How much attic ventilation does a roof need in Ontario?
Attics and roof spaces need unobstructed vent area of at least 1/300 of the insulated ceiling area, increased to 1/150 for low-slope roofs (below about 1-in-6). Vents must be distributed with at least 25 percent at the top and 25 percent at the bottom of the space.
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The ratio is calculated on net free vent area, so use manufacturer NFA figures rather than the gross size of soffit panels or roof vents. Balanced intake and exhaust matters more than raw totals: soffit intake blocked by insulation is the most common defect, which is why baffles at the eaves are effectively mandatory companions to loose-fill insulation. Cathedral ceilings and low-slope roofs need the doubled ratio plus clear vent channels in each rafter bay. Good ventilation limits winter condensation, ice damming and shingle overheating, but it cannot compensate for major air leakage from the house below.
For every 300 square metres of insulated ceiling, your attic needs about 1 square metre of clear vent opening, doubled for nearly flat roofs. Split the vents between the soffits at the bottom and vents near the ridge so air actually flows through and keeps the attic dry.
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This page is provided for general information. Building code requirements depend on your specific project, occupancy and municipality, and code editions change. Always verify requirements against the current Ontario Building Code and confirm with your local building department.