# Roof Drain (Internal Drain)

> An outlet set into the field of a low-slope roof that collects rainwater and carries it down through pipes inside the building.

- URL: https://roof.wiki/roof-drain/
- Topic: Low-Slope & Flat Roof Systems
- Also known as: rainwater outlet; internal drain; roof outlet; primary drain
- Regions: Global
- First published: 2026-11-02 · Last reviewed: 2026-11-02
- Cite as: "Roof Drain (Internal Drain)." roof.wiki. https://roof.wiki/roof-drain/

## Key facts

- **UK term:** Rainwater outlet
- **Recommended minimum (UK):** Two outlets, or outlet plus overflow
- **BS 6229:2025 sump limit:** About 0.72 m2 per outlet
- **Sizing standard (UK/EU):** BS EN 12056-3

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.

## Sources

1. Falls and drainage — Bauder. https://www.bauder.co.uk/roof-systems/refurbishment-projects/solving-the-challenge/improving-drainage-falls
2. BS 6229:2025 - The key changes and implications for flat roof design — Bauder. https://www.bauder.co.uk/technical-centre/standards-and-knowledge/bs-6229-2025-key-changes
3. Major Projects: Zero falls flat roof policy — LABC Warranty. https://labcwarranty.co.uk/blog/major-projects-zero-falls-flat-roof-policy/?hsLang=en
4. How to avoid ponding on flat roofs — Soprema UK via SpecifiedBy. https://www.specifiedby.com/soprema-uk/news/how-to-avoid-ponding-on-flat-roofs