Building components · Drainage Elements
Gully
Explains the gully as the point at which water enters a drainage system, which is a different job from providing access to a run that is already carrying 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 gully is
A gully is the opening through which surface water enters a buried drainage system. It sits at a low point in a surface — a yard, a driveway, a path, or the ground beneath a downpipe — and water is brought to it by the falls in the surface around it.
Most gullies are trapped: the body below the grating holds a standing water seal so that air from the drainage system cannot rise back through the opening. That seal, and the debris the grating and the body collect, are what make a gully a maintained element rather than a hole in the ground.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not an inspection chamber. That entry states the boundary from the other side: a gully admits surface water into the system, while a chamber provides access to a run that is already carrying it.
- It is not a channel drain. A channel intercepts water along a line and does not depend on falls directing water to a single place; a gully collects at a point and only works where the surface delivers water to that point.
- It is not a silt trap. A trap sits within a run to protect what is downstream of it; a gully sits at the boundary between the surface and the system and is where water enters it at all.
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.
- yard gully
- trapped gully
- back inlet gully
- road gully
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- The falls in the surrounding surface, since a gully collects at a point and water that never reaches that point is not collected.
- Being set at or below the surface draining into it, because a gully standing proud of that surface is simply bypassed.
- Its trap remaining charged, since a seal that dries out or is lost stops performing the job it is there for.
- Being cleared, because the grating and the body are where debris carried across the surface arrives and stops.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Point collection depends entirely on the surface delivering water to that point, which is why a gully in the wrong position drains very little of what falls around it.
Gratings and gully bodies fill with leaves and grit, and a gully whose grating cannot be lifted is one that will eventually stop admitting water.
A blocked or bypassed gully leaves water standing on the surface it was meant to drain, commonly against a wall or across a threshold.
Where a gully is set into a trafficked surface, the grating and the surround are both loaded by everything that crosses them.
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.
Clarifications
Commonly misunderstood
- That a gully and an inspection chamber are different names for the same buried box. One is an inlet and the other is an access point, and a system commonly needs both.
- That a gully which is not draining must be blocked at the gully. The obstruction is as often downstream, with the gully simply the first place it becomes visible.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Do the falls in the surrounding surface actually deliver water to this gully?
- Is the gully trapped, and what maintains the seal where it is used only intermittently?
- What loading crosses the grating, and are the grating and its surround built for it?
- How is the gully cleared, and can the grating be lifted from where it sits?
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 size, grating class, trap arrangement, fall or spacing, and none can be inferred from it.
- Surface water drainage layout and connection to below-ground drainage are regulated matters 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 →