Toilet Keeps Blocking? Why It Happens and How to Stop It

A toilet that keeps blocking has an underlying fault, and plunging it only clears the symptom. There are three usual causes: a flush that isn't strong enough to carry waste through the pan and pipe, something lodged in the trap that paper catches on, or a problem in the drain beyond the toilet. If only the toilet is affected, the cause is usually the flush or the pan. If the bath, shower or ground floor sinks also drain slowly or gurgle, the fault is further down, and a drain survey is the quickest way to find it.

Why a toilet keeps blocking

A toilet is designed to clear its contents in one flush. The Water Supply (Water Fittings) Regulations 1999 require every WC pan to be made and installed so that after normal use its contents can be cleared effectively by a single flush, and they limit new flushes to a maximum of 6 litres. That 6-litre limit has applied to new toilets since 2001, so modern pans rely on the shape of the pan and the speed of the flush rather than a large volume of water.

When one part is off, such as a cistern that doesn't fill fully, a trap narrowed by scale or a drain with a dip in it, waste gets part of the way and stops. Each flush adds a little more until the toilet blocks again.

Toilet or drain? How to tell

The single most useful question is whether anything else in the house is affected. Thames Water's own guidance makes the same point: if upstairs fittings are affected but downstairs works, the blockage is likely inside the home.

What you notice	Most likely cause
Only the toilet blocks; bath, basin and sinks drain normally	Weak flush, or something stuck in the pan or trap
Water rises high in the pan then slowly drops away	Partial blockage in the pan outlet or the pipe just beyond it
Toilet blocks and the bath or shower gurgles when it's flushed	Partial blockage or venting problem in the soil pipe
Ground floor toilet blocks, upstairs toilet causes it to rise	Blockage in the soil stack or the drain below it
Outside inspection chamber full or slow to empty	Blockage or fault in the underground drain
Neighbours on the same run having problems	Shared drain or sewer, often the water company's responsibility

If you have a macerator toilet (a small box behind the pan that pumps waste away), the causes are different. See our guide to macerator toilet problems.

A flush that's too weak

This is the most common reason a toilet keeps blocking when nothing else in the house is affected. Signs of a weak flush include a swirl that doesn't really pull waste away, paper left in the pan after flushing, or needing two flushes to clear it.

• The cistern isn't filling to the line. Every flushing cistern should be marked inside with a line showing the intended water level. If the fill valve has been set low, or has drifted, the cistern holds less water than the pan needs. Lift the lid and check. Most modern fill valves have an adjusting screw or clip to raise the level.
• The flush valve is leaking. If the seal on a push-button flush valve is worn, water trickles into the pan all the time and the cistern may never refill properly. You may also notice the water level in the bowl sitting low. Our guides to a low toilet bowl water level and toilet cistern problems cover this.
• A button that only half works. On dual-flush toilets, a sticking button or a stretched cable can mean you only ever get the small flush. See our guide to a toilet flush button that isn't working.
• A mismatched pan and cistern. If an older, larger-flush pan has been paired with a modern low-flush cistern, or a cheap replacement cistern fitted to a pan it wasn't designed for, the pan may never clear properly. This usually needs a matched pan and cistern.

Something stuck in the pan or trap

If the flush looks strong but the toilet still keeps blocking, something may be partly blocking the trap, the bend at the bottom of the pan. Paper and waste catch on it and build up.

• Objects. Toys, toothbrushes, air freshener clips, cotton buds and bottle tops can lodge in the trap and stay there for months. A plunger often moves the paper but leaves the object in place.
• Wipes and sanitary items. Thames Water warns that wet wipes, even those labelled flushable, block pipes. They don't break down like toilet paper and can snag in the trap or the pipe beyond.
• Limescale. London's hard water leaves scale in the trap and under the rim over the years. Heavy build-up narrows the outlet and roughens the surface so paper catches.
• The pan connector. The short pipe joining the back of the pan to the soil pipe can sag, kink or be fitted with too little fall, particularly after a new bathroom. Waste then sits in it.

A plumber can check the trap with a toilet auger or a camera, or take the pan off to clear it properly if needed. Our step-by-step guide on how to unblock a toilet covers the safe DIY methods first.

Problems in the drain

When the toilet keeps blocking and other fittings are slow too, the fault is usually past the toilet, in the soil pipe or the underground drain. Common causes include:

• A dip or sag in the pipe where the ground has settled, so waste sits in the low point.
• A displaced or cracked joint that catches paper.
• Tree roots getting in through joints. See our guide to tree root damage in London drains.
• Grease and scale building up in a drain shared with the kitchen.
• A long, shallow run from the toilet to the drain, which a low flush struggles to carry waste along. Approved Document H sets out the falls and sizes drains should be laid to, and older or altered pipework doesn't always meet them.

These don't go away with plunging or chemicals. Rodding or jetting clears the blockage, and a CCTV drain survey shows why it keeps happening, so the repair can fix the cause rather than the symptom.

What you can check yourself

1. Check the water line in the cistern. Lift the lid, let the cistern fill and see whether the water reaches the marked line. If not, adjust the fill valve.
2. Watch the flush. Look for a strong, fast rush of water. A slow trickle or a flush that stops early points to the flush valve or button.
3. Run the other fittings. Fill and empty the bath or run the shower and basin, then see whether the toilet water level rises or anything gurgles.
4. Lift the inspection chamber cover outside, if you have one and it's safe to do so. If it's full of water or waste, the blockage is in the drain beyond it. If it's clear and flowing, the problem is between the toilet and that chamber.
5. Ask the neighbours if you share a drain. If they're having problems too, contact Thames Water, as a shared drain or public sewer is usually theirs to clear.
6. Change what goes down. Only pee, poo and toilet paper, as Thames Water puts it. Bin wipes, cotton buds and sanitary items.

Avoid caustic drain cleaners. They rarely shift a physical obstruction and make the pan unsafe to work on.

When to call a plumber

Call a plumber if the toilet blocks more than once in a few weeks, if you've adjusted the cistern and the problem continues, if other fittings drain slowly or gurgle, if water rises in a ground floor toilet when upstairs fittings are used, or if the blockages started after a new bathroom was fitted. For advice on the signs that a single blockage needs a professional, see blocked toilet: when to call a plumber.

We charge £95 an hour for labour, plus a £60 call-out that's waived when the job runs over 2 hours. Parts are extra and we confirm the price before any work starts. Our blocked toilet, toilet repair and CCTV drain survey services cover homes and flats across London, and the pricing page explains how jobs are charged.

Frequently Asked Questions