Gully Types and Installation: Back-Inlet, Trapped and Yard Gullies

Quick Answer: A gully is the fitting that connects surface or waste water at ground level into the underground drainage system, with a water seal (trap) to stop foul air rising back out. The main UK types are the trapped gully (water seal, for general surface and waste water), the back-inlet (BIG) gully (a side inlet that takes a waste pipe or downpipe below the grating so it can't blow back), and the yard gully (a larger, often deeper unit for surface water in yards and hardstandings). Gullies are installed under Building Regulations Approved Document H, with the underground system designed to BS EN 752. Always confirm the trap seal and whether the gully serves foul or surface water before connecting.

Summary

The gully is the unglamorous workhorse of domestic and commercial drainage — the point where water leaves the visible world (a downpipe, a kitchen waste, a yard, a patio) and enters the buried pipe network. Every gully does two jobs: it provides an entry point into the drain, and it provides a trap — a water seal that stops the smell and gases of the drain rising back up into the open air or into a building. Get the gully type, the trap, and the foul-vs-surface-water connection right and the system works silently for decades. Get them wrong and you get drain smells at the back door, downpipes that "gulp" and overflow, surface water connected illegally into the foul sewer, or a frozen, blocked gully that floods the yard.

This matters to plumbers, drainage operatives, groundworkers, paving and driveway contractors, and anyone laying or connecting external pipework. It's particularly important when you're connecting a kitchen or appliance waste, a rainwater downpipe, or a yard/patio surface-water drain, because the choice of gully and where it discharges is governed by Approved Document H and by the absolute rule that foul and surface water must not be wrongly mixed on a separate system. A waste pipe that simply spills over a grating instead of being taken below it through a back-inlet is a classic, smelly, splashy installation fault.

The common misconceptions: (1) "any gully will do" — a back-inlet gully exists precisely so a waste or downpipe discharges below the grating and water seal, preventing splash, blow-back and debris build-up on top, which an open trapped gully doesn't do as well; (2) "the trap is optional outside" — the water seal is what keeps drain smells and rats from coming up, and a dry or wrong-depth trap is a common cause of external odour; and (3) "surface water can go in the foul drain" — on a separate (two-pipe) system that's a misconnection, overloads the foul sewer, and is not permitted under Approved Document H.

Key Facts

• Gully — a fitting connecting surface/waste water into the underground drain, incorporating a trap (water seal) to block foul air; sits below a grating or sealed cover.
• Trap / water seal — the U-bend of water in the gully; a typical gully trap seal is around 50mm deep, the same principle as an internal trap, kept topped up by use or rainfall.
• Trapped gully — the general-purpose gully with a built-in water seal, used for surface water and (with a back-inlet) waste; the open grating type takes rainwater and yard run-off.
• Back-inlet gully (BIG) — has a horizontal/angled side inlet that lets a waste pipe or downpipe discharge below the grating and into the water seal, preventing splash, blow-back and leaf/debris fouling; the standard choice for connecting downpipes and appliance wastes.
• Yard gully — a larger, deeper gully (often with a sediment/silt bucket) for surface water in yards, car parks and hardstandings; the silt bucket traps grit so it doesn't enter the drain.
• Hopper / hopper-head gully — an open funnel above the gully collecting several wastes (common on older properties); prone to smell and splashing — modern practice favours sealed back-inlet connections.
• Bottle gully — a compact modern surface-water gully with a removable bottle-shaped trap insert for easy cleaning, popular on patios and driveways.
• P-trap vs S-trap / horizontal vs vertical outlet — choose the outlet orientation (horizontal "P", vertical "S", or back/side) to match the buried pipe run.
• Foul vs surface water — on a separate system the two must stay separate; a downpipe into a foul gully, or a foul waste into a surface-water gully, is a misconnection prohibited under Approved Document H.
• Gratings and access — the grating/cover must be removable for cleaning; a sealed (anti-flood / non-return) gully is used where flooding back-up is a risk.
• Bedding and surround — gullies are bedded on and surrounded with concrete (typically a concrete surround/haunching) so they stay put under load and the connections don't move.
• Levels and falls — the gully grating sits slightly below the surrounding finished level so water falls toward it; the outlet connects to the drain at the correct gradient.
• Rodding/access — the connection from gully to drain should be accessible (via an inspection chamber/rodding point) so a blockage can be cleared. (See manhole construction.)
• Frost and leaves — exposed gully gratings collect leaves and can freeze; back-inlet and silt-bucket types reduce this; regular clearing is part of maintenance.
• Approved Document H / BS EN 752 — the regulatory and design framework for the gully's connection into the foul or surface-water system.

Quick Reference Table

Detailed Guidance

Trapped gully — the general-purpose unit

The trapped gully is the everyday surface-water and yard gully: an open grating over a built-in trap. Rainwater, yard wash-down and patio run-off fall through the grating, pass through the water seal, and into the drain — the seal stopping drain air rising back. It's simple and robust, but it has a weakness for waste and downpipe connections: anything discharged onto the open grating from above splashes, can blow debris around, and lets the falling water dislodge leaves sitting on the grating. That's exactly the problem the back-inlet gully solves, which is why open trapped gullies are best kept for collecting surface run-off rather than taking a piped waste or downpipe directly onto the grating.

Back-inlet gully (BIG) — the right way to connect a pipe

A back-inlet gully has a side inlet (often called a back inlet, hence BIG) that lets a waste pipe or rainwater downpipe be piped into the gully below the grating and into the water seal. This is the correct, tidy way to connect a kitchen sink waste, an appliance waste, or a rainwater downpipe to the underground drain: the discharge goes straight into the trap, there's no splash, no blow-back of foul air, and leaves/debris on top of the grating don't interfere with the piped flow. When you see a downpipe simply discharging onto an open gully grating and splashing the wall, that's the job that should have used a back-inlet gully. Choose the inlet orientation (left, right, or rear) and the outlet (P or S) to suit the pipe runs, and bring the waste/downpipe into the inlet with the correct fall.

Yard gully — surface water under load

Yard gullies are the bigger, deeper cousins used in yards, parking areas and commercial hardstandings where there's more water, more grit, and vehicle loading. They commonly include a silt/sediment bucket that catches grit and debris before it enters the drain — the bucket lifts out for cleaning, which is essential maintenance because a full silt bucket overflows and a clogged yard gully floods the area. Because they take vehicle loads, yard gullies need a properly rated grating/cover and a robust concrete surround. They handle surface water; don't connect foul waste into a surface-water yard gully on a separate system.

Installation — bedding, levels, falls and connection

Installing a gully is mostly about getting it set firmly, at the right level, and connected correctly:

• Excavate and bed the gully on concrete, then haunch/surround it in concrete so it can't move, settle or have its connections pulled apart by ground movement or load.
• Set the grating slightly below the surrounding finished surface (paving, slab, tarmac) so water falls toward and into it rather than past it.
• Connect the outlet to the drain at the correct gradient and orientation, with flexible/correct couplings to the buried pipe — and make sure there's a rodding point or inspection chamber downstream so the run can be cleared if it blocks. (See underground drainage and manhole construction.)
• Confirm the foul/surface-water destination is correct for a separate system before you make the connection — this is the point where misconnections happen.
• Fill and top up the trap so the water seal is present from day one, and check the grating/cover is removable for future cleaning.

Foul vs surface water — the misconnection trap

On a separate (two-pipe) drainage system, foul water and surface water travel in different pipes to different destinations, and they must not be mixed. Connecting a rainwater downpipe into a foul gully overloads the foul sewer in heavy rain; connecting a foul appliance waste into a surface-water gully sends sewage to a watercourse or soakaway. Both are misconnections, both breach Approved Document H, and both are a common, costly fault found in CCTV surveys. Before connecting any gully, establish whether the system is separate or combined and which pipe the gully discharges to, and match the connection accordingly. Surface water should follow the Approved Document H discharge hierarchy (soakaway/infiltration first, then watercourse, then sewer). (See soakaway sizing.)

Frequently Asked Questions

What's the difference between a trapped gully and a back-inlet gully?

Both have a water-seal trap. A trapped gully has an open grating that water falls through from above — fine for collecting surface run-off. A back-inlet gully adds a side inlet so a waste pipe or downpipe can be piped in below the grating, straight into the trap, with no splash or blow-back. Use a trapped gully to collect yard/patio surface water; use a back-inlet gully to connect a downpipe or appliance waste properly.

Why does my outside drain/gully smell?

Almost always a problem with the water seal. If the trap has dried out (a gully that rarely gets water), lost its seal, or was installed without one, drain air rises straight out. Top up or restore the trap, check the seal depth (~50mm), and make sure waste/downpipes discharge into a back-inlet below the grating rather than splashing on top. Persistent smell with a healthy trap can indicate a defect downstream worth a CCTV survey.

Can I connect my rainwater downpipe into any gully?

Connect it properly into a back-inlet gully, where the downpipe pipes below the grating into the trap — not splashing onto an open grating. Critically, on a separate drainage system the downpipe must go to a surface-water destination (soakaway, watercourse, or surface-water sewer following the Approved Document H hierarchy), not into the foul drain. Putting rainwater into the foul system is a misconnection and overloads the sewer.

What is the silt bucket in a yard gully for?

It traps grit, leaves and sediment before they enter the underground drain, where they'd otherwise build up and cause blockages. The bucket lifts out so you can empty it — that's a routine maintenance job. A neglected silt bucket fills up, the gully overflows, and the yard floods, so on yard and commercial gullies, emptying the silt bucket is part of keeping the drainage working.

Regulations & Standards

• Approved Document H (Drainage and waste disposal) — Building Regulations requirements for foul and surface-water drainage, including gully connections, traps, and the surface-water discharge hierarchy; the foul/surface separation rule.

• BS EN 752 — drain and sewer systems outside buildings: design, performance and layout of the underground system gullies connect into.

• BS EN 12056 — gravity drainage systems inside buildings; relevant where waste pipework runs internally before reaching the gully.

• BS EN 1433 / BS EN 124 — standards relevant to drainage channels and gully tops/gratings and their load classes (vehicle vs pedestrian areas).

• The Building Regulations 2010 — the legal framework Requirement H sits within.

• Building Regulations Approved Document H surface-water hierarchy — soakaway/infiltration, then watercourse, then sewer, for rainwater discharge.

• Approved Document H: Drainage and waste disposal — GOV.UK — foul and surface-water drainage, gullies, traps and discharge hierarchy

• BS EN 752 — Drain and sewer systems outside buildings (BSI) — external drainage system design standard

• Connect Right (misconnections guidance) — preventing foul/surface-water misconnections

• BS EN 124 — Gully tops and manhole tops for vehicular and pedestrian areas (BSI) — load classes for gully gratings and covers

• underground drainage — the buried pipe system gullies connect into

• manhole construction — inspection chambers and rodding access downstream of gullies

• soakaway sizing — surface-water disposal for rainwater gullies and downpipes

• foul drainage design — foul drainage layout and where foul gullies discharge