A roof-mounted exhaust fan should be installed on a purpose-built curb at least 200–300 mm above the finished roof surface, with the roof membrane turned up the curb and terminated under a metal counter-flashing, and with the discharge kept at least 3 m above the roof and 3–10 m away from any fresh air intake. The fan type follows the airstream: upblast for grease-laden kitchen exhaust, downblast for clean general extract, mushroom or hooded types for high-volume low-pressure roof ventilation, and inline or roof-mounted centrifugal units when ductwork resistance exceeds about 400 Pa.
Roof fan types and where each belongs
| Type | Typical airflow | Static pressure | Best application | Key limitation |
|---|---|---|---|---|
| Upblast centrifugal | 500–25,000 m³/h | 100–750 Pa | Kitchen grease exhaust, fume extract | Needs grease trough and clearance above roof |
| Downblast centrifugal | 500–30,000 m³/h | 100–600 Pa | Toilets, general building extract | Discharges onto roof; not for grease or corrosives |
| Mushroom / hooded axial | 2,000–60,000 m³/h | 50–300 Pa | Warehouse and factory roof ventilation | Low pressure capability only |
| Roof-mounted inline / tubular | 1,000–40,000 m³/h | 200–1,200 Pa | Long duct runs, smoke extract, laboratories | Taller profile, higher cost |
| Roof-mounted plenum / plug fan | 3,000–50,000 m³/h | 300–1,600 Pa | High-resistance systems with filtration | Largest footprint and weight |
Two selection errors dominate. The first is using a mushroom ventilator on a system that actually has 500 Pa of duct resistance; the fan will deliver perhaps 40–60 % of catalogue airflow. The second is fitting a downblast unit to kitchen exhaust, which deposits grease onto the roof membrane and voids most membrane warranties within a year.
Curb design and waterproofing
The curb, not the fan, is what keeps water out. Get it wrong and no amount of sealant will help.
- Curb height: 200 mm minimum above the finished roof surface for temperate climates, 250–300 mm as standard good practice, and 450–600 mm where annual snow accumulation is significant.
- Membrane termination: turn the roofing membrane up the full height of the curb and terminate it mechanically under a metal counter-flashing; never rely on adhesive alone at the top edge.
- Insulated curb: use a curb with integral insulation and a wood or thermally broken nailer so that condensation cannot form on the inner face in winter.
- Gasket: a continuous closed-cell foam or EPDM gasket between curb top and fan base flange, compressed by the fan's own fixings, typically 6–12 mm thick.
- Slope compensation: on pitched or tapered roofs use a pitched curb so the fan base sits level; a fan tilted more than 2–3° drains condensate the wrong way and loads the bearings unevenly.
- Drainage: keep the curb at least 300–450 mm clear of upstands, parapets and other curbs so water can drain past instead of ponding against it.
For grease exhaust, add a grease collection trough or cup with a capacity of at least 0.5–1.0 litre, plus an absorbent pad, and specify a hinged base so the fan can be tipped back for duct cleaning without breaking the roof seal.
Discharge location and separation distances
Where the air goes matters as much as how much moves. Standard practice is to discharge vertically upward at 8–12 m/s outlet velocity, at least 3 m above the roof level, and to keep a minimum separation from intakes: commonly 3 m for general exhaust, 5–8 m for toilet and kitchen exhaust, and 10 m or more for laboratory fume and hazardous discharge. Discharges must also clear the recirculation cavity of any nearby parapet or higher building — a rough rule is to stay outside a zone of one-third the height of the obstruction upwind of it.
- Keep the fan at least 3 m from any roof edge or working platform, or fit a guard rail if closer.
- Do not place exhaust discharges under a higher wall with openable windows within about 6 m.
- Allow 1.0–1.5 m of clear access on all sides for belt changes and impeller cleaning.
- Where the fan serves a smoke extract duty, verify the certified temperature class — commonly 300 °C for 60 minutes or 400 °C for 120 minutes — and use fire-survival cabling.
Structural, electrical and noise considerations
Roof fans are eccentric, vibrating, wind-loaded masses on top of a waterproof surface. A 20,000 m³/h upblast unit typically weighs 90–180 kg, and wind uplift on its hood can exceed its own weight in a coastal design wind, so the curb must be structurally connected to the deck and the fan bolted to the curb on all four sides. Fit vibration isolators sized for 90–95 % isolation efficiency at running speed, and always fit a flexible connection between duct and curb. Provide an external isolator switch within sight of the fan, and a service receptacle where roof access is difficult.
Noise is normally the reason a roof fan gets complaints. Sound power rises steeply if the fan is selected far from its best efficiency point, so keep the duty within 10 % of peak efficiency and expect 65–80 dB(A) at 3 m for typical building extract units. Where a residential facade is within 20 m, add a discharge silencer or choose a larger, slower-running fan — halving fan speed with a larger wheel at the same duty typically removes 8–12 dB. Keruite manufactures roof exhaust fans at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory, and every impeller is balanced to G2.5 and performance-tested before shipment.
When to choose which roof fan
- Choose an upblast centrifugal fan whenever the airstream carries grease, oil mist or condensable vapour, because the discharge must be thrown clear of the roof surface.
- Choose a downblast centrifugal fan for clean, dry general extract at 100–600 Pa where cost and low profile matter and nothing in the airstream can stain the roof.
- Choose a mushroom or hooded axial ventilator when the duty is high volume at very low resistance — typically direct roof-mounted factory or warehouse ventilation below 300 Pa with no ductwork.
- Choose a roof-mounted inline or tubular fan when duct resistance exceeds about 400 Pa, when the fan doubles as smoke extract, or when a laboratory discharge needs a high, well-defined exit velocity.
- Choose a plenum or plug fan on a roof-mounted housing when filtration, heat recovery or 1,000 Pa-plus resistance is involved and space on the roof allows the larger footprint.
- Choose a belt drive over direct drive only when the duty must be trimmed by changing pulleys after commissioning; otherwise direct drive with a variable-speed motor is more reliable and needs no belt inspection.
FAQ
How high should a roof fan curb be?
A minimum of 200 millimetres above the finished roof surface, with 250 to 300 millimetres as normal good practice. In regions with significant snow accumulation, raise the curb to 450 to 600 millimetres so drifting snow cannot reach the fan base flashing.
What is the difference between upblast and downblast roof fans?
Upblast fans discharge vertically upward and are required for grease, oil mist and condensable vapours because the plume is thrown clear of the roof. Downblast fans discharge down onto the roof and suit only clean, dry general extract such as toilet and corridor ventilation.
How far must exhaust discharge be from a fresh air intake?
Common practice is at least three metres for general building exhaust, five to eight metres for kitchen and toilet exhaust, and ten metres or more for laboratory fume discharge. The discharge should also sit outside the recirculation cavity of nearby parapets or taller structures.
Can a roof ventilator handle ducted systems?
Mushroom and hooded axial ventilators only work below roughly 300 pascals, so a ducted system will run at 40 to 60 percent of catalogue airflow. For duct resistance above about 400 pascals use a roof-mounted inline, tubular or plenum fan instead.
How do you stop a roof exhaust fan from leaking?
Waterproofing is done by the curb, not by sealant. Turn the roof membrane up the full curb height, terminate it mechanically under a metal counter-flashing, and compress a continuous closed-cell or EPDM gasket between curb top and fan base flange.