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Unblocking a clogged waste pipe

A clogged waste pipe from a sink, basin, shower or bath etc can usually be detected by the slow draining of water from it; the only problem is that waste pipes can be become increasing clogged over time so the draining will have got gradually slower and slower so it won't always become immediately obvious.

Once you realise you have a clogged waste pipe, it can to be attacked in a number of ways. If the outlet is completely blocked, or the water drains very slowly, start by using a plunger, then, if necessary, dismantle and clean the waste trap and finally move on to using a chemical drain cleaner.

After the blockage has been cleared, it is a good idea to put a small quantity of chemical cleaner down the plug hole every month or so just to keep the waste pipe clear in future.

Using a Plunger

A plunger is only really effective when there is water in the waste pipe back up into the basin/sink. Often a blockage may be due to some form of physical obstruction in the pipe - possibly a build up of hair, grease or kitchen waste. It only needs a small piece lodged within the pipe and this will catch other things in the waste water to build up a substantial obstruction over time.

A plunger works by pushing and pulling the blockage within the waste pipe, until it becomes dislodged and can be carried away down the pipe.

  1. Cover the plug hole with the open end of the plunger and fill the sink/basin or whatever with enough cold water to completely cover the rubber cup of the plunger.
  2. Block any overflow using a wet cloth - this will prevent air or water escaping from the waste pipe. Depending upon the design of the overflow, it may be possible to securely push the wet cloth into it, otherwise, the cloth may need to be held in place using one hand.
  3. Holding the plunger vertically, push it down hard and then shapely draw it up. Repeat this three or four times.
  4. Move the plunger away from the plug hole and see how fast the water is draining away.

If the water is still not draining away very fast, the blockage may be more substantial or may have been pushed further along the waste pipe.

Rather than just keep using the plunger, it is worth checking the waste trap for an obstruction.

Cleaning the Waste Trap

The waste trap is fitted under the plug hole and provides a water seal to prevent odours coming back into the room from the drains. Over a period of time, all sorts of debris can collect in the trap and reduce the drainage.

Most modern waste traps have a cover at the bottom which can be unscrewed by hand to gain access; others need to be completely disconnected from the plug hole outlet and waste pipe; older waste traps, usually on lead pipe work, have a relatively small access plug at the bottom of the bend which is released by using a wrench. See our page on waste traps to identify the different modern types available.

  1. Remove as much water as possible from within the sink/basin by using a cup to lift it out - this reduces the amount of water which be released when the trap is opened.
  2. Place a container under the waste trap to collect any water that will be released which the traps is opened/disconnected. Space may be limited under the traps, so you may need to use something shallow and wide rather than a bucket.
  3. Open or disconnect the trap as appropriate for the type.
  4. Let the contents of the trap drain into the container. Depending upon the type of trap:
    • On old lead pipe work - Use a length of bent pipecleaner (a short length of net curtain wire, with the plastic removed, males an ideal cleaning tool) to probe inside the waste pipe either side of the access hole to dislodge/catch any muck in the trap.
    • Where only the bottom cap is removed - Clean out the part of the trap still attached to the plug hole using a rag and small brush as carefully as possible. Take the removed bottom cap to another sank/basin and clean out the inside under running water.
    • Where the whole trap is removed - Take it to another sank/basin and clean out the inside under running water. If possible, unscrew the various parts of the trap so that each part can be thoroughly cleaned.
  5. Refit the waste trap to the waste pipe and run some hot water through it to rinse out any remaining muck.

Using Chemical Cleaners

Chemical cleaners are powerful chemicals which are just poured through the plughole and into the waste pipe. They are usually corrosive and will break down any waste debris that has built up on the inside of the waste pipe. Chemical drain cleaners come as either a liquid or in a granular form which needs to be mixed with water before use - both types are effective but the unused granules have a limited shelf life once the container is opened - so don't start by buying too large a packet.

The problem with liquid chemical cleaners is that they only clean the surface they flow over - i.e. in a waste pipe, the lower half. However foaming chemical cleaners expand once inside the pipe to fill it and these clean away debris all around the inside of the pipe.

Always follow the manufacturers instruction for mixing (if necessary) and using; often these specify a period to leave the chemicals in the pipe work before flushing through with clean water. Most cleaners are only suitable for use with mains sewer drainage.

Different types of chemical cleaners are used for different pipe materials (i.e. plastic or copper etc) and they must only be used with the appropriate type of pipe.

Care also must be taken to avoid getting the chemicals on other items (floors, table surfaces etc) and appropriate gloves etc worn to keep it away from the skin - always read and follow the instructions carefully.


Finally, once the blockage has been removed, run plenty of hot water down the waste pipe to clear away any muck remaining in the pipes.