What’s Causing This?

Hair and soap scum accumulation — The most frequent cause of blocked bath and shower drains. Hair binds with soap residue to form a dense plug inside the trap or waste pipe. See our guide to unblock a bath drain for step-by-step removal.

Grease and food debris in kitchen drains — Cooking fat poured down the sink solidifies as it cools, trapping food particles and gradually narrowing the pipe. This is the leading cause of blocked kitchen sinks. Our guide to unblock a kitchen sink walks you through clearing it safely.

Foreign objects or wet wipes in the toilet — Non-flushable items including wet wipes, cotton pads, and sanitary products are a common cause of toilet blockages. Follow our guide to unblock a toilet before calling a plumber.

Leaves, silt, and debris in outside drains — External drain covers accumulate fallen leaves, soil, and grit, particularly in autumn. Blockages here can cause water to back up into the property. Read our full guide to unblock an outside drain.

Collapsed or misaligned underground pipework — Older clay pipes can crack, sag, or become offset at joints, causing recurring blockages that cannot be cleared by hand. This requires professional CCTV survey and repair — see the “When to Call a Tradesman” section below.

How to Diagnose the Exact Cause

1. Identify which drain is blocked. Test every sink, bath, shower, and toilet in the property. If only one fixture is slow or blocked, the obstruction is local to that appliance’s trap or waste pipe. If multiple fixtures on the same floor are affected, the blockage is likely in a shared branch drain.
2. Check all ground-floor fixtures and the outside drain. If ground-floor toilets and sinks are backing up while upper floors drain freely, the main soil stack or underground drain is likely blocked. Lift the nearest external inspection chamber cover — if it is full of water, the blockage is downstream of that point.
3. Look inside the trap. For sinks and basins, remove the trap (the U-shaped pipe beneath the plughole). If it is packed with debris, the trap itself is the blockage. If it is clear, the obstruction is further along the waste pipe.
4. Inspect the plughole for visible debris. For baths and showers, remove the drain cover and use a torch. A compacted mat of hair is often visible just below the surface and can be hooked out directly.
5. Check the toilet pan for a slow-drain pattern. If water rises high before draining slowly, the blockage is in the toilet trap or the soil pipe close to it. If water fills the pan and does not drain at all, the obstruction is complete and closer to the pan exit.
6. Rule out a collapsed pipe. If the blockage recurs within days of being cleared, or if you notice ground subsidence near drain runs, suspect a structural pipe fault. A drainage contractor can confirm this with a CCTV inspection.

How to Fix It

Hair and soap scum in bath or shower drain: Remove the drain cover, pull out the hair plug with a drain hook or bent wire, then flush with hot water. For stubborn build-up, a bicarbonate of soda and vinegar treatment can help. Full method: Unblock a Bath Drain.

Grease blockage in kitchen sink: Use a cup plunger first. If ineffective, remove and clean the trap, then use a drain snake to clear further down the waste pipe. Avoid caustic chemical drain cleaners on a regular basis — they can damage older pipework. Full method: Unblock a Kitchen Sink.

Toilet blocked by foreign object: Use a toilet plunger (flange type) with firm, steady strokes. A toilet auger (closet snake) can dislodge objects the plunger cannot reach. Never use caustic chemicals in a toilet. Full method: Unblock a Toilet.

Blocked outside drain: Lift the inspection chamber cover, remove standing debris with gloves, then use a set of drain rods to break up and push the blockage to the next chamber. Hose down the chamber once clear. Full method: Unblock an Outside Drain.

Collapsed or cracked underground pipe: This cannot be fixed by DIY. A drainage contractor must carry out a CCTV drain survey to locate the fault and determine whether lining or excavation is needed. See the “When to Call a Tradesman” section.

When to Call a Tradesman

Call a qualified drainage contractor or plumber if: sewage is backing up into the property through multiple fixtures simultaneously, as this indicates a main drain failure that is a health hazard under the Public Health Act 1936; the outside inspection chamber is overflowing and rodding has not cleared it; you suspect a collapsed, cracked, or root-invaded underground pipe; or the blockage has recurred more than twice in a short period despite being cleared. In England and Wales, homeowners are responsible for drains within their property boundary, while shared sewers are the responsibility of the water company — check with your water supplier if the blockage appears to be in the shared sewer (gov.uk and Ofwat publish guidance on this boundary). Always use a drainage contractor registered with the National Association of Drainage Contractors (NADC) or a plumber on a recognised trade register. Working inside a confined inspection chamber without appropriate safety equipment is dangerous — the HSE classifies deep inspection chambers as confined spaces requiring specialist precautions.

How to Prevent It Happening Again

• Fit a hair-catcher or fine mesh drain cover over bath and shower plugholes and clean it after every use — this alone prevents the majority of bathroom drain blockages.
• Never pour cooking fat or oil down the kitchen sink; collect it in a container and dispose of it with general waste or at a local recycling centre. Wipe greasy pans with kitchen paper before washing.
• Only flush the three Ps — pee, poo, and paper — down the toilet. Wet wipes, cotton wool, and sanitary products must go in the bin, even if packaging claims they are flushable.
• Clear leaf litter from external drain covers and gullies each autumn, and flush outside gullies with a bucket of hot water monthly to prevent grease and silt from setting. For ongoing maintenance advice, see our guide to cleaning drains without chemicals.