High-Capacity Anti-Vortex Roof Drain Outlet — Aluminium Vertical Spigot, 50-160mm

Most flat roof outlets are sized for a domestic patio or a garage — a 50mm or 75mm bore, rated for the rainfall a small roof will ever see. Scale that same outlet up to a large commercial deck, and the maths stops working: the outlet becomes the bottleneck, not the roof. In a heavy downpour, water backs up faster than it can leave, ponds against the upstands, and in the worst case overflows the parapet before the standard outlet has cleared it.

This is a large-capacity, anti-vortex gravity outlet built specifically for that problem. It uses a baffled dome or flat grate to break up the swirling vortex that normally chokes a plain outlet's flow, so the pipe underneath can run far closer to its true full-bore capacity. It is still a straightforward gravity outlet — no pumps, no siphonic priming, nothing bespoke to specify — just a better-engineered casting that clears more water per minute through the same size of pipe.

Specification

Why choose an anti-vortex outlet over a standard gravity outlet

A plain gravity outlet lets water spin as it drops into the pipe. That swirl drags air down with it, which cuts the effective flow because the pipe is running part-full of air rather than water. The baffle inside an anti-vortex grate breaks that spin before it starts, so the outlet can push a genuinely higher volume of water down the same diameter of pipe in the same downpour. For a roof designer or contractor, that means fewer outlets are needed to hit the same drainage design flow — or the same outlet count copes safely with a larger catchment area and a more severe storm event.

Frequently asked questions

Is this a syphonic drainage system?
No. This is a gravity outlet with an anti-vortex baffle, not a syphonic (engineered, primed) system. Syphonic drainage is a bespoke, project-specific design with no standard off-the-shelf price. This outlet is a standard stocked product that installs the same way as any gravity roof outlet — no design calculation, priming or specialist commissioning required.

What's the difference between the dome grate and the flat grate?
The dome grate sits proud of the roof surface and is the standard choice for most flat roofs — it has more strainer area exposed, so it copes better with leaves and debris. The flat grate sits level with the surrounding surface and is intended for outlets on a trafficked or paved area (a roof terrace or car park deck) where a raised dome would be a trip hazard or get damaged by vehicles.

How many of these does my roof need?
That depends on your roof area, local rainfall intensity and pipe run design, which is a job for your roofing contractor or drainage designer working to BS EN 12056-3. As a guide, each outlet here will generally cope with a larger catchment area than an equivalent-diameter standard domestic outlet, but we can't give a safe outlet count without your roof's dimensions and drainage design — call our technical team if you'd like help working through it.

Can this replace a blocked or undersized outlet on an existing roof?
Yes, on a refurbishment the existing outlet can usually be cut out and this unit bonded into the membrane in its place, provided the pipework below is the same bore or can be adapted. On a re-roof this is exactly the point at which to upsize, since the outlet is far cheaper to change now than once the new membrane is down.

Does it come with a leaf guard?
The dome grate itself acts as a coarse strainer against leaves and debris. For roofs near trees we'd still recommend a routine gutter-and-outlet clearance as part of annual roof maintenance — no outlet, however well designed, stays clear indefinitely without one.

Installation & ordering guidance

Select the diameter to match your downpipe or drainage run, and the grate type to match how the roof area around the outlet will be used. The outlet bonds into the waterproofing membrane in the same way as a standard gravity outlet: cut the membrane to the collar, lap it over the outlet flange, and bond with contact adhesive and uncured flashing tape for a fully watertight joint, positioned at the lowest point of the roof. Because pricing and lead time step up meaningfully with diameter, it's worth confirming your required bore with your drainage design or downpipe size before ordering — our technical team is on hand if you're not sure. Call 02031510153 or email sales@rubberroofuk.uk.

If your roof only needs a small, domestic-scale outlet rather than this large-capacity range, see our EPDM Roof Drain Outlet with Leaf Grate. If you need a threaded (screw-fit) connection rather than a spigot, see our Aluminium Vertical Threaded Roof Drain Outlet.