I find that plumbing is, for the most part, reasonably logical and straight forward. Generally as long as you’ve only got water coming out where you want it you’re good to go. One area that is totally confusing though is domestic waste pipes. There’s a myriad of sizes and technologies to choose from and they seem to all be slightly incompatible. In this article I’ll try and untangle the mass a little bit.

I’m not going to pretend this is the final word in domestic waste pipes. This is just what I’ve learnt while fixing up a few houses and doing some reading. With that said there’s some useful information here.

Note that this article is based on the domestic waste pipes found in the UK. The joining systems are probably similar elsewhere in the world but the pipe sizes may be different. The pipe sizes we have in the UK are based on British Standards sizes.

Domestic Waste Pipes

There are three commonly available pipe joining systems and three common sizes. The pipe joining systems are solvent weld, push fit and compression. The common sizes are 32mm, 40mm and 50mm. So far so easy but here’s where the fun starts…

The sizes of the pipes usually have very little to do with the actual size of the pipe. For example a 40mm solvent weld pipe will have an OD of 43mm and an ID of 38mm – why it’s sold as 40mm is anyone’s guess. A 40mm push fit pipe will usually have an OD of 40mm but not always.

To confuse matters further waste pipes traditionally were measured using their ID whereas pipes in general, for example water pipes, are measured across their OD. As far as I can tell there is currently a mixture of measuring ID and OD for waste pipes, DIY stores often sell by OD but plumbers merchants will sell by ID or nominal size.

The upshot of this is that a solvent weld pipe won’t fit in a push fit socket and vice versa. Fortunately not all is lost because we have a third pipe joining system – compression fittings. Compression fittings are the ones that have a screw lock and removable soft rubber washer. Compression fittings save the day because they will usually seal on both solvent weld and push fit pipe which allows you to switch between one system and the other. When looking for a compression fitting that will switch between systems check to see if it’s sold as universal, I believe most are but better to be safe than sorry.

So what do you choose?

I’ve used all three systems  and at the moment I prefer solvent weld.

Done correctly (clean pipes, good application of weld compound) you end up with a joint that will never fail as is only slightly larger than the pipe you are working with. The only down side of solvent weld is that you need to make sure you’ve got some weld compound handy as it goes off but it’s not very expensive.

I don’t like the push fit system. The push fit by necessity has to be tight and it concerns me that it’ll damage the fitting pushing it home. It’s not really any faster than solvent weld and while in theory you cold reuse the parts in reality it never happens.

I find the compression system has it’s place under sinks where the pipe work might be rerouted or moved. Sink traps and pipes are the ones that block most commonly in my experience and they need maintenance. Compression fittings can easily be taken apart and put back together. The main downside of the compression system is that the fittings are huge, they barely clear the wall when properly clip in. This means that any tight runs become hard work. The other downside is that they are the most expensive fittings system but it’s not that much more.

Overall the advice is this: pick one system and stick with it for the whole system. If you’ve got two bathrooms feel free to use different systems in each but within a plumbed area just pick one and stick to it.

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