Roof exhaust fans come in upblast and downblast versions, and picking the wrong one causes problems that cost real money to fix later.
An upblast fan throws air straight up and away from the roof. A downblast unit pushes it outward and down across the roof surface instead.
Use an upblast fan wherever the discharge carries grease, fumes or odour, since it throws the plume clear of the roof and away from intakes. Use a downblast fan for clean general ventilation, where its lower profile and simpler construction suit the duty. Most codes require upblast for commercial kitchen extraction.
What each arrangement does
| Upblast | Downblast | |
|---|---|---|
| Discharge direction | Vertically upward | Outward and down over the roof |
| Plume height | High, clears the building | Low, stays near the roof |
| Grease handling | Designed for it, with a drain | Unsuitable |
| Re-entrainment risk | Low | Higher |
| Profile | Taller | Lower |
| Typical use | Kitchen, fume, odour | General clean ventilation |
A downblast fan discharging near an intake will send exhaust straight back into the building, which is an easy mistake to make on a crowded roof.
Why grease duties require upblast
Kitchen exhaust carries grease, and grease landing on a roof creates two problems.
It degrades the roof membrane, which is expensive to repair and often invalidates a warranty. It is also a fire risk, since a grease-coated roof surface near a discharge point is exactly what you do not want.
Upblast kitchen fans include a grease collection point and a drain to a container. A fan without one simply lets grease run down the outside of the curb and onto the roof.
Where downblast still belongs
- General warehouse and factory ventilation with clean air.
- Toilet and washroom extract, where odour dispersal is modest.
- Roof spaces and plant rooms.
- Applications where a low profile matters visually.
- Sites with no rooftop air intakes nearby.
- Finally, budget-driven general ventilation duties.
Downblast units are simpler, cheaper and lower, which is why they remain common on general ventilation. Our roof exhaust fan range covers both types.
Installation points that matter
- Use a properly flashed roof curb rather than sealing onto the deck.
- Set the curb height to suit the roof build-up and any snow load.
- Fit a backdraft damper where reverse flow would cause problems.
- Keep discharge points well clear of fresh air intakes.
- Provide safe access for cleaning, since roof fans get neglected.
- Finally, check wind loading, particularly on tall upblast units.
Access is the item most often skipped. An upblast grease fan that cannot be reached safely will simply never be cleaned.
Motor position and airstream
On most roof exhaust fans the motor sits outside the airstream, which is a genuine advantage.
That arrangement keeps grease, moisture and heat away from the windings, so service life improves considerably. Meanwhile the motor is accessible from above without disturbing the ductwork below.
Our commercial kitchen exhaust guide covers the whole system, from canopy to discharge.
Maintenance realities
Roof fans suffer from being out of sight, and upblast units in particular.
Nobody walks past one. Nobody hears it change. So unless a schedule exists, a rooftop fan runs untouched until it stops, which is usually on the busiest evening of the year.
Grease units need cleaning on a schedule set by cooking volume, often quarterly. Bearings need checking annually, since weather exposure shortens life. Meanwhile hinged bases make the difference between a job somebody does and a job somebody postpones.
For grease duct and cleaning requirements, NFPA 96 is the reference in North America, and local codes apply elsewhere.
One planning note for new buildings. Decide where the upblast discharges sit before the intakes are positioned, rather than the other way round. Moving a roof curb after the membrane is laid costs far more than moving a line on a drawing.
Roof exhaust FAQ
Can I use a downblast fan for a kitchen?
No, and most codes prohibit it. Grease discharged across the roof damages the membrane and creates a fire risk, so upblast with a grease drain is required.
How far from an air intake should a discharge be?
As far as the roof allows, and codes usually set a minimum. Prevailing wind matters too, so a compliant separation can still cause re-entrainment on a bad day.
Is upblast noisier?
Not inherently, though the discharge is directed upward, so noise travels further. Where nearby buildings overlook the roof, consider an attenuator.
Do roof fans need weather protection?
The housing provides it, though the motor and electrical connections still need proper sealing. Corrosion protection matters, particularly on coastal sites.
Upblast throws the plume clear, downblast keeps things low and cheap. Grease, fumes and nearby intakes all point firmly toward upblast.
