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Blocked Outside Gully
The grated drain outside the kitchen or bathroom wall is full and slow, and water pools around it.
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What's happening?
A gully is the trapped, grated opening in the ground where waste pipes from a kitchen sink, a washing machine or a bathroom discharge into the underground drain — and in many houses it also takes a rainwater downpipe. Because it is open to the air it catches everything: leaves, grit, moss off the roof, and the fat that comes out of a kitchen sink and cools the instant it hits cold water. That combination silts the trap and then the outlet, so waste backs up and stands in the gully instead of flowing away. It is one of the most common drainage faults on a UK house and one of the most preventable, and it is also frequently misdiagnosed as a blocked kitchen sink when the sink itself is perfectly clear.
What usually causes it?
- Fat, oil and grease from the kitchen sink solidifying in the gully trap
- Leaves, moss and roof grit washed in through an open or broken grating
- A missing gully grating, so everything from the garden goes straight in
- A waste pipe discharging above the grating rather than below it, so debris is not carried through
- Silt built up in the trap over years with no cleaning
- A blocked outlet downstream of the gully, so it fills from below
- Roots or a defect in the short length of pipe between the gully and the first chamber
Is it dangerous?
Moderate. Standing foul water outside a kitchen wall is a hygiene issue and attracts flies and rodents; grease-slicked paving is a slip hazard. The bigger practical risk is water finding its way where it should not — a gully that surcharges next to a wall can push water under a door threshold or against a damp-proof course, and a gully shared with a rainwater downpipe will overflow across the ground every time it rains hard.
Can I fix it myself?
Yes, and this is one of the more satisfying jobs on the list. Lift the grating and scoop the debris out with a gloved hand or a trowel into a bag — not into the garden and not back into the drain. Reach down into the trap and clear the silt and grease from the bottom. Flush it with a bucket of hot water and washing-up liquid, or a proper drain-safe degreaser, and repeat until it runs away freely. Replace a broken or missing grating. If it fills again as fast as you empty it, the blockage is downstream and rodding or jetting is needed.
When should you call a drainage engineer?
Call a drainage engineer if the gully refills from below after you have cleared it, if it backs up when the washing machine runs, or if clearing it makes no difference to a slow kitchen sink. Also call if the gully is cracked, has lost its trap, or the surrounding ground has sunk, since that points at a defect in the pipe underneath rather than at debris.
What will the drainage engineer actually do?
- Clear and de-silt the gully and its trap and flush it under pressure
- Rod or jet the outlet from the gully to the first inspection chamber
- Check the discharge pipes terminate correctly below the grating
- CCTV the short run where the gully refills from below, to look for a defect or root ingress
- Replace a cracked gully, a missing grating or a failed trap
- Advise on separating a rainwater downpipe where a shared gully cannot cope with storm flow
Frequently asked questions
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