A roof curb is a raised framed structure built up from the roof deck, providing an elevated, weathertight base for mounting rooftop equipment like HVAC units, skylights, or exhaust fans, keeping the equipment’s connection point above the roof surface where water flows.
This raised design prevents water from pooling directly against equipment mounted at roof level, instead allowing water to flow around and past the curb’s base as it would across any other part of the roof surface.
Curbs are more commonly associated with flat or low-slope commercial roofing than typical steep-slope residential applications, though similar principles apply to skylight curbs on residential roofs.
What Does a Roof Curb Involve?
- A framed, raised structure built on the roof deck
- Flashing integrating the curb with the surrounding roofing membrane
- A weathertight seal where equipment mounts to the curb
- Adequate height to keep equipment above expected water flow
Why Are Roof Curbs Important?
By elevating equipment above the roof’s water-shedding surface, a properly built and flashed curb prevents the water pooling and infiltration risk that would occur if heavy equipment were mounted directly on or too close to the roof deck without this protective elevation. This detail is particularly critical on flat or low-slope roofs, where water doesn’t shed as readily as on a steep residential roof, making adequate curb height and flashing even more consequential.
Inadequate curb flashing is a documented common source of commercial roof leaks, since the transition between the flat roofing membrane and the vertical curb sides requires careful, often custom-fitted flashing work to remain reliably watertight.
Roof Curb vs. Roof Penetration
A roof curb is a specific raised structure supporting mounted equipment. Roof penetration is the broader category of any opening or interruption through the roof surface, of which a curb-mounted piece of equipment represents one particular, more substantial example.