Sump (roof drainage)
A roof sump is a recessed collection point in a flat roof or box gutter that gathers water to the outlet and downpipe, with the membrane clamped into it to stop leaks.
Ask Chalkline about this →A sump is a recessed collection point in a flat roof, box gutter, or paved area that gathers water and feeds it into the drainage outlet and downpipe. The waterproofing membrane is dressed down into it and secured with a clamping ring or to a puddle flange.
On a flat or low-pitch roof, or in a box gutter, water needs a defined low point to collect and drain away. The sump is that recess, set below the surrounding surface so the falls run water to it, with the outlet and downpipe at its base. The detail that matters is the waterproofing: the membrane must be carried down into the sump and clamped (a clamping ring) or sealed to a puddle flange, so water cannot track underneath it and into the structure. A sump also sets the reference level for any overflow provision.
Get it wrong and it leaks or ponds. An undersized sump, one set too high (so water does not actually fall to it), or a poorly dressed membrane junction is a classic source of flat-roof leaks and standing water.
For a builder the practical points are: size and locate the sump to the hydraulic design, set it as the genuine low point so the falls deliver water to it, and get the membrane-to-sump junction right because it is a known failure point. Pair the sump with a separate overflow so that a blocked outlet sheds water safely rather than flooding the roof and finding its way inside.
Also known as: Roof sump, rainwater sump.
Category: Roofing / Drainage.
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References
- Drainage outlet (Chalkline) (verified 2026-06-01)
Last updated: 2026-06-01. Verified: 2026-06-01. Quarterly review for currency.