Roof Cowls & Weatherproof Vent Cowls

A roof cowl is a weatherproof termination fitting for vertical duct runs that penetrate a roof, allowing air to exhaust or intake freely while preventing rain, wind, birds and debris from entering the system. We stock the Systemair RAW series in 8 sizes from 200mm to 1000mm — the industry-standard commercial roof cowl for extract fan outlets, MVHR roof terminals, kitchen extraction, and general building ventilation. All cowls include integrated bird guards as standard, are suitable for both flat and pitched roof installations, and are compatible with rigid and flexible ductwork. All sizes in stock with next-day delivery available.

Circular Weather Louvres / Square Weather Louvres / Roof Fans / Ducting

What is a roof cowl used for?

A roof cowl is fitted to the top of a vertical duct that exits through a roof. It allows ventilation air to 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. They're 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 8 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, giving flexibility where duct sizes have been uprated or where stocking is limited. For smaller domestic applications (100mm to 150mm) or for non-standard sizes, contact us on 0161 818 2050 and we can source appropriate alternatives.

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 designed to create a low-pressure zone at the outlet that aids exhaust flow — the shape 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. All our roof cowls are active exhaust/intake terminals suitable for mechanical ventilation systems.

Roof cowl vs 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 up. A weather louvre is used where the duct exits horizontally through an external wall — the louvre fits into the wall opening and vents straight out. For most commercial buildings you'll have both: roof cowls on kitchen and plant room extracts that rise to the roof, and weather louvres on wall-mounted intakes and extracts. Cowls give better weather protection for vertical runs; louvres give a cleaner architectural finish for 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 provides the weather seal between the cowl flange and the roof membrane. On flat roofs, a minimum 150mm upstand is typical to keep the cowl clear of standing water and snow. On pitched roofs, an appropriate flashing or proprietary roof adaptor is used to maintain the weather seal. Check individual product specifications 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, preventing birds, rodents, leaves and large debris from entering 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 — contact us 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 the Building Engineering Services Association (BESA) guidance. The cowl itself does not provide fire resistance or grease arrestance — those are handled upstream. Larger commercial kitchen systems typically use 400mm, 500mm or 630mm cowls depending on extract volume.

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. Electrical or controls connections are not required — roof cowls are passive terminals. Installation should be carried out by a competent roofer or M&E contractor to maintain the roof weather seal.