Why Does My Toilet Keep Blocking?

A toilet that keeps blocking almost always has a cause beyond the pan. The common ones are wipes or sanitary items that never break down, a weak or badly set-up flush, a partial blockage further down the drain, and damage such as a displaced joint, root ingress, deformed pitch fibre or a section of pipe laid too flat. Plunging clears the symptom but not the cause, which is why the toilet blocks again a week or a month later. Our drainage plumbing engineers deal with the pan and soil pipe, and a CCTV drain survey finds the faults you can't see.

If you need to clear a blocked toilet right now, start with our guide on how to unblock a toilet. This guide is about why it keeps happening.

Why does my toilet keep blocking?

Because something is slowing the flow or catching the waste every time you flush. One blockage is usually down to what went into the pan. Repeat blockages, especially with careful use, usually mean the pipe itself is part of the problem.

Think of it in three zones:

1. The toilet itself: what is being flushed, how much paper, and how well the cistern and pan flush.
2. The pipework behind it: the pan connector, the branch pipe and the soil stack inside the house.
3. The underground drain: the run from the house to the inspection chambers and on to the sewer.

The further down the problem is, the less a plunger can do about it.

What are the real causes of repeat toilet blockages?

These are the seven causes we look for when a toilet keeps blocking, and how to tell them apart.

Could it be what's being flushed?

Yes, and it's the first thing to rule out. Water UK's blockages factsheet, citing its Wipes in Sewers Blockage Study (12 December 2017), says 93% of the material in the sewer blockages studied was wipes. It also says drains and sewers were only designed to carry human waste, wastewater and loo roll.

Wipes don't break up the way paper does. One or two can pass through a good drain, but they snag on any rough joint or root and then catch everything that follows. In a house with children or several people using one toilet, it's worth asking directly. Our guide to what not to flush covers the full list, including "flushable" wipes.

Can a low-flush toilet keep blocking?

It can contribute, especially when it's paired with an older drain. Under paragraph 25 of Schedule 2 to the Water Supply (Water Fittings) Regulations 1999, no flushing device fitted to a WC pan may give a single flush of more than 6 litres, and on a dual-flush cistern the smaller flush can't be more than two-thirds of the larger one.

Most modern toilets clear well on that volume. Problems come when:

• the cistern's water level has been set low, or the flush valve is worn and lets water dribble away;
• the half flush is used for everything, including paper;
• a new pan has been fitted to an old drain without checking how the two line up;
• the pan connector is too long, kinked or sags, so waste sits in the dip.

If the bowl doesn't clear fully on a full flush, or you always need to flush twice, look at the cistern before you blame the drain.

Is it a partial blockage further down the drain?

Often, yes. A drain can be partly blocked for months, with water squeezing past a build-up of paper, fat or scale, before it blocks fully. Each flush leaves a little more behind.

The test is simple. Find the nearest inspection chamber outside, lift the cover if you can do so safely, and get someone to flush the toilet. Waste should run through the channel at the bottom and away within seconds. If the chamber fills, or the flow backs up and drains slowly, the blockage is downstream of that chamber. If the toilet also gurgles when the bath or basin empties, that's another sign the shared pipe is restricted. Our guide to signs of a blocked or collapsed drain lists the other clues.

Can a damaged drain make a toilet keep blocking?

Yes, and this is the cause most often missed. A drain that is cracked, has a joint that has slipped out of line, or has roots growing through it will catch paper on every flush. Jetting clears it, but the edge or the roots are still there, so it blocks again.

The usual culprits:

• Displaced joints. Older clay drains are laid in short lengths, each with its own joint. Ground movement can push one out of line, leaving a lip that catches waste.
• Root ingress. Roots find their way into joints looking for water and spread inside the pipe. See our guide to tree roots in drains.
• Pitch fibre. Common in houses built from the 1950s to the 1970s, it can blister and go oval with age. Our guide to pitch fibre pipes explains what to look for.
• Poor fall. Approved Document H says a drain carrying WC waste should be at least 100mm in diameter, and its Table 6 recommends minimum gradients for foul drains of 1 in 40 to 1 in 80, depending on the flow. A section laid too flat, or one that has sunk, lets solids settle.

What if the drain is shared with neighbours?

Then it may not be yours to fix. Since 1 October 2011, privately owned sewers and lateral drains that connected to a public sewer on 1 July 2011 have been transferred to the sewerage companies, as Defra's private sewers transfer guidance explains. Citizens Advice says you don't have to maintain or repair lateral drains you share with your neighbour.

If your neighbours are getting blockages too, or the chamber that fills serves more than one house, report it to the water company: Severn Trent on 0800 783 4444 across most of Nottinghamshire, Derbyshire and Leicestershire, or Anglian Water across much of Lincolnshire. Our guide on who is responsible for blocked drains covers the detail.

What can you check yourself?

Work through this before you call anyone:

1. Watch a flush. Does the bowl empty fully with one full flush? Does water rise before it falls?
2. Look inside the cistern. Is the water level near the mark on the overflow or flush valve? Does it hold, or trickle into the pan?
3. Check behind the pan. Is the connector straight and firmly seated, with no damp at the base?
4. Run the other fittings. Do the basin, bath and shower drain freely, or gurgle when the toilet is flushed?
5. Lift the outside chamber. Is it clear and dry between flushes, or holding waste?
6. Ask the household what goes down the toilet, and the neighbours whether they have had problems.

If the answer points inside the bathroom, a plumber or our drainage plumbing team can sort the pan and cistern. If it points outside, the next step is a camera.

When is a CCTV drain survey the sensible next step?

When the toilet has blocked more than two or three times in a few months, when you've ruled out what's being flushed and the flush itself, or when jetting has cleared the drain but it blocks again. A camera shows exactly what is catching the waste and where, so you pay to fix the cause rather than clearing the same blockage over and over.

Our CCTV drain surveys start from £149 + VAT and come with the footage, photos, a written report and a drain map. If the survey shows the fault is in a shared or lateral drain, we tell you, so you can report it rather than pay for a repair that isn't yours. Our guide to what a CCTV drain survey report shows explains the codes and grades.

When to call a professional

Call a drainage engineer if the toilet blocks again within a few weeks of being cleared, if more than one fitting is slow or gurgling, if the outside chamber fills when you flush, or if an object has gone past the trap. Stop using the toilet if waste is coming back up into the bowl or anywhere else in the house.

Our blocked drain service starts from £120 + VAT, and drain repairs from £250 + VAT with a 12-month workmanship guarantee. There's no call-out fee and we agree the price before any work starts. We cover Nottingham, Mansfield, Derbyshire, Leicestershire and Lincolnshire, 7 days a week from 6am to 8pm, with emergency call-outs 365 days a year. Call 07300 600 950 or book online.

Sources

• Water UK: Cutting through confusion on blockages (factsheet, PDF)
• The Water Supply (Water Fittings) Regulations 1999, Schedule 2, paragraph 25
• Approved Document H: Drainage and waste disposal (gov.uk)
• Defra: guidance on the private sewers transfer regulations (PDF)
• Citizens Advice: who is responsible for repairing drains and sewers