How do I prevent ice damming on my addition roof in Ottawa?

Ice damming is a common and damaging problem on addition roofs in Ottawa, and preventing it starts with understanding why it happens. An ice dam forms when heat escaping through the roof melts snow on the upper part of the roof. The meltwater runs down to the eaves, where the roof extends past the heated part of the building and is colder, and refreezes into a ridge of ice. That ridge traps more meltwater behind it, which can back up under shingles and into the roof structure, causing leaks, mould and rot. Ottawa winters swing above and below freezing often enough to keep this cycle going.

The most effective prevention is strong insulation and air sealing at the ceiling. For an addition, the Ontario energy rules require a compliance package or the addition table, and the verified Zone 1 addition value for a ceiling with an attic is R-60. Air sealing matters just as much as insulation, because warm, moist air leaking into the attic is what melts the snow. For a vaulted or cathedral ceiling without an attic, your designer will specify an assembly that controls both heat loss and air leakage; ask how they will achieve it.

Roof ventilation works alongside insulation. A well-ventilated roof keeps the roof deck closer to the outdoor temperature, which reduces uneven melting. Ventilation usually means intake at the soffits and exhaust at or near the ridge, with baffles keeping insulation from blocking the airflow at the eaves. Your designer or contractor will size the ventilation to meet the Building Code.

At the roof covering, a self-adhering waterproof membrane along the eaves is the backstop if a small ice dam does form. Ask your contractor how far up the roof they plan to run it and what they use in valleys.

The spot where the addition roof meets the existing house is especially vulnerable. Valleys, different insulation levels between old and new, and complex flashing at the tie-in all create risk. Ask your contractor how they will keep insulation consistent across the transition and how they will flash the valley. Heat cables in valleys or at eaves are sometimes used as a supplement in problem spots, but they are not a substitute for good insulation, air sealing and ventilation.

Ask each contractor to explain their roof details in the quote, line by line. You can browse addition contractors in the Ottawa Construction Network directory at justynrookcontracting.com and compare quotes.

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