Push fit or solvent weld waste pipe?

Push fit or solvent weld, which should I use?

Push fit where you may need to take the joint apart. Solvent weld where the pipe is buried, boxed in or under a floor. Push fit uses a rubber seal inside the socket and pulls apart with hand pressure. It tolerates movement, it forgives a joint that is slightly out of square, and it comes apart to clear a blockage. Solvent weld is glued with cement that fuses the two surfaces. Once it has gone off the joint is permanent and the way out is a hacksaw. Solvent fittings are slimmer, so they fit behind a bath panel or inside a stud wall where a push fit socket will not.

How do I tell what is already in the wall?

Look at the socket and at the material. A push fit socket is bulky, with a visible cap and a rubber seal inside. A solvent socket is short, plain and glued. Push fit waste pipe is polypropylene: waxy to the touch, slightly flexible, and it goes pale when you scratch it hard. Solvent weld pipe is MuPVC, which is harder, shinier and sounds sharper when you tap it. This matters more than anything else on the job, because solvent cement will not stick to polypropylene. Glue a fitting to push fit pipe and you get a joint that looks right and leaks the first time the bath drains. If you are not certain, cut a 20mm sliver off the end and bring it to the counter.

Is it 32mm or 40mm?

32mm for a basin, 40mm for a sink, bath, shower or appliance. Both are outside diameter measurements and both systems use the same two sizes. Past the maximum branch length in the table you need a bigger pipe or a branch vent, or the trap will siphon.

Appliance	Pipe	Max branch length	Fall	Min trap seal	Clip spacing
Basin, bidet	32mm	1.7m	18 to 90mm per metre	75mm	500mm horizontal, 1.2m vertical
Sink, bath, shower	40mm	3m	18 to 90mm per metre	75mm	500mm horizontal, 1.2m vertical
Washing machine, dishwasher	40mm	3m	18 to 90mm per metre	75mm	500mm horizontal, 1.2m vertical
WC	110mm	6m	18mm per metre minimum	50mm	900mm horizontal, 1.2m vertical

What fall does waste pipe need?

Between 18mm and 90mm per metre. Aim for 20mm per metre and you sit inside the range with room to spare. Too shallow and the pipe will not self-cleanse, so solids settle and you have a blockage within a year. Too steep and the water runs away from the solids, which gets you to the same place more slowly. A short steep branch can also pull the trap seal out and leave you smelling the drain. Set it with a 1m spirit level and a 20mm packer under one end.

How do I make a joint that does not leak?

Cut square and deburr, whichever system you are using. A rough end tears the seal or leaves a channel through the cement.

• Push fit: chamfer the outside edge about 15 degrees, wipe the seal, lubricate with silicone grease, push fully home, then pull back 10mm so the pipe can expand.
• Solvent weld: clean both faces, brush cement lengthways onto the pipe end and inside the socket, push home with a quarter turn, hold for 15 seconds, wipe the excess off.

Never use oil or washing up liquid as a push fit lubricant, because both attack the rubber. Leave solvent joints at least an hour before running water, and read the tin, because cold weather can double that.

Can I join push fit to solvent weld?

Yes, in one direction only. A push fit fitting grips solvent weld pipe, because both are the same 32mm or 40mm outside diameter. You cannot glue a solvent fitting onto polypropylene push fit pipe. Where the systems have to meet mid-run, put the push fit fitting at the changeover. Reducers from 40mm to 32mm are made in both systems, so buy the one matching the fitting it goes into.

How often does waste pipe need clipping?

Every 500mm horizontally and every 1.2m vertically, with a clip within 150mm of each fitting. That is where the weight hangs. Unclipped pipe sags between supports, the sag holds standing water, and that is where the blockage starts. Clips are also what stops push fit joints creeping apart over the years.

What if the existing pipe is imperial?

Measure across it. If it reads 38mm, it is 1.5 inch imperial and 40mm fittings will not seal on it. Imperial 1.25 inch measures close to 32mm and will usually take a 32mm push fit fitting. Houses plumbed before the early 1980s are the ones to check. Adaptors exist for most combinations, but which one depends on what you actually have. Ring us with the measurement, or bring the offcut in.