Run a new waste pipe for a washing machine or dishwasher — HomeGraft

Run a new waste pipe from your washing machine or dishwasher to the drain, keeping the right fall and joints.

WP-01Plan the pipe run and connection pointDecide where the new pipe will join the drainage system - usually a boss fitting on an existing waste pipe, or a tee near the kitchen or bathroom stack. Measure from the appliance to that point and sketch the route, keeping it as short and straight as possible. Mark where the pipe will need to pass through a wall, floor or cupboard back so you know what to check before drilling. The whole run needs a steady fall towards the drain, so plan a path that lets it slope down all the way.

WP-02Buy the pipe, trap and connection fittingsWashing machines and dishwashers normally use 32mm pipe, though some older stacks are 40mm - check yours before buying. Get a boss adaptor or access fitting to connect into the existing pipe, a trap with at least a 75mm water seal to stop smells coming back up, and enough straight pipe and bends to complete the route. Add solvent weld cement, PTFE tape, and pipe clips to support the pipe roughly every 500mm on the flat and 1200mm vertically.

1 each · 32mm solvent weld waste pipe (3m length)

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1 each · 32mm boss adaptor or access fitting

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1 each · 32mm trap, 75mm water seal

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1 tin · Solvent weld cement

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1 roll · PTFE tape

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1 pack · 32mm pipe clips (pack of 10)

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1 tube · Silicone sealant

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WP-03Work out what's behind the wall before you drillTap along the surface - a dull, solid sound means brick or block, while a hollow sound usually means plasterboard on a stud wall, or dot-and-dab board fixed with adhesive blobs over solid masonry behind. Run a stud detector or a strong magnet across the wall to find the line of screw or staple heads marking a timber or metal stud. Drill a small pilot hole in an inconspicuous spot - pale dust means timber, fine grey swarf means metal. A wall that gives slightly and feels ridged under the surface is probably lath and plaster, common in houses built before the 1930s. Knowing this now decides which fixing you buy, not after.

1 each · Stud detector or strong magnet

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WP-04Check for cables and pipes, then drill or cut the routeCables and pipes can run almost anywhere in a wall or floor, not just in the safe zones you might have read about. Use a cable and pipe detector to scan the route, but still treat the wall as if something is there - detectors miss cables that are deep, in metal conduit, behind metal-backed plasterboard, or confused by a metal stud frame. Before drilling, find your stopcock (usually under the kitchen sink) and check it turns freely, so you can shut the water off fast if you hit a pipe. If you have any doubt at all, isolate the circuit at the consumer unit before drilling, and turn the water off at the stopcock before cutting into anything that could carry water. Drill a 40mm hole for 32mm pipe, or 45mm for 40mm pipe, through the centre of a timber stud, or use a core drill through masonry.

RULES

1 each · Cable and pipe detector

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1 each · 32mm/40mm drill bit or core drill bit

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1 each · Fire-rated pipe sleeve

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KNOW THE RULES

If the pipe passes through a fire-separating wall or floor - such as between flats, or into a loft above a room below - Building Regulations Approved Document B requires the gap to be fire-stopped afterwards with the correct sleeve or sealant.

WP-05Connect into the existing waste pipeIf an old appliance is already connected, turn off its water supply and place a bucket under the pipe to catch any residual water. Cut a short section out of the existing waste pipe and fit the solvent weld boss adaptor, leaving the full setting time stated on the cement tin before testing. Near the base of a stack, a push-fit boss with a rubber seal gives a quicker, re-workable joint instead.

RULES

1 each · Bucket

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1 pack · Cloths or rags

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KNOW THE RULES

Altering or connecting into existing drainage falls under Building Regulations Part H. A simple appliance connection through a standard boss usually doesn't need Building Control involvement, but check with your local authority if you're altering the drain itself or joining a shared stack.

WP-06Run the pipe and set the fallLay the pipe along the marked route, joining lengths with solvent weld or push-fit joints as you go. Waste pipe needs a steady fall towards the drain - aim for 18mm to 90mm of drop per metre, roughly a 1:40 to 1:80 slope. Check the fall with a spirit level as you fix each clip, and avoid any dips where water or debris could sit and start to smell.

1 each · Spirit level

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2 each · 32mm bend fittings

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WP-07Fix the pipe clips - the right fixing for your wallThe pipe itself is light, but a clip that pulls out will let the pipe sag and lose its fall. Into a timber stud, use a short wood screw driven into the centre of the stud. Into a metal stud, use a self-drilling metal-stud fixing or a small toggle - never a plastic wall plug and never an ordinary wood screw, both strip straight through 0.5mm steel. Into plasterboard between studs, or over dot-and-dab, use a toggle or hollow-wall anchor rated for at least the pipe's weight, long enough to also reach the solid wall behind on dot-and-dab. Into solid brick or block, drill a 6mm hole to the plug's stated depth and use a plastic wall plug with a matching screw. Into lath and plaster, screw into the timber lath itself where you can feel it behind the plaster, or use a plasterboard-style fixing that spreads the load without cracking the surface.

1 pack · Wood screws (pack of 10)

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1 pack · Self-drilling metal stud fixings (pack)

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1 pack · Hollow-wall or toggle anchors (pack)

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1 pack · Wall plugs, masonry (pack)

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WP-08Fit the trap and standpipeFit the trap to the appliance end of the pipe, keeping at least a 75mm water seal so drain smells can't come back up. If using a standpipe, cut it to stand 600mm to 900mm above the floor and secure it to the wall with a clip near the top, leaving enough pipe above the trap for the appliance's outlet hose to hook over.

WP-09Test every joint for leaksBefore pushing the appliance back into place, run a few litres of water through the new pipe from a jug, checking every joint by eye and by touch for damp patches. Leave solvent weld joints the full time stated on the tin before testing - rushing this is the most common cause of a leak. Fix any drips now, while you can still reach the pipe easily.

WP-10Connect the appliance and finish upPush the appliance's outlet hose fully onto the standpipe or trap connection and secure it with the clip supplied, then slide the appliance back into place without kinking the new pipe or hose. Run a short wash or rinse cycle and check the connection again while water is flowing. Clear away offcuts, wipe down your tools, and vacuum up any dust left from drilling.

1 each · Hose clip

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