When a roof fails early in Toronto, the cause is in the attic more often than on the roof. Curling shingles, rotted sheathing, ice dams, mould on the underside of the deck and frost on the nails in January are all symptoms of the same thing: an attic that is too warm and too damp because it cannot breathe.
What ventilation is for
An attic has two jobs in this climate. In winter it must stay cold, so the roof deck stays below freezing, snow does not melt from underneath, and ice dams do not form at the eaves. In summer it must shed heat and moisture, so the deck does not cook the roofing from below and humid air does not condense on cool framing at night.
Both jobs depend on the same mechanism: cool outside air enters low, at the soffits or eaves, warms slightly, rises, and leaves high, at the ridge or through roof vents, carrying heat and moisture with it. If either end of that path is blocked, the whole system stops.
What the code requires, and why it is only a start
Ontario Building Code section 9.19 requires unobstructed vent area of at least 1/300 of the insulated ceiling area, split roughly evenly between high and low vents, for most sloped roofs. For a typical 150 m² Toronto attic that is about 0.5 m² of net free vent area, half at the eaves and half at the ridge.
The code is a minimum for new construction. It assumes a well-sealed ceiling, continuous soffit intake and insulation that stays clear of the eaves. Almost no older Toronto home meets those assumptions. Knee walls, dormers, cathedral sections, pot lights and finished third floors all interrupt the airflow path, and in our experience an older home needs closer to 1/150 and, more importantly, an intake path that actually exists.
The five mistakes we find most often
- Blocked soffits. Insulation pushed to the eaves, or original soffits covered in aluminum with too few perforations. The ridge vent is then exhausting nothing. Fix: rigid baffles at every rafter bay and adequate intake vents.
- Mixed exhaust types. A ridge vent plus gable vents plus a powered fan. Air takes the shortest path, so the fan pulls from the gable vent and the soffits never move air. Fix: one exhaust strategy, balanced with intake.
- Bathroom and kitchen fans venting into the attic. Astonishingly common in homes renovated in the 1980s and 1990s. Each shower adds litres of water vapour to the attic. Fix: insulated ducts to the exterior, through the roof or a gable, with a proper hood.
- Unsealed ceiling penetrations. Pot lights, the attic hatch, plumbing stacks and chimney chases leak warm, moist house air into the attic all winter. Ventilation cannot keep up. Fix: air sealing before anything else. See our ice dam guide.
- No ventilation at all in cathedral or low-slope sections. Common in additions and third-floor conversions. Fix: continuous vent channels above the insulation, or a properly designed unvented assembly with spray foam and a vapour retarder.
How to tell if your attic is breathing
- In winter: frost on nail tips or the underside of the sheathing, icicles at the eaves while neighbours have none, or snow that melts in patches over the heated space.
- In summer: an attic that is far hotter than outside by mid-afternoon, or a top floor that will not cool at night.
- Any time: dark staining or mould on the deck, rusted fasteners, insulation that is damp or compressed, or a musty smell at the hatch.
- A simple test: on a still winter day, hold a piece of tissue at a soffit vent from inside the attic. It should be drawn toward the roof space.
Warranty note. Most shingle, slate, cedar and metal manufacturers make adequate ventilation a condition of their material warranty. An unventilated attic can void the warranty on a brand-new roof. We measure and correct ventilation on every re-roof for this reason.
Ventilation and premium roofs
Slate, clay tile and metal are far more tolerant of a warm deck than asphalt, but the deck and framing beneath them are not, and a 150-year roof deserves a deck that will last as long. Cedar is the exception that demands ventilation on both sides: we install it on a batten system so the shakes themselves can dry, over an attic that is itself vented. Our proposals include a ventilation calculation to Ontario Building Code 9.19, and any intake or exhaust corrections are drawn and priced before work begins.