roof curb

A roof curb is a prefabricated or site-built raised frame, typically made of steel or aluminum, that sits on the roof deck and provides a stable, weathertight mounting base for mechanical equipment. It isolates the equipment from direct contact with the roof membrane, protecting both the unit and the building envelope from water infiltration where penetrations occur. HVAC rooftop units, exhaust fans, skylights, and ventilation ductwork all rely on curbs to transition from the interior building systems to the exterior environment without compromising the roof's integrity.

Standard roof curbs are manufactured in common sizes such as 24 by 24 inches, 30 by 30 inches, and larger bespoke dimensions to accommodate specific equipment footprints. The frame is typically 12 to 18 inches tall, with mounting feet that bolt or weld to the roof deck structure. Many curbs include a sloped weather collar that directs water away from the equipment base, and flashing is installed beneath and around the curb to seal the roof membrane penetration. The interior perimeter of the curb often features a drip edge or lip that prevents water from pooling at the equipment interface.

Common failure points and variations

Improper flashing installation is the leading cause of roof leaks at curbed equipment. If the flashing is not sealed correctly to both the roof membrane and the curb frame, capillary action draws water under the flashings during wind-driven rain. Curbs made from galvanized steel can rust if the protective coating is damaged during installation or if the unit vibrates against the frame over time. Aluminum curbs are lighter and more corrosion-resistant but less rigid; oversized equipment mounted on an undersized aluminum curb can sag slightly, causing pooling. Custom-formed curbs with integral ductwork flanges are used in retrofit installations where standard sizes do not fit the available roof space.

The term curb derives from its function as a raised edge or border, much like a street curb defines the pavement boundary. In roofing trade language, the curb is distinct from a simple mounting pad or equipment stand because it incorporates weather protection into its design. Some specifications refer to it as a equipment curb or roof-mounted unit curb to distinguish it from other building curbs.

Installation depth and materials must satisfy local building codes and the roofing manufacturer's warranty. Many warranty clauses void coverage if penetrations are not made with approved curbs and flashing systems. The curb also affects thermal expansion of the roof membrane around the penetration, so adequate clearance and flexible sealing are critical in climates with large seasonal temperature swings.