A roof cowl is the weatherproof termination for a vertical duct that penetrates a roof, letting air exhaust or enter freely while keeping rain, wind, birds and debris out of the system. We stock the Systemair RAW series in eight sizes from 200mm to 1000mm, the standard commercial roof cowl for extract fan outlets, MVHR roof terminals, kitchen extraction and general building ventilation. Every cowl includes an integrated bird guard, suits flat and pitched roofs, and connects to rigid or flexible ductwork. All sizes are in stock for next day delivery. If you need a smaller domestic size or something non-standard, call 0161 399 3825 or email hello@efans.co.uk and we will source it.
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Frequently asked questions
What is a roof cowl used for?
A roof cowl fits to the top of a vertical duct that exits through a roof. It lets ventilation air flow in or out while keeping rain, birds, leaves and debris from entering the duct. Typical uses include commercial kitchen extract terminations, toilet and bathroom extract discharge, MVHR supply and exhaust roof terminals, plant room ventilation, warehouse and industrial roof extract, and general building ventilation. It is the standard external termination for any duct that exits through a roof rather than a wall.
What sizes of roof cowl do you stock?
We stock the Systemair RAW series in eight standard duct diameters covering most commercial and industrial applications: 200/250mm, 300/315mm, 350mm, 400/450mm, 500/560mm, 630/710mm, 800mm and 1000mm. The dual-size cowls fit either of the stated diameters, which helps where duct sizes have been uprated or where stocking is limited. For smaller domestic applications from 100mm to 150mm, or for non-standard sizes, call 0161 399 3825 and we can source an alternative.
What is the difference between a roof cowl and a roof vent?
The terms are often used interchangeably, but there is a practical difference. A roof cowl is a directional or domed fitting shaped to create a low-pressure zone at the outlet, which aids exhaust flow and helps extract fans work more efficiently by reducing back-pressure. A roof vent is a broader term covering any roof-mounted ventilation terminal, including passive ridge vents, soffit vents and static roof vents. Everything on this page is an active exhaust or intake terminal suitable for mechanical ventilation systems.
Roof cowl or weather louvre, which do I need?
The choice comes down to orientation. A roof cowl is used where the duct exits vertically through the roof: the cowl sits on top of the duct and vents horizontally or upward. A weather louvre is used where the duct exits horizontally through an external wall, fitting into the wall opening and venting straight out. Most commercial buildings need both: cowls on kitchen and plant room extracts that rise to the roof, louvres on wall-mounted intakes and extracts. Cowls give better weather protection on vertical runs; louvres give a cleaner architectural finish on wall terminations.
Are roof cowls suitable for flat roofs?
Yes. The Systemair RAW range is designed for both flat and pitched roof installations. The cowl mounts to a roof kerb or upstand that forms the weather seal between the cowl flange and the roof membrane. On flat roofs a minimum 150mm upstand is typical, keeping the cowl clear of standing water and snow. On pitched roofs an appropriate flashing or proprietary roof adaptor maintains the seal. Check the individual product specification for installation dimensions and any roof pitch requirements.
Do roof cowls come with a bird guard?
Yes. Every Systemair RAW roof cowl includes an integrated bird guard as standard, keeping birds, rodents, leaves and large debris out of the duct. This meets Building Regulations Part F requirements for external ventilation openings. For food production, clean room or pharmaceutical facilities that need a finer insect mesh, the bird guard can be supplemented with an additional fine mesh screen. Get in touch for specification advice.
Are roof cowls suitable for kitchen extract?
Yes, roof cowls are the standard termination for commercial kitchen extract systems. For grease-laden extract from commercial cooking, the cowl should be paired with appropriate upstream grease filtration and a duct access point for cleaning, in line with DW172 and BESA guidance. The cowl itself provides neither fire resistance nor grease arrestance, both of which are handled upstream. Larger commercial kitchen systems typically use 400mm, 500mm or 630mm cowls depending on extract volume.
Can the same cowl be used for supply and extract?
Yes. A roof cowl is a passive terminal with no fixed airflow direction, so the same fitting works on an intake or an exhaust duct. What matters is that the cowl is sized for the duct and that its free area does not throttle the system: an undersized cowl adds static pressure and will pull the fan off its duty point. On MVHR installations where supply and exhaust both terminate at roof level, keep the two terminals far enough apart that exhaust air is not drawn back into the intake.
How is a roof cowl installed?
The cowl fits over a roof-mounted duct upstand or kerb and is fixed via a flanged base. On flat roofs, a timber or proprietary metal kerb is built into the roof structure and sealed to the roof membrane before the cowl is mounted. On pitched roofs, a flashing kit or roof adaptor provides the transition between the cowl and the tiles or sheeting. No electrical or controls connection is required, since roof cowls are passive terminals. Installation should be carried out by a competent roofer or M&E contractor so the roof weather seal is maintained. See our RAW roof cowl accessories for kerbs, adaptors and fixings.
