How do I clear a blocked outside drain?

Clearing a drain or gully that is backing up outside, and knowing when it becomes the water company's problem.

Difficulty: intermediate
Time: 1 hr 30 min
Read: 2 min
Safety: warning

Short answer

Lift the drain cover and look. If the chamber nearest the house is full but the next one is empty, the blockage is between them and drain rods will clear it. If every chamber is full, the blockage is further down the shared sewer and that is usually the water company's responsibility.

An outside drain backing up is unpleasant but usually simple to locate: you check the inspection chambers in sequence and find the first one that is full. That tells you exactly which stretch of pipe is blocked and whether it is yours to clear.

Safety

Drain water carries bacteria. Wear waterproof gloves, avoid splashing, keep it away from your face, and wash thoroughly afterwards. Never enter a drain chamber. Replace covers securely — an open drain is a serious fall hazard, especially for children. If sewage is backing up into the house, contact your water company immediately.

What you'll need

Drain rods
Waterproof gloves
Bucket
Hose
Drain cover lifting keys (optional)
Eye protection

Step by step

Find and lift the inspection chamber covers, starting nearest the house.Lifting keys make this far easier. Some covers are heavy — get help.
Check which chambers are full and which are empty.The blockage is between the last full chamber and the first empty one. This is the whole diagnosis.
If every chamber including the last is full, stop and call your water company.The blockage is in the shared sewer, which is usually their responsibility and free to clear.
Assemble drain rods, screwing each section on tightly and turning clockwise only.Turning anticlockwise unscrews the rods and leaves a section stuck in the drain.
Feed the rods into the pipe in the direction of the blockage, pushing and turning.Use a plunger or corkscrew head. Steady pressure with rotation, not brute force.
Work at the blockage until the water level drops suddenly.It usually goes all at once. Stand clear of the chamber when it does.
Flush the whole run through with a hose for several minutes.It carries the loosened debris away rather than letting it re-settle downstream.
Clean the rods and everything else thoroughly, and replace the covers securely.An unsecured drain cover is a genuine hazard.

Tips

Always turn drain rods clockwise. Anticlockwise unscrews them and a lost rod section makes the problem considerably worse.
Fat, wet wipes and nappies cause the majority of domestic drain blockages. None of them should go down a toilet or sink.
If drains block repeatedly in the same place, there may be a damaged or root-invaded pipe. A drainage survey with a camera is worth the cost at that point.

Common mistakes

Turning rods anticlockwise — They unscrew and you leave a section stuck in the drain — a much bigger problem than the original blockage.
Rodding before checking which chambers are full — You may be working on a clear stretch while the real blockage is elsewhere, or on a shared sewer that is not yours to clear.
Not wearing protection — Drain water carries bacteria and it splashes when the blockage clears.

Questions people ask

Whose responsibility is a blocked drain?

Broadly, pipework within your property boundary serving only your property is yours; shared sewers and pipes beyond the boundary are usually the water company's. Checking which chambers are full tells you which side of that line the blockage is on.

What causes most outside drain blockages?

Fat poured down sinks, wet wipes, and tree roots entering damaged joints. The first two are avoidable; the third needs a camera survey to confirm.