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Cross-market term

Gullies and surface water inlets

A trapped opening at ground level where water enters a drain — from a downpipe, from a waste pipe, or from the surface — with a water seal that stops drain air coming back out of it.
  • 4Markets
  • NoSame thing everywhere
  • 3Calculators

What it is called, by market

One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.

  • United Kingdom

    Close, not identical

    gully

    also gulley, back inlet gully, yard gully, road gully, trapped gully

    One word covering several fittings. A BACK INLET gully takes a waste or rainwater pipe below the grating; a road gully is a much larger chambered unit with a silt bucket.

  • Australia

    Close, not identical

    gully

    also gully trap, surface inlet pit, overflow relief gully

    The overflow relief gully is an Australian particularity with no British equivalent: a deliberately low, easily lifted grating outside the house so that a blocked drain surcharges into the garden rather than through a shower waste.

  • United Statesyours

    Same word, different thing

    area drain

    also catch basin, yard drain, trench drain, floor drain

    A CATCH BASIN has a sump BELOW the outlet to trap grit, which an ordinary British gully does not — so the two are not interchangeable, and a catch basin requires periodic emptying that a gully does not.

  • Canada

    Same word, different thing

    catch basin

    also area drain, yard drain, sump

    As the United States, with frost depth and grit from winter salting making the sump a practical necessity rather than an option.

The governing standard, by market

  • United Kingdom

    Approved Document H / BS EN 1253

    Foul and surface water drainage, and the classification and testing of gullies for buildings.

  • United States

    IPC Chapter 11 / ASTM C913

    Storm drainage and area drain requirements, and precast concrete water and wastewater structures.

Calculators for this

Each works in either measurement system, and the terminology on the page follows whichever market you have selected.

Sump Pit & Catch Basin Concrete Calculator →Drain Pipe Slope Calculator →Storm Drain Pipe Slope Calculator →

Frequently asked questions

What is a back inlet gully, and why does it matter which one is fitted?
It is a gully where the incoming pipe discharges BELOW the grating, straight into the trapped chamber, rather than spilling onto the grating from above. The difference is entirely practical. A pipe discharging over an open grating is exposed to leaves and debris landing on the same grating, it splashes, and in a hard frost the flow can ice over the grating and back up. A back inlet takes the pipe under the grating so the discharge is enclosed, which is why it is normally required for a waste pipe from a kitchen or a washing machine and is the better arrangement for a rainwater downpipe. Substituting a plain gully for a specified back inlet one looks like the same item on an invoice and is a different fitting doing a worse job.
Is a catch basin the same as a gully?
Close in position and different in design, and the difference is maintenance. Both sit at ground level and take surface water into a drain. A CATCH BASIN has a sump — a chamber extending below the level of the outgoing pipe — so grit, sand and leaf litter settle out and stay there instead of travelling into the system. That is valuable where road grit or winter salting loads the runoff, and it is a liability if nobody ever empties it, because a full sump stops catching anything and eventually stagnates. A typical British domestic gully has no such sump: the trap holds a water seal and the flow carries debris through. So a drawing calling for catch basins is calling for a structure with a maintenance schedule, not just a grating.
Why does a gully smell, and why is that not usually the gully's fault?
Because the water seal has gone, and the commonest reason is that nothing has run into it. A gully's trap holds a small volume of water against drain air, and that water evaporates — so a gully serving a downpipe in a long dry spell, or one serving a waste pipe from an appliance nobody uses, loses its seal in weeks and becomes an open hole into the drain. The fix is to pour water into it, and if it smells again within days the problem is different: either the trap is damaged, or the drain itself is surcharging and pushing air up. The second diagnostic is whether nearby gullies smell too. One gully is its own trap; several at once is the system, and that is a blockage rather than a seal.