Building components · Drainage Elements
Chamber Benching
Explains benching as the surface that keeps a chamber self-cleansing, which is the opposite of what a silt trap is shaped to do.
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 benching is
Benching is the formed floor of a drainage chamber: the surface built up around the open channel running through it, falling toward that channel from the chamber walls. Flow entering the chamber stays in the channel, and anything that leaves the channel is returned to it by the fall of the surface around it.
Its purpose is negative as much as positive. A chamber floor that is flat, or that dips, holds water and collects the solids the flow is carrying; a benched floor gives those solids nowhere to rest. It is also the surface anyone working in the chamber stands on, which makes its shape and its finish working matters rather than cosmetic ones.
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. The two are shaped for opposite ends: a trap has a sump below its outlet so that solids settle out and stay there, while benching is shaped so that nothing settles at all.
- It is not an inspection chamber. The chamber is the access structure; benching is the floor formed within it, and chambers are met both with it and without it.
- It is not a channel drain. A channel drain is set into a surface to intercept water crossing it; the channel within benching carries a run that has already been collected.
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.
- haunching
- channel haunching
- chamber floor benching
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- The channel it is formed around, since benching works by returning flow to that channel rather than by carrying flow itself.
- Falling consistently toward the channel, because any flat or reversed area is exactly where solids will collect.
- Its finish, since a rough or broken surface holds what a smooth one sheds.
- Being formed against the chamber structure without a gap, because water finding its way behind benching is no longer in the drainage system.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Benching is what makes a chamber self-cleansing, so a chamber with poor benching becomes a place where the system collects rather than a place where it is reached.
It is the surface anyone entering or working in the chamber stands on, and it is wet whenever the run is flowing.
Benching is worked by flow, by grit and by whatever is used to clear a blockage, and once broken it begins holding what it was formed to shed.
It is shaped by hand around a channel, late in the construction of the chamber, and the quality of that shaping is not recoverable once the cover is 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.
Clarifications
Commonly misunderstood
- That benching is a finish applied for appearance. Its shape is the working part, and a flat chamber floor behaves quite differently from a benched one.
- That a chamber collecting solids must be undersized. A chamber that holds solids commonly has a floor that lets them rest.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Does the benching fall toward the channel across the whole chamber floor?
- Is the channel through the chamber continuous with the pipes entering and leaving it?
- Is the benching sound, or has it been broken where clearing equipment has been used?
- Where a chamber is entered, what is being stood on and what condition is it in?
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 fall, profile, thickness or material specification, and none can be inferred from it.
- Below-ground drainage construction, including chamber detail, is a regulated matter requiring qualified design.
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 →