Building components · Layers & Barriers
Drainage Layer
Explains drainage as a layer whose purpose is conveying rather than resisting water, and why it needs an outlet.
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 layer is
A drainage layer provides a path within a build-up along which water can move. It is used where water is expected — behind a retaining wall, above waterproofing, beneath a green roof — and where the response is to let it flow rather than to hold it back.
It only works if it leads somewhere. A drainage layer with no outlet becomes a reservoir, and the pressure it was installed to relieve builds up in it instead.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a barrier. A barrier resists water; a drainage layer conveys it, and the two are commonly used together with the drainage on the wet side.
- It is not a filter layer, although drainage layers commonly incorporate one. Filtration keeps solids out of the drainage medium so it does not silt up.
- It is not a ventilated cavity, which moves air rather than water, though a single space sometimes does both.
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.
- drainage medium
- drainage composite
- drained cavity layer
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Having an outlet, since drainage without discharge is storage.
- Filtration, because a drainage layer that fills with fine material stops draining.
- Continuity to the outlet, as an interruption creates a dam within the layer.
- The load it carries, since a compressed drainage layer has lost its void space.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Relieving water pressure is frequently more effective than resisting it, which is why drainage appears alongside waterproofing rather than instead of it.
Silting is the usual failure mode, and it is progressive and invisible until the drainage stops working.
The outlet is the part that can be inspected and cleared; the layer itself generally cannot be.
Compression during backfilling or loading removes the void space the layer depends on.
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.
Junctions
Interfaces this element takes part in
Junctions where the condition between this element and another governs how the construction behaves. These name the junction and the subjects it raises; they are not details, arrangements or approved ways of building.
Protection Over Waterproofing
Where a protective layer covers waterproofing, in the interval when the waterproofing is most vulnerable and least replaceable.
Raises: Moisture · Drainage · Buildability · Weathering
Where it is met
Application contexts
Building contexts where this element is commonly encountered. Association is not recommendation: appearing here does not mean the component suits every instance of that context, or any particular one.
- Basement
- Boundary Structure
- Deck & Terrace
- Site Drainage
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 a drainage layer keeps water out. It moves water to where it can leave, which is a different mechanism.
- That drainage removes the need for waterproofing. The two work together, with drainage reducing what the waterproofing has to resist.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Where does this drainage layer discharge, and is that outlet accessible?
- What filtration keeps solids out of the drainage medium?
- What loading will the layer carry, and will it retain its void space under it?
- How is the layer continuous to the outlet without interruption?
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
- No thickness, flow capacity, filtration specification or outlet arrangement is stated here.
- Drainage design is professional work informed by the water expected and the ground conditions.
Envelope Layers & Barriers
Continuous positioned layers within a build-up — insulation, separation, protection, drainage and root barriers — named by the job they do rather than the product used.
Browse all layers & barriers entries →