Building components · Roof Carcass & Edges
Outlet Sump
Explains the sump as shaped build-up around a drainage point, distinct from the outlet fitting it serves and from the general falls of the roof.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a outlet sump is
An outlet sump is a depression formed in the layers of a roof or deck immediately around a drainage outlet. It is created in the build-up — by recessing the deck, by tapering the insulation down toward the point, or by setting a preformed unit into the layers — rather than being part of the outlet fitting itself.
It exists because a broad fall across a roof still leaves the last part of the journey undecided. A surface falling toward one corner delivers water to the area around an outlet set flush in it, and water then sits alongside the outlet rather than entering it. The sump makes that last part positive: within its area, every direction is downhill toward the drain.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not the rainwater outlet. The published outlet entry describes the point at which water leaves the roof and enters the pipework, and treats it as the constriction that governs the chain. The sump is the shaped recess in the surrounding build-up that brings water to that point; a roof can have an outlet of ample capacity with no sump around it and still hold water beside it.
- It is not a channel drain. A channel drain intercepts water along a line and is set into a surface people use; a sump concentrates water to a single point and exists below the surface of the build-up, where its shape is fixed at the moment the waterproofing goes over it.
- It is not a fall or a firring arrangement. Firrings and tapered layers set the plane of the whole roof; the sump is a local correction in the immediate area of a drain, and a roof with entirely conventional general falls may still be formed with one.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- roof outlet recess
- outlet recess
- tapered sump
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Having the depth available in the build-up to form a recess without losing the layers it is formed in.
- How the waterproofing is dressed down into it and clamped or bonded at the outlet, since the sump concentrates water at exactly the point where the layer is penetrated.
- Being kept clear, because a sump collects whatever the roof collects and delivers it to a single point where it stops.
- Whether anything laid over the roof — insulation above the membrane, ballast, paving or a walkway — preserves the path into the recess or bridges it.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
The purpose is entirely local: it governs the behaviour of the last part of the route, which is the part a general fall across a roof cannot determine.
Water is concentrated at the same position as a penetration through the waterproofing, which makes this one of the few places where the wettest point and the most vulnerable point coincide by design.
Debris carried across the roof arrives here and stays, so a sump is a position to be checked rather than one to be formed and forgotten.
It is formed in layers that are covered as soon as the waterproofing goes on, which means it cannot be added or deepened afterwards without opening the roof.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Related elements
Neighbouring components
Typed relationships to other elements. There is deliberately no generic “related” category: an edge that cannot be classified does not exist.
Clarifications
Commonly misunderstood
- That water standing beside an outlet means the outlet is too small. It commonly means the surface around the outlet does not fall into it, which is a shaping problem rather than a capacity one.
- That a sump can be introduced later. It is formed in the build-up beneath the waterproofing, so creating one means taking the waterproofing and the layers beneath it apart at that point.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Does the surface fall into each outlet from every direction, or only in the general direction of it?
- How is the waterproofing taken down into the recess and secured at the outlet?
- What depth is available in the build-up to form the recess, and what is displaced to make it?
- How is each sump reached for inspection and clearing once the roof is finished?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- This entry describes a position in a build-up. It states no size, depth, fall, outlet spacing or capacity, and none can be inferred from it.
- How many outlets a roof has, where they sit and how the surface is shaped around them are drainage design determinations for that roof, verified against local requirements.
Roof Carcass & Edges
The battens, boards and edge elements that finish a roof structure and prepare it to receive a covering.
Browse all roof carcass & edges entries →