Building components · Drainage Elements
Perforated Underdrain
Explains a pipe that collects along its length rather than conveying from a point, and why its surround is part of 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 underdrain is
A perforated underdrain is a pipe with holes or slots along it, laid within a permeable surround. Water in the surrounding ground enters through the openings all along its length, and the pipe then carries it away.
The pipe alone does very little. What makes it work is the surround: a permeable medium that lets water reach the pipe from a wide area, separated from the surrounding soil so that fines do not migrate in and blind it.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a conventional drain pipe. A sealed pipe conveys water introduced at defined points; a perforated pipe collects along its length.
- It is not a drainage layer. A drainage layer is a plane; the underdrain is a linear collector within or beneath one.
- It is not a channel drain. A channel intercepts surface flow; an underdrain collects water already in the ground.
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.
- land drain
- perforated pipe
- filter drain
- french drain pipe
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Its permeable surround, which is the collecting element rather than the pipe itself.
- Separation between that surround and the surrounding soil, without which fines migrate in and the drain blinds.
- Having an outfall, since a collector with nowhere to discharge fills and stops collecting.
- Its position relative to the water it is meant to intercept, which follows from where that water actually is.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Underdrains intercept water moving through the ground, which is a different problem from surface water and needs a different response.
Blinding by fines is the usual failure mode, it is progressive, and it cannot be reversed once the surround is contaminated.
The pipe can sometimes be rodded from a chamber; the permeable surround cannot be cleaned at all.
The separation layer around the surround is placed early and is easily damaged during backfilling.
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.
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
- Site Drainage
Clarifications
Commonly misunderstood
- That the pipe is the drain. The permeable surround is what collects; the pipe conveys what the surround delivers.
- That a land drain solves a wet garden. Where the water is coming from, and where the drain can discharge to, both have to be established first.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- What water is this drain intended to intercept, and is it positioned to do so?
- What separation protects the permeable surround from fines in the surrounding soil?
- Where does the drain discharge, and does that outfall have capacity?
- Can the pipe be rodded, and from where?
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 pipe size, gradient, surround grading or spacing is stated here.
- Land drainage design depends on ground conditions and on a legitimate discharge point, both of which are professional matters.
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 →