How do I solvent weld a plastic waste pipe joint?

Solvent cement does not glue the two parts together. It softens the surface of both, they flow into one another, and as the solvent leaves you are left with a single continuous piece of plastic. That is why a good solvent weld is stronger than the pipe and a bad one leaks straight away: there is no middle ground and no gradual failure.

Safety

What you'll need

• solvent cement matching the pipe material
• the matching pipe cleaner or primer
• a fine-toothed saw or pipe cutter
• a deburring tool, a file or a knife for the chamfer
• clean lint-free cloth and nitrile gloves
• a pencil for the witness mark

Step by step

1. Identify the pipe material before you buy anything.PVC-U, ABS and MuPVC all solvent weld, each with its own cement, and some cements cover more than one. Polypropylene — usually a slightly waxy, flexible pipe — cannot be solvent welded at all and uses push-fit joints with rubber seals. Using cement on polypropylene simply does nothing.
2. Cut the pipe square.A pipe cutter or a fine-toothed saw against a mitre block. A cut that is off square means the pipe cannot seat fully in the socket and part of the joint is unsupported.
3. Deburr the inside and chamfer the outside edge.The internal burr causes blockages and traps debris. The external chamfer lets the pipe enter the socket without scraping the cement off the socket wall as it goes in, which is a genuinely common cause of a dry patch.
4. Dry-fit and mark the socket depth.Push the pipe fully home dry, mark the pipe at the socket mouth with a pencil, and withdraw it. That witness mark tells you at a glance during assembly whether the pipe is fully home and, afterwards, whether it has crept back out.
5. Check the alignment while it is dry-fitted.Once the cement is on you have seconds, not minutes. Mark the rotational position of the fitting as well as the depth if the direction matters, which it usually does on a branch or a bend.
6. Clean both surfaces with the matching cleaner.The cleaner removes mould release, swarf, grease and the glaze on the surface, and it slightly softens the plastic ready for the cement. Wipe the pipe end and the inside of the socket, and let it flash off for a moment.
7. Apply cement to both surfaces, along the pipe rather than round it.An even coat on the pipe end over the full socket depth, and a thinner coat inside the socket. Brushing lengthwise rather than circumferentially leaves fewer ridges. Enough to wet everything, not so much that it pools in the fitting.
8. Push fully home with a quarter turn and hold.Insert straight, twist about a quarter turn to spread the cement, push to the witness mark and hold firmly for around thirty seconds. The softened plastic will try to push the pipe back out and it will do so the moment you let go.
9. Wipe the squeeze-out ring and then leave it completely alone.A ring of cement at the socket mouth means the joint is full. Wipe the excess off the outside, then do not move, twist, load or test the joint. Follow the cure time on the tin before running any water — it is longer than people expect, and longer again in the cold.

Tips

• Work out the whole run dry first and assemble in an order that lets each joint be pushed fully home. A joint you cannot push into is a joint that will leak.
• In cold weather the cement thickens and cures slowly. Keep the tin somewhere warm before you start and allow considerably longer before testing.
• Do not use solvent cement to try to seal a leaking push-fit or compression joint. Those seal on a rubber ring or an olive, and cement neither bonds nor seals there.
• Never solvent weld into an existing joint that is wet. Drain the run, and put a rag in the pipe if it is still weeping — cement will not fuse through water.
• Keep the lid on the tin between joints. The solvent evaporates fast and a half-used tin left open thickens to the point of being unusable.

Common mistakes

• Skipping the cleaner because the pipe looks clean — New pipe carries mould release and the surface has a glaze on it. The cleaner is part of the chemistry, not a tidiness step, and joints made without it are the ones that weep.
• Letting go too soon — The interference fit plus the softened plastic pushes the pipe back out of the socket. It creeps a few millimetres, the joint is short, and it leaks. The witness mark shows this immediately.
• Using the wrong cement for the plastic — ABS and PVC cements are not interchangeable, and neither does anything at all to polypropylene. The joint looks made and has no strength.
• Running water the same afternoon — The joint reaches handling strength long before it reaches full strength, and the cure time on the tin assumes reasonable warmth. Cold and damp extend it considerably.

If it doesn't work

Questions people ask