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What are the pipes under the sink for?

Under a kitchen sink you will usually find hot and cold supply pipes with small shut-off valves, a trap and waste pipe carrying water to the drain, and sometimes appliance connections. In Cheshire, learn which valve is which before a leak starts, and call a plumber for anything you cannot tighten safely.

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By the QuoteZila editorial team. Editorial policy.

What should you find under a kitchen sink?

Open the cupboard under a kitchen sink in Cheshire and you will usually see the same family of pipes. Whether your home is in Crewe, Nantwich, Sandbach or further afield, knowing which pipe does what saves panic when something drips. Anyone in the UK can post or register on QuoteZila, and this page is a simple guide to what is there.

From the top down, you will usually see:

  • Hot and cold supply pipes, rising to the tap. They may be rigid copper or plastic, with flexible braided hoses for the last bit.
  • Isolation valves on those supplies, small valves you can turn with a screwdriver or a little handle.
  • The waste, a plastic fitting under the plughole, often with a separate overflow pipe from a slot near the top of the bowl.
  • The trap, a U-bend or a round bottle shape that holds water.
  • The waste pipe, running from the trap through the wall or floor to the drain.
  • Appliance connections, such as a valve for a washing machine or dishwasher, and a spigot on the trap for its drain hose.

What do the isolation valves do?

The small valves on the hot and cold pipes let you turn off the water to the tap without shutting off the whole house. Find them now, before you need them. A slotted valve is usually off when the slot is across the pipe and on when it is in line with it. A valve with a handle works the same way.

Valves that have not been touched for years can stick. Turn them gently. If one will not move, or starts to weep when you turn it, stop and use the main stop tap instead. A plumber can replace a stiff valve in a short visit.

If you cannot find any valves under the sink, the tap may be fed straight from the main pipes. That is a good reason to have valves fitted the next time a plumber is there.

What do the trap and waste pipe do?

The trap holds a small pool of water in its bend. That water seal stops smells and gases from the drain coming back into the kitchen. It also catches some debris before it reaches the pipe in the wall.

The waste pipe carries water from the trap to the drain or the soil stack. It needs a steady fall to work. If the pipe sags, water and grease can sit in the low point and build up a blockage.

If the sink drains slowly or gurgles, the trap is the first place to look. Our blocked sink guide goes through clearing it. Put a bowl underneath before you unscrew anything, and keep the washers in order.

What should you do about a leak?

First, find where it starts. Dry every joint with a cloth, run the tap, and watch. Then:

  • A drip at a trap joint. Hand-tighten the plastic nut. If it still leaks, the washer may be missing, twisted or worn. Fit a new one of the same size.
  • A drip at a supply connection. Turn the isolation valve off. Compression fittings on copper sometimes need a small extra turn with a spanner. Do not overtighten. If it still weeps, call a plumber.
  • Water from a flexible hose. Braided hoses can fail with age. Turn the valve off and replace the hose, or ask a plumber.
  • Water from the sink itself. The seal between the sink and worktop, or between the waste fitting and the bowl, may have failed.

Do not ignore it. GOV.UK guidance on damp and mould says damp can be caused by leaking water from waste pipes, overflowing sinks, burst pipes and other leaks [S1]. It says fixing leaking pipes can help remove the source of moisture [S1]. A wet cupboard base swells and rots, and water can travel under the units.

When should you call a plumber?

Call a plumber if a valve will not turn or leaks when you turn it, if a copper joint keeps weeping, if the waste pipe is cracked inside the wall, or if water is coming up through the floor. Call straight away if a supply pipe has burst. Turn off the main stop tap first.

Replacing a sink, tap or trap like for like does not need building regulations approval. GOV.UK says most repairs and like-for-like replacements of sinks do not need to be told to a building control body [S2].

When you compare quotes for a plumber, Citizens Advice says a quote is a fixed price, while an estimate is just a rough guess [S3]. It says to be cautious if a price is a lot lower than others, because they might not be quoting for exactly the same work [S3]. Our plumbers page covers sink repairs, and tap replacement costs and kitchen sink replacement costs explain related prices.

Honest status: QuoteZila is new. We're starting in Cheshire, beginning with Crewe, Nantwich and Sandbach, and growing area by area. Anyone in the UK can still post a job or register a business. How fast you get quotes depends on how many local businesses have joined near you. Businesses can register and receive jobs straight away. We don't check insurance, qualifications or registrations, so please check these yourself before you hire.

Jobs you might post

These are examples, not taken from real job posts.

  • Small leak at a compression joint under the kitchen sink. Customer has turned the valve off and wants the joint fixed.
  • No isolation valves under the sink. Plumber to fit valves on the hot and cold supplies while replacing the tap.
  • Cupboard under the sink smells and the base has gone soft. Plumber to find the leak and say whether the waste pipe needs replacing.

Get a plumber for a sink leak

A photo of the pipes under the sink and a note of where the water appears helps a plumber quote. Your job goes to up to five matched local businesses [QZ2]. QuoteZila is free for customers and local businesses [QZ1].

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Questions about under-sink pipes

Which valve turns off the water to the sink?

Look on the hot and cold pipes just below the tap for small valves with a slot or a little handle. A slot turned across the pipe usually means off, and along the pipe usually means on. If there are none, use the main stop tap for the house.

Why does the cupboard under my sink smell?

Often it is the trap. The water sitting in the bend stops drain smells coming up. If the trap is partly blocked, or the seal at a joint has failed, smells can get through. A clean trap and sound washers usually fix it. If not, ask a plumber to check the waste.

Is a small drip under the sink a problem?

Yes, deal with it. GOV.UK guidance on damp and mould says damp can be caused by leaking water from waste pipes and overflowing sinks. A drip into a cupboard base can rot the chipboard and spread under the floor before you see it.

Can I replace the trap myself?

Often, yes, because a plastic trap usually unscrews by hand. Put a bowl underneath, keep track of the washers and their order, and fit the new one the same way. If the joint into the wall is cracked or glued, ask a plumber.

Does QuoteZila check plumbers?

No. We don't check insurance, qualifications or registrations, so please check these yourself before you hire. You can report a quote or a business from your job page.

This page is general information, not professional or legal advice. QuoteZila does not check Gas Safe, WaterSafe, NFRC, NICEIC or other registrations against the register on your behalf: look them up yourself.

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Sources

  1. QZ1: QuoteZila product fact. QuoteZila pricing line.
  2. QZ2: QuoteZila product fact. QuoteZila FAQ How does getting quotes work.
  3. S1: Understanding and addressing the health risks of damp and mould in the home, GOV.UK (MHCLG, DHSC and UK Health Security Agency). Accessed 2026-10-31. https://www.gov.uk/government/publications/damp-and-mould-understanding-and-addressing-the-health-risks-for-rented-housing-providers/understanding-and-addressing-the-health-risks-of-damp-and-mould-in-the-home--2
  4. S2: Building work, replacements and repairs to your home, GOV.UK (Ministry of Housing, Communities and Local Government). Accessed 2026-10-31. https://www.gov.uk/government/publications/building-work-replacements-and-repairs-to-your-home/building-work-replacements-and-repairs-to-your-home
  5. S3: Before you get building work done, Citizens Advice. Accessed 2026-10-31. https://www.citizensadvice.org.uk/consumer/getting-home-improvements-done/before-you-get-building-work-done/