Rainwater outlet

A channel that collects rainwater run-off from a roof and conveys it to outlets and downpipes. Gutters may be eaves-mounted, formed as valley or parapet gutters within the roof, or integrated into metal roofing systems, and are sized by rainfall calculation.

A vertical pipe conveying rainwater from a roof outlet or gutter down to the drainage system at ground level. Downpipes are made from cast iron, steel, aluminium or plastics, and their number and size are determined by roof catchment area and design rainfall intensity.

The designed slope built into a nominally flat roof so that rainwater drains towards gutters or outlets. Falls may be formed by the structure, screeds, firring pieces or tapered insulation, and design gradients are normally set steeper than the minimum finished fall to allow for construction tolerances and deflection.

Rainwater that remains standing on a flat roof after the rest of the surface has drained, typically caused by inadequate falls, structural deflection or blocked outlets. Persistent ponding adds load, accelerates deterioration of some coverings and can invalidate system guarantees.

A flat roof designed to attenuate rainfall by temporarily storing water on the roof and releasing it at a controlled rate through restricted outlets, reducing peak loads on drains and watercourses. Blue roofs form part of sustainable drainage (SuDS) strategies and place high demands on the reliability of the waterproofing and the structure.