toward the cold overhang.

4. 04

   It refreezes at the eave

   A ridge of ice builds, water ponds behind it, and it works its way up under the shingles into the assembly.

The key point: the shingles are not the failure. Shingles shed water flowing downhill — they are not designed to hold standing water backing uphill.

A roof rake removes the fuel for the next dam and is worth doing on the lower few feet after a heavy snowfall. Heat cables melt a channel through the ice so water has an escape route. Neither addresses why the roof is warm, and both add their own problems — rakes strip granules if used aggressively, and cables age, fail and get expensive to run.

1. 01

   Air sealing first

   Seal every attic penetration — pot lights, bath fans, plumbing stacks, wire runs and the attic hatch. This is the cheapest and highest-impact step and most homes have never had it done.

2. 02

   Insulation second

   Bring attic insulation up to modern levels and keep it out of the soffit with baffles so intake air can still move.

3. 03

   Ventilation third

   Balance intake at the soffits with exhaust at the ridge so the deck stays close to outdoor temperature all winter.

4. 04

   Membrane fourth

   At the next re-roof, run ice-and-water shield well past the code minimum at the eaves and through every valley.

Ice dams are an attic problem that shows up on the roof. Fix the attic and the icicles stop on their own.

Bathroom and kitchen fans that terminate inside the attic are one of the most common problems we find in Durham Region homes. They dump warm, humid air directly against the cold sheathing, which drives both ice dams and attic frost that rains down on the insulation during a thaw.

• Every fan should terminate through the roof or wall with an insulated duct and a damper.
• Dryer venting should never terminate in the attic under any circumstance.
• Check for a white frost layer on the underside of the sheathing in February — that is condensation, not a leak.

Contain the water inside, photograph everything, and call for steam removal rather than mechanical chipping. Sudden ice-dam water damage is often covered by Ontario homeowner policies; repeated damage on a roof with known ventilation problems frequently is not, which is another reason to fix the cause once.