What are the Toronto building code requirements for bathroom exhaust fan ductwork?

The Ontario Building Code requires every bathroom in a Toronto home to have mechanical exhaust ventilation rated at a minimum of 50 CFM, and the exhaust duct must terminate at the building's exterior — never into the attic, soffit, crawlspace, or any other enclosed space. These requirements apply to new construction, renovations where exhaust is being added or replaced, and any situation where a building permit is pulled for bathroom work.

The 50 CFM minimum applies to a standard residential bathroom. For larger bathrooms or ensuites with a separate shower enclosure, higher CFM ratings are appropriate — a common guideline is 1 CFM per square foot of bathroom floor area, with a 50 CFM minimum. Bathrooms with jetted tubs, steam showers, or multiple fixtures benefit from 80 to 150 CFM fans to manage the additional moisture load. In Toronto's climate, where winter heating creates very dry indoor air and summer humidity can be excessive, properly sized bathroom exhaust is critical for controlling condensation on windows, preventing mould growth on grout and drywall, and maintaining healthy indoor air quality.

The duct material matters. Bathroom exhaust duct should be rigid or semi-rigid aluminum or galvanized steel — smooth-wall duct provides lower airflow resistance and does not trap moisture the way corrugated flex duct does. If flex duct is used for a bathroom exhaust run, it must be ULC-rated, pulled taut with no sags or kinks, and supported every 4 feet to prevent sagging. Sagging flex duct creates low points where condensation pools, eventually dripping back through the fan housing onto the bathroom ceiling. This is an extremely common problem in GTA homes, especially in attic-routed exhaust runs where the temperature difference between warm bathroom exhaust and cold attic air during winter generates heavy condensation.

The duct must slope upward from the fan toward the exterior termination to allow condensation to drain outward rather than back toward the fan. The exterior termination point needs a proper vent cap with a damper flap — not a simple open pipe — to prevent cold air backdraft during Toronto winters and to keep pests, rain, and snow out. Wall termination is generally preferred over roof termination because roof penetrations are more prone to leaks and are harder to maintain, though roof termination is sometimes necessary depending on the bathroom's location in the home.

One of the most common code violations found in Toronto homes is bathroom exhaust vented into the attic. This is never acceptable under the OBC, yet it is found in a surprising number of GTA homes, including relatively recent builds where shortcuts were taken. Warm, moisture-laden exhaust air dumped into a cold attic creates severe condensation, mould growth on roof sheathing, ice damming, and wood rot. If your Toronto home has bathroom exhaust venting into the attic, extending the duct to a proper exterior termination should be a priority — this repair typically costs $300 to $800 per fan in the GTA, including duct extension, exterior cap, and proper sealing.

Any electrical work associated with bathroom exhaust fan installation — wiring, switching, timer controls — must be performed by an ESA-Licensed Electrical Contractor in Ontario. For new fan installations or replacements that involve duct modifications, a ductwork professional can ensure the run is properly sized, routed, insulated, and terminated. Find qualified contractors through the Toronto Construction Network.