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How to install a sink trap

How to install a sink trap

Jeremy, Construction site supervisor, Cardiff

Guide written by:

Jeremy, Construction site supervisor, Cardiff

151 guides

Waste traps are fitted to sinks and toilets to prevent bad odours from escaping and to stop any solid objects from getting stuck in your pipes. Sink traps, in particular, are very straightforward to install or replace. Read on for our step-by-step guide to installing a sink trap.

Important features

  • Connecting the trap to the plug
  • Connecting the trap to the drain pipe
  • Connecting a bottle trap
  • Checking for leaks
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Installing or replacing a sink trap is a pretty straightforward plumbing job. Unlike most plumbing tasks, such as installing a tap, the water in a waste trap is not pressurised. Instead, when the trap is filled, atmospheric pressure is equal on both sides.

This means there is less chance of a leak. However, you still have to follow a few basic rules. Sink drainage is controlled by gravity, meaning your pipes need to have a minimum slope of 1 to 3 centimetres per linear metre. Otherwise, your sink will not drain correctly. As such, the sink trap should not be installed too low down.

The following sanitary appliances all contain a trap:

The role of a trap

Traps are designed to prevent bad smells from escaping. The trap retains a small amount of water which is used to block odours coming from the drain. It also helps to hold onto any solid matter contained in waste water, such as:

  • hair;
  • grease;
  • food waste in kitchen sinks;
  • residue from personal hygiene products.

Sink traps are designed to be an accessible and removable part of your plumbing system. It is always preferable to have a blockage in the trap rather than in a pipe hidden behind the wall or in the floor!

Washing machines and dishwashers feature a trap that is attached to the PVC drain pipe. The trap is installed like aany otherPVC connection with the use of a special PVC glue.

For all other sanitaryware (toilets, sinks, etc.), the trap is screwed between the unit and the drain pipe,creating a junction between the two.


  1. Connecting the trap to the plug
  2. Connecting the trap to the drain pipe
  3. Connecting a bottle trap
  4. Checking for leaks

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Kitchen sinks

Every sink features a plug which is used to fill up and drain the unit. The base of the plug is threaded. At the top of the trap, you'll find a screw coupling.

A flat, rubber O ring will generally be positioned inside this coupling. You simply need to screw the coupling onto the plug to make the connection watertight. Start by tightening it by hand and finish off with an adjustable spanner as it gets harder to turn.

Be careful not to screw too tightly, however, to avoid damaging or breaking the plastic. You just need to compress the rubber O ring slightly. The joint does not have to cope with pressurised water as the water flows naturally by gravity.

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The process of connecting the trap to the drain pipe is more or less the same.

  • Insert the screw coupling onto the drain pipe.
  • Slide the compression seal onto the pipe. This seal may be made of nylon or Teflon – in which case it will generally be white or transparent – or rubber (generally black in colour).
  • Check the length of the drain pipe to ensure that you are able to fit the trap correctly. If required, use a hacksaw or pipe cutter to cut the pipe so that it fits into the trap and is well-aligned with the plug.
  • Next, insert the drain pipe into the trap.
  • Move the seal as close as possible to the trap.
  • Move the coupling into position and tighten it. This will compress the seal, creating a watertight joint between the drain pipe and the trap.

If you are installing a bottle trap, you may have to fit the base which is the part of the trap designed to contain a small amount of water and any solid matter. If you are replacing an existing trap, be sure to place a bucket below the trap to catch any water as you remove it. The bottom of the trap may feature a thin rubber washer and is designed to simply screw onto the base of the trap. Generally speaking, tightening by hand should suffice.

Once your trap is in place, you will have to check it is watertight. To do so:

  • Insert the plug.
  • Fill the basin with water.
  • Open the plug.

Check that all connections are watertight by ensuring the water drains away quickly. If you notice any leaks, ensure that all parts are firmly tightened.

Do not apply silicone to a leaking trap. Instead, check that your connections are screwed together tightly, replace a washer or seal or fit a new trap. Silicone is not suitable for use with a trap. This is because traps are designed to be accessible and removable and should not be glued into place.

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Silicone sealant

Required skills

Replacing or installing a new trap is a straightforward process that can be carried out by just about anyone.

Time required

5 minutes

Number of people required

1 person

Tools and equipment

Personal protective equipment (PPE)

Please note: this is a non-exhaustive list; Be sure to match your personal protective equipment to the job at hand.


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Guide written by:

Jeremy, Construction site supervisor, Cardiff, 151 guides

Jeremy, Construction site supervisor, Cardiff

Electrician by trade, I first worked in industrial estates where I installed, wired and fixed a large number of electrical installations. After this, I managed a team of electricians for this type of work. 10 years or so ago, I turned to building and construction. From the modest family home, to gyms and theatres, I have been able to coordinate, audit and organise all sorts of construction sites. For 4 years now, I am restoring and building an extension to a bungalow in the heart of the Welsh countryside. My experience in manual work and my knowledge means I am proud to be of service. Terraces, interior design, roofing, plumbing, electrics, anything goes! My wife, daughter and I, built almost everything we have from scratch! So to answer all of your questions and advise you on choosing your tools? Easy!

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