A vertical roof outlet — also called a bottom outlet — is a rainwater outlet that takes water straight down through the roof deck. Immediately below the roof it connects to a vertical downpipe, sometimes via a short swept bend. It is the most common solution on flat roofs of sheds, garages, extensions and larger industrial roofs.
Vertical outlet or side outlet
A flat roof has two basic options:
- Vertical (bottom) outlet — water passes vertically through the roof plane. The connection is short, the flow is good, and the outlet can be positioned anywhere in the roof
- Side outlet or spitter — water passes horizontally through the parapet or upstand. Useful where you do not want to penetrate the deck, for instance on a timber roof
Components
- Outlet body — a plastic, stainless steel or lead unit with a flange or membrane skirt that is dressed into the waterproofing
- Leaf guard or gravel guard — a domed grate or strainer basket keeping leaves, moss and gravel out of the downpipe
- Downpipe — the vertical rainwater pipe to the drain or to a soakaway
Where an outlet passes through the roof of a heated space, an insulated version is used. Without insulation, the cold pipe is a thermal bridge on which condensation forms, causing damp patches and mould on the ceiling below.
Points to watch
- Position it at the low point. Falls must run to the outlet; an outlet on a high point leaves standing water permanently
- Always provide an overflow. If the outlet blocks, water has to be able to escape — preferably somewhere visible on the elevation so the fault is noticed. An overflow is a normal requirement on flat roofs
- Keep the grate clear. With trees nearby, checking twice a year is realistic; a blocked outlet is the most common cause of ponding on a flat roof
- Think about the load. Water that cannot escape weighs 10 kg per m² per centimetre of depth — a ponded area can overload a lightweight roof
- Detail the connection properly. The flange belongs bonded between the layers or beneath the cap sheet, not laid loosely over the top
Related terms
- Rainwater outlet
- Side outlet
- Overflow
- Leaf guard
- Flat roof
- Falls
- Parapet
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