Building components · Substructure & Ground
Drainage Sump
Explains the sump as the point a system is drained from when gravity alone will not take water away, and why it is a permanent dependency rather than a fitting.
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 drainage sump is
A drainage sump is a formed low point into which water is led so that it can be taken away from one place. It is met where water arrives below the level it has to reach: in a basement, in a light well, behind a retained structure, or at the low end of a drained cavity.
Collecting water is only part of what a sump is for. A sump exists because the water cannot leave by gravity, so something has to lift it — which is the point at which the arrangement stops being passive and begins depending on equipment that can fail, lose power, or go uninspected.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a silt trap. A trap is placed within a run to hold solids back from what is downstream of it; a sump is the end of a route, and what gathers there is water that has nowhere lower to go. The published trap entry carries 'sump' as an alias, because the settling volume in its base is also called one; the word names a shape there and a destination here.
- It is not an outlet sump on a roof. A roof outlet sump is a local depression formed in the deck so that water reaches the outlet; a drainage sump is a collection point from which water has to be removed, commonly by pumping.
- It is not an outfall. An outfall is where a system discharges into a sewer, a watercourse or the ground; a sump is where water is gathered before it can be taken to such a point.
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.
- sump pit
- collection pit
- pump pit
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- What removes the water from it, since a sump without a working means of removal simply fills.
- A power supply where removal is by pump, and what happens to that supply in the conditions that produce the most water.
- Being inspectable, because a sump is commonly the least visited part of an arrangement and the first to be found failed.
- Being kept free of silt, since solids arriving with the water settle at exactly the lowest point.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
A sump converts a gravity problem into an equipment problem, which is a real answer and a permanent dependency at the same time.
Pumps, floats and non-return arrangements are consumable items in a wet, silty environment, and they sit behind a cover that is rarely lifted.
Everything within a sump is permanently or intermittently submerged, which is the shortest-lived environment in the whole drainage arrangement.
Where a sump serves a habitable below-ground space, the consequence of it not working is water inside the building rather than water standing outside it.
Materials
Commonly formed from
Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.
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.
Clarifications
Commonly misunderstood
- That a sump drains a space. It collects water; something else removes it, and that is the part that fails.
- That a sump is a fallback for an unusual event. Where it is the only route out, it is in use whenever water arrives, and its condition is a continuous matter.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- What removes water from this sump, and what happens if it stops?
- Is there any alarm or indication that the sump is filling or that removal has failed?
- How is the sump inspected and cleared, and when was that last done?
- Where a power supply is relied on, what is the position during a power failure?
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. It states no depth, capacity, pump duty, discharge rate or alarm provision, and none can be inferred from it.
- Below-ground water management, including pumped drainage serving habitable space, is a regulated matter requiring qualified design and periodic verification.
Substructure & Ground Elements
The prepared surfaces, layers and buried members between undisturbed ground and the first structure above it.
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