A roof curb provides both structural support and weatherproofing for a rooftop exhaust fan; height and flashing decide its performance.
Structural Role
The curb raises the fan above the roof membrane so airflow is not blocked by parapets or snow and so the unit sits on a defined, level, load-bearing frame rather than the fragile roofing that was never meant to carry equipment weight.
A rooftop fan lives in the worst of both worlds: weather above and a sealed building below, so the curb is the interface that must be structural and watertight at once. Height, material, and flashing are the three details that decide whether it stays put and dry.
Height clears the membrane and any snow load so airflow is not blocked, and the common 300-450 mm range keeps the unit stable while clearing the roof build-up; a taller curb needs bracing against wind and seismic overturning because a narrow plinth is easier to tip. The height is set by the roof, not a catalogue default.
Flashing is the waterproofing: it wraps the curb base into the specific membrane, built-up or single-ply, and is the single most important curb feature for a dry interior. A curb cap or equipment rail lets the fan be levelled and later changed without disturbing the seal below, limiting the cost of a future swap-out to the fan alone.
Penetrations for conduit and drain through the curb are themselves leak paths if left unfinished, and a small uncapped hole can admit more water over a year than the main joint would. Each is sealed and flashed, and in seismic zones the curb and fan are tied down in three axes so the unit cannot shift or topple during an event.
The company's 80,000 m² production site builds exhaust equipment under ISO 9001:2015 with CCC certification, and its engineers respond to selection enquiries within 24 hours to specify curb height, material, and flashing against the roof type and the wind or seismic load the site must resist.
Snow and parapet geometry set the practical height: in climates with deep snow the curb must clear the drift, not just the design snow load, and a parapet or roof equipment nearby can recirculate exhaust unless the fan sits high enough above the wake. The curb is detailed with the roof profile and the local wind so the inlet draws clean air and the unit is not sitting in its own discharge, which would cut capacity and load the fan for no gain.
Curb height commonly falls in the 300-450 mm range, enough to clear the membrane and insulation while keeping the unit stable; taller curbs need bracing for wind and seismic load, because a tall, narrow plinth is more easily overturned.
The curb must carry the fan's weight and its reaction loads, including the torque of a spinning rotor and wind uplift, so it is built from steel or a specified composite, not just wood that would rot and lose strength under weather exposure.
For seismic zones the curb and fan are tied down with restraints that limit movement in three axes, preventing the unit from shifting or toppling during an event and protecting both the equipment and the roof below it.
Weatherproofing
Flashing wraps the curb base into the roof membrane so water runs off rather than into the building; the detail must suit the specific roofing type, from built-up to single-ply, and is the single most important curb feature for a dry interior.
A curb cap or equipment support rail lets the fan be set and levelled independently of the roofing, so future fan replacement does not disturb the waterproofing below, limiting the cost and disruption of a later change-out.
Condensate and drain paths matter on cooled exhaust; the curb should not trap water against the membrane, which would accelerate weathering and leaks, and any drain through the curb must itself be flashed and sealed to remain watertight.
Penetrations for conduit and drains through the curb must themselves be sealed, because each hole is a potential leak path if left unfinished, and a small uncapped conduit can admit more water over a year than the main joint would.
| Item | Specification | Note |
|---|---|---|
| Curb height | 300-450 mm | Clear membrane |
| Material | steel | Load bearing |
| Flashing | to membrane | Weatherproof |
| Seismic | 3-axis tie | Restraint |
| Cap | equipment rail | Level + service |
| Penetration | sealed | No leak path |
Selection Checklist
- Set curb height to clear the membrane and any snow load.
- Build the curb from steel or rated composite, not wood.
- Flash the base into the specific roof membrane type.
- Add seismic restraints where the site requires them.
- Provide a curb cap so the fan is levelled and serviceable.
- Seal every penetration through the curb.
About the Manufacturer
Keruite Air Conditioning Group, headquartered in Dezhou, Shandong, has more than 30 years of HVAC manufacturing experience and operates an 80,000 m² production site with its own performance testing laboratory. Products are built under ISO 9001:2015 quality management and carry CCC certification, and are supplied to customers in over 50 countries.
Our engineers respond to selection enquiries within 24 hours and can provide project-specific sizing, performance curves, and documentation to support specification and procurement.