Roof Drain (Internal Drain)

A roof drain, called a rainwater outlet in the UK, is a fitting installed in the roof surface, usually at a low point, that collects water and conducts it into a leader or downpipe running through the building. Internal drains let large roofs drain without needing every slope to reach the perimeter.

Components

• Drain body or bowl set into or onto the deck
• Clamping ring or flange that grips the membrane, or a spigot outlet welded or bonded to it
• Strainer or dome that keeps out gravel and leaves
• Sump formed in the insulation or deck so the outlet sits below the surrounding roof surface

Placement and number

Drains go at the lowest points of the roof, after allowing for deflection: on steel and timber decks the low point is often at mid-span, not at columns. UK guidance recommends at least two outlets on even small roofs, or an outlet plus overflow, so a single blockage cannot flood the roof. Outlets are recessed or set in sumps to avoid ponding caused by membrane laps building up around them. BS 6229:2025 limits sumps to about 0.72 m2 (7.8 sq ft) per outlet, between 400 mm and 1,200 mm (16-47 in) across, and internal gutters linking outlets should be at least 500 mm (20 in) wide.

Sizing

Drain and pipe sizes depend on rainfall intensity and the roof area served. In the UK and Europe calculations follow BS EN 12056-3; in the US they follow the plumbing code. Siphonic systems, which run pipes full to create suction, are used on very large roofs.

Maintenance

Blocked drains are a leading cause of ponding and of structural overload. Strainers should be cleared during routine inspections, typically in spring and autumn and after storms.