Building components · Drainage Elements
Rainwater Outlet
Explains the outlet as the constriction that governs the whole drainage chain above it.
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 is
An outlet is where collected water enters the pipe system. On a flat roof it is a fitting set into the deck and connected through the waterproofing; on a pitched roof it is where the gutter connects to the downpipe.
It is the narrowest point in the chain, which makes it the point that governs. A gutter of ample capacity draining through an inadequate or blocked outlet backs up regardless of how large the gutter is.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a gutter. The gutter collects and conveys along the roof edge; the outlet is where water leaves it.
- It is not a downpipe. The downpipe is the vertical run below; the outlet is the connection into it.
- It is not an overflow. An overflow is the deliberate secondary route used when the outlet cannot cope.
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.
- outlets
- roof outlet
- roof drain
- gutter outlet
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Being kept clear, since it is the point where debris carried by the water arrives and stops.
- Its connection through the waterproofing on a flat roof, which is a penetration through the layer keeping the building dry.
- The number and position of outlets relative to the roof area and its falls.
- Whether a secondary route exists for when the outlet is blocked or exceeded.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
A flat roof outlet is a designed penetration through the waterproofing, and it is where water is deliberately concentrated — the two together make it the most demanding point on the roof.
Outlets collect debris by design, and roofs where outlets cannot be reached are roofs where they are not cleared.
A blocked outlet on a flat roof causes water to accumulate, and accumulated water is a load the roof structure may not have been designed for.
The junction between an outlet and the waterproofing is a detail that is worked repeatedly by thermal movement and by water.
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.
Materials
Commonly incorporates
Materials this element commonly contains as a distinct constituent part, rather than being made of.
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.
Membrane to Roof Outlet
Where the waterproofing is deliberately penetrated at exactly the point water is concentrated.
Raises: Moisture · Drainage · Movement · Structural transfer
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.
- Deck & Terrace
- Flat Roof
- Site Drainage
Clarifications
Commonly misunderstood
- That an outlet is a fitting. On a flat roof it is a penetration through the waterproofing at the point water concentrates.
- That a blocked outlet is a nuisance. On a flat roof it can load the structure with accumulated water.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- How many outlets serve this roof, and what area does each take?
- How is the outlet connected through the waterproofing?
- Is there a secondary route for when an outlet is blocked or exceeded?
- How are the outlets accessed for clearing?
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 size, capacity, spacing or connection arrangement is stated here.
- Roof drainage design, including outlet provision and overflow arrangements, is professional work subject to regulation.
Drainage & Rainwater Elements
The gutters, outlets, downpipes, channels, chambers and traps that collect water, carry it and let it be inspected.
Browse all drainage elements entries →