An emergency roof drain — also called an emergency overflow — is a second drainage route on a flat roof that only comes into action when the normal rainwater outlets can no longer cope. Its opening sits slightly higher than the normal outlet: in ordinary use it does nothing, but in a blockage or a cloudburst water visibly pours out of it.
Why it is needed
A flat roof is never perfectly level; under load the structure deflects. If water stands on it, the roof deflects further, so more water collects. This self-reinforcing process is called ponding and is a well-known cause of roof collapse. The emergency drain breaks the cycle: water leaves before the load becomes critical.
An emergency drainage provision is a standard requirement for new flat roofs in Dutch building regulations, with rainwater drainage capacity determined to NEN 3215. In the UK, rainwater drainage and overflow provision are covered by BS EN 12056-3 and its national annex.
Types
| Type | Description |
|---|---|
| Spout / scupper | A pipe or channel through the upstand, visible on the facade |
| Rectangular overflow | A wide slot cut in the upstand |
| Lowered roof edge | Part of the edge is deliberately set lower so water spills there |
| Separate overflow system | A second set of roof outlets with its own pipework |
Key rules
- Never connect it to the sewer — an emergency drain must discharge freely onto ground or a paved surface, because during heavy rain the drainage system is itself overloaded
- Put it where it shows — a spout dripping over a footpath is a free alarm that the main outlet is blocked
- Set the level carefully — the invert sits above the normal water level but well below the top of the upstand
- Design the structure for it — the roof must be able to carry the depth of water that builds up before the emergency drain starts to work
Maintenance
Check the normal roof outlets at least twice a year (spring and after leaf fall) and clear leaves, moss and gravel. Clean leaf guards and gravel guards at the same time. If a spout drips when it is not raining hard, the main outlet is almost certainly blocked.
Related terms
- Rainwater drainage
- Flat roof
- Roof upstand
- Gutter
- Scupper
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