What is the proper exhaust duct termination height above a Toronto flat roof?

Exhaust duct terminations on flat roofs in Toronto must be at least 18 inches (450mm) above the roof surface according to Ontario Building Code requirements, with additional height requirements depending on the type of exhaust and proximity to air intakes or property lines.

The Ontario Building Code Part 6 (Mechanical) establishes specific termination heights to prevent exhaust air from re-entering the building through windows, air intakes, or HVAC equipment, and to ensure proper dispersion above the roof surface. For most residential exhaust systems (bathroom fans, dryer vents, general ventilation), the minimum 18-inch height applies when the termination is at least 10 feet from any air intake, operable window, or property line.

Kitchen range hood exhaust requires greater clearances — typically 3 feet above the roof surface and 10 feet horizontally from any air intake, window, or door. High-capacity commercial-style range hoods (600+ CFM) may require even greater separation distances. The code also requires that exhaust velocity be sufficient to prevent downdrafts — typically 1,500 feet per minute minimum exit velocity for grease-laden kitchen exhaust.

Special considerations for Toronto's flat roof climate: Toronto's frequent wind direction changes and the urban heat island effect around flat roofs can create complex air pressure patterns that pull exhaust back down to roof level. Many experienced GTA sheet metal contractors recommend exceeding minimum code heights by 6-12 inches, especially for bathroom exhaust in dense urban areas where neighboring buildings create wind turbulence. Proper termination caps are critical — use wind-resistant caps designed for flat roof applications that prevent rain entry while maintaining adequate exhaust flow.

Make-up air considerations: If your kitchen exhaust exceeds 400 CFM (which triggers Ontario's make-up air requirement), the make-up air intake must be positioned to avoid drawing in the exhaust air. This often requires careful coordination of exhaust and intake locations during the design phase, especially on smaller flat roof areas where separation distances are limited.

Professional installation is essential for flat roof exhaust terminations because improper flashing and roof penetrations are the leading cause of flat roof leaks in the GTA. The termination must be properly flashed with a roof curb, sealed with appropriate roofing membrane, and positioned to avoid ponding water areas. Any work involving roof penetrations typically requires both a mechanical permit and coordination with a qualified roofer familiar with Toronto's flat roof membrane systems.

Need help finding a ductwork contractor experienced with flat roof exhaust installations? Toronto Ductwork can match you with professionals who understand both the mechanical code requirements and proper flat roof integration techniques.