DIY Doctor UK DIY Forums

I have a shower tray to which I have fitted a new wast trap, and I want to replace the rest of the waste pipework.

The problem is the huge variety of pipe and fitting dimensions - none of this stuff appears to be designed to fit together properly. Conversely, when you buy any pipe or fitting marked "15mm", you know for sure it will all fit together OK.

The internal diameter of the waste trap outlet is 44mm, and it has an external thread for a compression joint.

The external diameter of waste pipes labeled as "40 mm" is:

* "Normal" 40mm waste pipe is actually 41mm outer diameter.
* "Solvent weld" 40mm waste pipe is actually 43mm outer diameter. Why is this different?

But, I don't want to use solvent weld, I want to use normal waste pipe with compression fittings.

I have some "40mm" compression elbows and straight connectors: these have an internal diameter of 43mm.

The threaded external ring from a "40mm" straight connector will screw onto the waste trap, but there seems to be a lot of play between the "40mm" pipe (actually 41mm) and the waste fitting (actually 44mm).

I also need to get round some bends, and I have purchased a flexible waste pipe, assuming that this will do the trick - some luck!

One end of this has a threaded compression fitting that is slightly too small to take a "normal" waste pipe - the maximum external diameter pipe it will take is about 38mm.

The other end of this flexible fitting is plain, and has an external diameter of 36mm.

I can't imagine what this is supposed to fit.

To summarise the confusing array of dimensions:

* 36mm - plain end of flexible waste pipe
* 38mm - compression fitting end of the flexible waste pipe
* 41mm - the external diameter of "normal" waste pipe
* 43mm - the external diameter of "solvent weld" waste pipe
* 43mm - the internal diameter of "40mm" compression connectors
* 44mm - the internal diameter of the waste trap exit - will take a "40mm" compression fitting.

Can anyone explain this vast range of sizes?

Don't worry too much, it's all fairly straightforward.

Pipes come in two sizes 1 &1/4" or 1 & 1/2" now referred to as 32mm and 40mm. I will ignore 32mm as the rules apply as for 40mm.

All modern push fit couplings are the same size regardless of make. Solvent weld is a slightly larger size to avoid non solvent pipe being used in error.

Compression fittings are designed slightly larger for two reasons. Firstly, to allow joins where the pipes are joined slightly at an angle or offset, and secondly to allow coupling of solvent and pushfit as well as older imperial pipe sizes. Joints may feel loose, but will always produce a watertight seal.

If I remember right, screw threads do vary between manufacturers but should not prove a problem as all fittings are supplied complete.


I hope this helps to keep you dry and sane!

hi it doesnt matter what system you buy,solvent,compression or push fit it will all fit into the trap,you then tighten it up with the compression nut and carry on from there so if push fit is your option it will work
bob