Back Inlet Gully vs Hopper Head: Where Each is Used

Summary

Choosing between a back inlet gully and a hopper head is one of the most common drainage queries on extensions, kitchen relocations and shower-room work. Get it wrong and the Building Control inspector will reject the connection — the rule is well-established but easily missed because old housing stock is full of hopper-head waste connections that were acceptable in their day but no longer comply.

The fundamental rule: foul waste must be trapped, sealed and not exposed to open air at the discharge point. A back inlet gully meets this rule because the waste pipe enters below the grating and discharges into a trap. A hopper head fails this rule because waste discharges into open air, allowing smells and insects to escape (the original purpose of the trap), and is therefore not permitted for foul connections in new work.

Hopper heads remain entirely valid for rainwater — that's what they were designed for. The confusion arises when an installer plumbs a basin or shower into an existing hopper head because it's the easiest connection; this used to be tolerated, but today is a Building Regs failure.

Key Facts

• Back inlet gully (BIG) — a gully with a 110mm or 150mm side inlet (waste connection) and a top grating, trapped below ground.
• Front inlet gully — gully with a top grating only, no side inlet. Used purely for surface water at ground level.
• Hopper head — open funnel at the top of a downpipe, untrapped, used for rainwater from gutters or as a junction point for upper-storey waste.
• Trap seal depth — gullies must have a minimum 50mm water seal per BS EN 1253 to prevent foul-air ingress.
• Approved Document Part H1 — Foul water drainage. Sets requirements for branch discharge pipes, traps, and ventilation.
• AD Part H, paragraph 1.18 — discharge of waste branch into a gully must be below the grating and above the water level in the trap.
• Hopper heads for waste — implicitly disallowed since the 1990s; modern Part H requires trapped connection.
• Waste pipe diameters — 32mm (basin), 40mm (sink, shower, washing machine), 50mm (kitchen sink with disposal). All must be trapped.
• Trap types — bottle, P-trap, S-trap, running trap. All must provide 50mm minimum seal.
• BIG materials — typically PVC, vitrified clay or cast iron. PVC dominates modern work.
• BIG depths — measure invert depth at the chamber, then size BIG to drain at minimum 1:80 gradient to chamber.
• Building Control inspection — connections are checked when the first-fix drainage is open. Don't backfill before inspection or you'll be digging it up.
• Rainwater hopper modern use — still valid as a junction at first-floor level where two rainwater downpipes combine before continuing to ground.
• Combined drainage — most older UK properties have combined foul/surface water; new build areas typically have separate systems. Check the property's system before connecting anything.

Quick Reference Table — When to Use Which

Detailed Guidance

Back inlet gully construction and depth

A back inlet gully has three connections:

PLAN VIEW — BACK INLET GULLY
============================

                    [GRATING] (top)
                       │
                       ▼
                    ┌──────┐
   SIDE INLET ──→  ─┤      ├
   (waste pipe)     │ TRAP │
                    └──┬───┘
                       │
                       ▼
                  OUTLET to foul drain
                  (110mm, falls to chamber)

Key dimensions for a domestic install:

• Side inlet — typically 50mm or 110mm depending on application (32mm/40mm waste pipes need to enter via a reducer)
• Trap seal — 50mm minimum (BS EN 1253)
• Outlet diameter — 110mm standard, falls at 1:40 to 1:80 to chamber
• Set above paving — grating should sit 5-10mm above adjacent paving so surface water runs in, not out
• Surrounding paving — slope toward grating with a 1:80 fall

Installation sequence

For a typical extension drainage with a new BIG:

1. Excavate trench from new chamber to BIG location at 1:40 fall.
2. Bed 110mm PVC pipe on pea gravel.
3. Install BIG, ensuring grating at correct level (set above future paving level).
4. Connect waste pipe from inside building into side inlet. Use a deep-seal trap inside if total run is over 3m.
5. Pressure-test the foul system before backfilling.
6. Notify Building Control for inspection if extension or new connection (Section 106 of WIA 1991 may also apply for connection to public sewer).
7. Backfill with pea gravel to 100mm above pipe crown, then granular fill.
8. Concrete or paving over.

Why hopper heads are no longer acceptable for waste

Three failure modes in old hopper-head waste installations:

1. Foul air escape — open hopper releases sewage smells into the air, particularly in summer.
2. Insect/rodent ingress — open hopper provides entry to drains.
3. Splash and overflow — waste discharging into a hopper can splash onto walls; in winter the splash can freeze and block the downpipe.

Pre-1990s housing stock often has bathroom waste from a first-floor basin discharging into a hopper at the eaves; this connects directly into a soil and vent pipe (SVP) below. While these can usually be left in place under a "like-for-like" repair, any modification triggers an upgrade to current standards. So if a homeowner replaces the basin, the installer must reroute waste through a BIG or trapped internal connection — not back into the hopper.

Sizing for waste from multiple appliances

When a single BIG receives waste from more than one appliance:

For more than 3 fittings on a single BIG, install two BIGs or route via a chamber with a 110mm branch.

Hopper heads — where they remain valid

Hopper heads are still appropriate for:

• Gutter-to-downpipe transitions at eaves level
• Combining two rainwater downpipes into one
• Overflow from a flat roof outlet where the overflow point is well above the level of any habitable opening
• Some balcony drainage designs

Always confirm the hopper discharges to a surface water drain or soakaway, not to the foul drain (where the property has a separate system — older combined systems accept both).

Condensate from boilers

A condensing boiler condensate pipe routed externally must:

• Be insulated against freezing (BS EN 14336 / industry guidance from the Heating and Hotwater Industry Council)
• Discharge into a trapped connection — BIG with an air gap is acceptable
• Use 32mm or 40mm pipework rather than the manufacturer's standard 22mm where external runs exceed 3m
• Include a condensate neutraliser where local drainage requires (some areas/private treatment plants)
• Never discharge into a hopper

A frozen condensate is the single most common winter boiler callout. Using a BIG with a properly designed trap and insulated external run eliminates 90% of these failures.

Frequently Asked Questions

Can I connect a washing machine waste to an outside hopper if there's nowhere else?

No — not for new work or modifications. Approved Document Part H requires foul waste to be discharged into a trapped connection (gully or trapped branch). A hopper has no trap and discharges into an open downpipe. Common options when a BIG seems impossible: install an internal trapped connection to the existing SVP, or fit a new BIG into the existing drain run with a short trench. Either is acceptable; the hopper option is not.

Do I need Building Control approval to install a new BIG?

If you're installing as part of a notifiable project (extension, conversion, new appliance connection involving drain modification), Building Control will inspect the drainage as part of the project sign-off. For a like-for-like replacement of an existing BIG, no notification is normally required, but the work must still comply with Approved Document Part H. If in doubt, submit a Building Notice.

Can a BIG be installed inside a property?

Yes, but it's uncommon. Internal BIGs are sometimes specified in kitchens and utility rooms in new builds where the slab construction permits. The grating must be sealed against splashing and odour — easier and cheaper to fit a trapped connection internally (e.g. a P-trap on a basin, S-trap on a kitchen sink) and run the waste underground to an external chamber.

What's the minimum water seal depth required?

50mm per BS EN 1253. Modern UK practice favours 75mm seals where pipe pressure changes could siphon the trap (long branch runs, multi-storey systems). A 50mm seal can be siphoned by a single washing-machine pump-out if the system isn't vented; 75mm is the safe default.

How do I deal with an existing hopper-head waste connection during a kitchen refit?

Treat it as a fault that needs correcting under "modification triggers upgrade". The cleanest fix: cut the waste pipe back, install a new BIG at ground level, connect the waste run from inside the building to the BIG side inlet, and cap or remove the old hopper-head connection. Cost typically £200–£450 in materials plus 0.5-1 day of labour. Add it as a separate line item on the quote with a clear explanation — most customers accept it once they understand it's a regs failure.

Regulations & Standards

• Building Regulations Approved Document Part H1 — Foul water drainage. Sets connection, trap and ventilation rules.

• Building Regulations Approved Document Part H3 — Rainwater drainage. Hopper heads sit under H3 for rainwater use.

• BS EN 1253-1:2015 — Gullies for buildings — design requirements. Trap seal depths.

• BS EN 752:2017 — Drain and sewer systems outside buildings.

• BS EN 12056-2:2000 — Gravity drainage systems inside buildings. Sanitary pipework, layout and calculation.

• Water Industry Act 1991 — Section 106 governs connection to public sewers.

• Heating and Hotwater Industry Council (HHIC) — Condensate Disposal guidance — boiler condensate routing best practice.

• Approved Document H — Drainage and waste disposal (gov.uk)

• Building Regulations 2010 — Schedule 1 Part H (legislation.gov.uk)

• HHIC — Condensate Disposal — industry guidance for condensing boilers

• BSI — BS EN 12056 series — drainage standards

• Wavin / Polypipe technical guides — manufacturer guidance on BIG installation

• drainage cctv surveys — surveying older waste connections

• inspection chambers and manholes — chamber design and depth

• foul drainage repair — common remediation approaches

• hot water storage — related hot water system drainage