Gully
/ ˈɡʌli /
Also known as: yard gully, trapped gully, back-inlet gully, surface water gully
Definition
A gully is a drainage inlet fitting set into the ground that collects surface water or external appliance waste and discharges it to the underground drain. It incorporates a water-filled trap to prevent foul air rising back through the open grating. Common types: the yard gully (open-top with a removable grating, collects surface runoff); the back-inlet gully (BIG, with a side pipe connection for appliance waste above the trap level). In a separate drainage system (now standard for new developments), surface water gullies must connect only to the surface water drain - connecting foul appliances to a surface water drain is a criminal offence under the Water Industry Act.
In practice
Gullies require periodic maintenance - the trap can become blocked with silt, leaves, and debris, and the grating can become clogged, causing surface water to back up. Lifting the gully grating and clearing the trap with a gloved hand or a small drainage rod is straightforward but often neglected. A gully that smells of drains even when recently cleaned suggests the trap has dried out (evaporated) - pouring a bucket of water into the gully refills the trap. For a gully that has been dry for a long time, adding a small amount of cooking oil on top of the water helps to slow evaporation in warm weather.
When a downpipe terminates above a yard gully (the shoe of the downpipe discharges into the open top of the gully), the gap between the pipe and the grating should be screened or hooded to prevent leaves and debris from bypassing the grating and blocking the underground drain. Modern practice for new installations connects the downpipe directly to the underground pipe via a sealed connection, bypassing the open yard gully altogether - this eliminates the open water surface that can freeze and cause splash-back issues.