Should I vent my Toronto home bathroom exhaust fan through the roof or the soffit?

Always vent through the roof or a gable wall — never through the soffit. While soffit venting might seem easier and cheaper, it creates serious moisture problems in GTA homes that far outweigh any installation savings. The exhaust air you are pushing out of your bathroom is warm, moisture-laden air, and when it exits through a soffit vent, it gets drawn right back into the attic through the soffit intake vents that are part of your roof ventilation system. You are essentially pumping bathroom humidity directly into your attic.

This is a particularly damaging problem in Toronto's climate. During winter, that warm, moist exhaust air enters the cold attic space and condenses on the underside of the roof sheathing. Over a single heating season, a soffit-vented bathroom fan can deposit litres of moisture onto your roof decking. The result is mould growth on the plywood sheathing, wood rot, deteriorating roof structure, and ice damming where the moisture freezes near the roof edge. Home inspectors in the GTA flag soffit-vented bathroom exhausts as a deficiency on virtually every inspection report, and it can become a deal-breaker during a home sale.

A roof cap termination is the preferred method for most GTA homes. A proper roof cap installation involves running rigid or semi-rigid metal duct from the fan housing up through the attic and out through a purpose-cut hole in the roof, sealed with a flashed roof cap that prevents rain and snow from entering. The duct must be insulated in the attic space with a minimum of R-8 duct wrap to prevent condensation from forming on the cold exterior of the duct during winter. Without insulation, a bathroom exhaust duct running through a -15 degree attic will sweat profusely, dripping water back into the fan and onto the bathroom ceiling below.

Gable wall termination is an excellent alternative if your attic layout allows for a reasonably short, straight run to a gable end. This avoids penetrating the roof membrane entirely, eliminating any future leak risk at the roof cap. It is especially practical in homes with accessible gable ends on the same side of the house as the bathroom.

A few practical considerations for the installation: slope the duct slightly downward toward the exterior termination point so any condensation that does form drains outward rather than back into the fan housing. Use a minimum 4-inch duct — 6-inch is better for longer runs. Ensure the roof cap or wall cap has a damper flap that closes when the fan is off to prevent cold air backdraft in winter, which is a common complaint in Toronto homes. The total cost for a professional bathroom exhaust fan installation with new ductwork through the roof typically runs $400-$800 in the GTA. Any electrical work for wiring the fan requires an ESA-Licensed Electrical Contractor. If your home currently has a soffit-vented bathroom fan, rerouting it through the roof is one of the smartest moisture-control investments you can make.