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Blocked Drains: Causes, Fixes and When to Call a Plumber

Backed-up water in a drain inspection chamber

A blocked drain usually starts as a slow sink, a gurgle or a smell, and it rarely clears itself. Most household blockages sit in your own pipework and you can try a plunger and a few safe steps first. If the water still will not go, if more than one fixture is affected or if waste is coming back up, it is time for a professional drain clearing service. This guide covers the signs, the causes, what to try, who is responsible and what to expect when you call a blocked drain plumber.

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TL;DR

  • Slow draining, bad smells and a toilet that is hard to flush are the classic early signs of a blockage.
  • For a sink, bath or shower, start with a plunger, then a suitable unblocker, then clean the U-bend trap.
  • If cleaning the trap does not work, or several drains are affected, call a professional who can use cameras and high-pressure jetting.
  • Drains that serve only your property and sit within its boundary are normally your responsibility. Shared drains and public sewers belong to the water company.
  • Only pee, poo and toilet paper go down the toilet. Cooled fat goes in the bin, not down the sink.

Table of Contents

How to tell you have a blocked drain

Blockages build up gradually, so the early signs are easy to ignore. Thames Water says you may have a blockage if the toilet is hard to flush, the sink or bath empties very slowly, or there is a bad smell from your drains.

In practice, look out for these:

  • Slow drainage. Water lingers in the basin, bath or shower tray, then leaves with a sluggish swirl.
  • Gurgling. Air is struggling to get past a partial blockage, so you hear it in the pipes when another fixture is used.
  • Smells. Food, grease and waste trapped in the pipe start to rot and the smell comes up through the plughole.
  • Pooling water. Water collecting around an outside gully or a manhole cover is a sign the drain is not carrying flow away.
  • Several fixtures affected. If the bath gurgles when you flush the toilet, or the shower drains badly when the washing machine runs, the problem is probably further along the pipe than a single trap.

One fixture slow means the problem is likely in its own waste pipe or trap. Several fixtures slow means a shared run of pipe is partly blocked, and that is where you should stop experimenting and call someone.

What causes blocked drains

The causes are surprisingly ordinary. Drains in homes are usually no more than four inches (100mm) wide, and CCW reports that things which should not be flushed or poured down sinks and toilets account for 75% of all sewer blockages. Anything that is not pee, poo or toilet paper can catch and build up.

The usual culprits are:

  • Fat, oil and grease. They look liquid when poured away, then cool and solidify inside the pipe, trapping food scraps and forming a thick layer that narrows the bore.
  • Wet wipes. Even wipes labelled flushable do not break down like toilet paper, and they snag on anything rough inside the pipe.
  • Sanitary products and nappies. These swell and hold together, and they are among the most troublesome blockages, as Northern Ireland Water notes.
  • Hair and soap scum. In baths and showers, hair binds with soap residue and forms a plug just below the grate or in the trap.
  • Food waste and coffee grounds. They settle in kitchen pipes and bind with grease.

Older properties in London add their own factors: long runs of pipe shared between flats, awkward bends in converted buildings and ageing pipework that has roughened with years of use. These do not cause a blockage on their own, but they make one more likely to form and harder to shift.

What you can safely try first

For a blocked sink, bath or shower, a sensible order of attack avoids damage and saves a callout when the blockage is minor. Which? advises starting with a plunger, then a drain unblocker, and if the drain is still blocked, removing and cleaning the U-bend trap.

Blocked kitchen sink

  1. Remove standing water with a jug or cup so the plunger has a good seal.
  2. Block the overflow with a damp cloth and plunge firmly with steady strokes.
  3. If the water still sits there, try an unblocker made for kitchen sinks and follow the label.
  4. Place a bucket under the trap, unscrew it, empty it and clean it out. Wear gloves, because what comes out is rarely pleasant.

If the kitchen sink is the problem, our guide to a blocked kitchen sink goes through the steps and the signs that it is time for a plumber.

Blocked bath drain

For bathroom drains, Which? suggests pouring in 500ml of hot tap water, using a plunger, then applying an unblocker designed to dissolve hair and soap scum, and finishing with another 500ml of hot water. Bath drains tend to clog with hair just below the plughole, so lift the plug and clear what you can see before you start.

Steps for safely clearing a blocked bath drain

Blocked shower drain

WaterSafe Register lists several options for a shower drain:

  • Flush with hot water, but not boiling.
  • Use a plunger for about 20 seconds.
  • Use one cup of baking soda followed by one cup of vinegar, leave for an hour, then flush with hot water.
  • Lift the cover and remove the blockage by hand with gloves, using a screwdriver if the cover needs unscrewing.
  • Use a chemical drain cleaner sparingly and follow the manufacturer’s instructions.

Most shower blockages are hair and soap. Lifting the grate and pulling out the clump often fixes it in minutes. Put a hair catcher over the drain afterwards.

What not to put down a blocked drain

A few well-meant fixes make things worse. Which? advises against pouring fats, oils and grease or coffee grounds down the sink, and warns that bleach can react with vinegar and ammonia. Never mix drain products or follow one with another without rinsing thoroughly, and keep the room ventilated when you use any chemical.

A few more habits to avoid:

  • Do not keep pouring chemical unblocker into a drain that has not responded. If the pipe is fully blocked, the liquid just sits there and becomes a hazard for whoever opens the trap next.
  • Do not use boiling water on a shower drain, as WaterSafe Register recommends hot but not boiling.
  • Do not push a stiff wire or coat hanger down a pipe you cannot see. You can scratch the pipe or shove the blockage deeper.

If you have already used a chemical product, tell the plumber when they arrive so they can protect their skin and eyes when they open the trap.

When to call a blocked drain plumber

Call a professional when your own attempts have not worked or when the signs point beyond a single fixture. Which? says that if you have removed and cleaned the trap and U-bend and the drain is still blocked, it may be time to call a professional. It also notes that a drainage expert may use cameras or sonic tracking devices to find blockages deeper in the pipes, then clear them with high-pressure jets.

Call straight away if:

  • Water or waste is coming back up through a toilet, shower or outside gully.
  • More than one fixture is draining slowly or gurgling.
  • A smell persists after you have cleaned the trap.
  • The blockage keeps returning within days or weeks. A repeat blockage often means a partial obstruction, a damaged section or a build-up that plunging cannot reach.
  • You can see water pooling around an outside drain or manhole.

A proper callout should start with finding where the blockage is, then clearing it and confirming the water runs freely. Our drain unblocking service covers homes and businesses across London. For stubborn build-ups, high-pressure water is often the right tool, and we explain how that works in our guide to a drain jetting service.

If the water is spreading, call the emergency plumber line rather than waiting for the next free appointment. Water that is backing up into a property can damage floors, plaster and belongings within hours.

A note on boilers and drains

Some readers who search for drain problems are also dealing with a boiler that will not fire or switch on. That is a separate fault and not something to fix by taking pipes apart. Heating and gas appliances need a Gas Safe registered engineer, which is what our gas works team provides. If a leak is the cause of a wet patch rather than a blocked drain, our leak detection service can trace where the water is coming from.

Who is responsible for a blocked drain

Responsibility depends on where the blockage sits, and it is worth knowing before you pay for a callout.

GOV.UK states that from 1 October 2011, private sewers (drains shared between two or more properties) and lateral drains (drains serving one property but lying outside its curtilage) became the responsibility of water and sewerage companies. Where drainage serves one property only and is within its curtilage, it remains the owner’s responsibility. This applies in England.

Thames Water explains that homeowners are responsible for waste drainage pipes within their property boundary that serve just their property, while the water company owns and maintains public sewers under roads and footpaths and any sewers shared with neighbours.

Thames Water also lists signs that the blockage is in your home:

  • Your neighbours are not having problems.
  • Your property does not share a drain with others.
  • There is no other flooding in the local area.
  • Upstairs facilities are affected but downstairs ones work.
  • The drain or sewer access point is clear.

In Wales the rule is similar: Dŵr Cymru Welsh Water says pipes that only collect wastewater from your property and lie inside the boundary are your responsibility.

If flooding was caused by a public sewer, CCW says you are entitled to a refund of your annual sewerage bill, up to £1,000, and it advises checking whether your household insurance covers sewer flooding. For tenants, the landlord is usually the first call for a drain fault in the building, so report it in writing and keep the reply.

How much does it cost to unblock a drain?

CCW says that if the problem is a blockage or fault in your private drain, you will need to hire a drainage contractor, and that clearing a blocked drain can cost up to £250. Real prices vary with access, the position of the blockage, the time of day and whether a camera survey or jetting is needed.

We do not publish a price list, because every drain is different. What we do is give you a fixed price quote before any work starts, so you know the cost in advance and there are no surprises when the job is finished. You pay on completion.

When you compare quotes from any provider, ask what is included: the callout, the clearing method, a camera check afterwards and whether out-of-hours work costs extra.

Blocked drains in commercial properties

A blocked drain in a restaurant, office or shop affects trading as well as plumbing. Grease from kitchens, shared washrooms and large buildings with many users all raise the risk of blockages. Property managers often handle drains as part of wider planned and reactive maintenance, and a guide to commercial plumber services in London shows how that kind of cover is arranged across a building. For urgent clearing at a business premises, our emergency team is available around the clock.

Our view: fix the cause, not just the symptom

A blocked drain call usually begins with a customer who has already tried the plunger and a bottle of unblocker. That is a sensible start, and we say so. What changes the outcome is what happens next.

We think the job is not finished when the water goes down. It is finished when we know why it blocked. A slow shower in one flat can be hair in a trap. The same symptom in a terrace of old houses can be a build-up of fat further along the run. Treating both the same way means a second callout.

That is why we give a fixed price quote before work starts, and why our engineers explain what they found and what they cleared. You should never be surprised by a bill, and you should know what to do to keep the drain clear.

Our calls are answered 24/7, because drains do not choose convenient hours. Whether it is a blocked sink in the evening or an overflowing gully on a Sunday, our aim is a rapid response and a clear explanation. For gas appliances and boilers, our engineers are Gas Safe registered.

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Need a blocked drain cleared in London?

We clear blocked drains for homeowners, landlords and businesses across London, day or night. Every job is quoted at a fixed price before work begins, and you pay when it is complete. Book through our drain unblocking page or call out the team through the emergency plumber service if water is already backing up.

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We cover areas including Battersea, Notting Hill, Kensington, Chelsea, Acton, Earl’s Court and Wandsworth.

How to stop drains blocking again

Prevention is simple and costs nothing. Thames Water advises remembering the three Ps: pee, poo and toilet paper, and nothing else down the toilet.

In daily terms:

  • Wrap sanitary items, nappies, wipes and condoms in toilet paper and put them in the bin.
  • Let leftover cooking oil, fat and food cool, then put it in the bin.
  • Take chemicals, solvents, engine oil and paint to a local refuse or recycling site, never down a drain.
  • Scrape plates into the food waste or bin before washing up.
  • Fit a hair catcher in the shower and bath, and empty it after each use.
  • Run hot water down the kitchen sink after washing greasy pans.

Severn Trent Water gives the same advice: wait for fats, oils and greases to cool before binning them, and flush only pee, paper and poo. Its figures also remind us that sewer pipes are narrow, around 150mm, so a small habit repeated across a street adds up.

Landlords can help tenants by putting a short note by the bathroom and kitchen explaining what must not go down the drain. It is one of the cheapest ways to cut repeat callouts.

FAQ

What can I put down a drain to unblock it?

Start with hot water, not boiling for showers, and a plunger. A shop-bought unblocker suited to the fixture is the next step, followed by cleaning the U-bend trap by hand. Baking soda followed by vinegar, left for an hour, is another option for a shower drain. Never mix products, and avoid bleach with vinegar or ammonia.

Why is my drain blocked even though I only use the sink normally?

Grease, soap scum and food scraps build up slowly in a pipe and narrow it, even with careful use. Hair does the same in baths and showers. The blockage is often in the trap or just beyond it, so cleaning the U-bend is a good first check.

Am I liable for a blocked sewer pipe?

If the pipe serves only your property and sits within its boundary, it is generally your responsibility. Shared pipes and public sewers are the water company’s. Check the pipe layout and your water company’s guidance, and speak to your landlord if you rent.

How much does it cost to unblock a drain?

CCW says clearing a blocked drain can cost up to £250, though the price depends on the access and the method. We give a fixed price quote before starting any work, so you know the cost in advance.

When should I call a drain clearing service?

Call when cleaning the trap has not worked, when several fixtures are slow, when waste is coming back up or when the blockage keeps returning. A professional can use cameras to locate the problem and high-pressure jets to clear it.

Sources

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