Fitting Waste Pipes and Traps

In modern plumbing systems, the waste pipes which carry away used water are normally made from plastic. The waste pipe lead from the outlet of the bath, shower, etc, through the wall and into the soil stack.  Plastic waste pipes are joined using either push-fit, solvent-weld or compression joints. All of the waste systems in your house will have some type of trap installed. This is designed to create a barrier (of water) to stop smells from penetrating back into the house through the plugholes. Fitting or replacing the plastic waste pipes and traps is a fairly common plumbing project, and certainly one worth understanding properly.

Cutting Plastic Waste Pipes

Measure and mark the cut mark on the outside of the pipe using a pencil or thin marker pen. Hold the pipe firmly and use a junior hacksaw to cut through it at the point of your mark. You want the cut edge to be square, so take your time with the cut. If it makes things easier, you can wrap a piece of paper around the pipe with its edge at the cut mark. This should give you a straight guide for cutting.

When the pipe is cut, use a fine-toothed file to remove the burrs from both inside and outside of the cut edge. If you are planning to joint the pipe using a solvent-weld, it is also a good idea to wipe the end of the pipe with a damp cloth to remove any fine plastic dust. You can read more about different types of plastic plumbing pipes here.

Joining Plastic Waste Pipes

There are three different methods available to you when you come to join the cut sections of plastic waste pipe. All three are perfectly good for joining pipes, but push-fit and compression joints offer a bit more room for error. Solvent-weld joints really need to be perfect first time.

Push-Fit Joints
Make sure you have enough push-fit sockets ready before you start. Working on one joint at a time, spray the end of the cut pipe with Silicone Spray. Push the pipe into the socket, making sure that it slides in as far as it can go (the pipe will only go halfway into the socket). Spray the end of the next pipe and slide this into the socket next. As before, the pipe will only go as far as the middle of the socket. Rubber rings inside the socket will help to hold the pipe firmly in place, but you need to make sure that the pipe ends are both pushed very firmly into the socket.

Solvent-Weld Joints
Without applying any solvent, slide the socket over the end of the pipe and use a pencil to mark where you want it to be. Try to make sure that the pipe stops in the middle of the socket fitting. Remove the socket from the end of the pipe and apply the solvent cement from the pencil line to the cut end of the pipe. Slide the pipe into the socket, up to the pencil line. The solvent will react with the plastic in the pipe to create a waterproof joint, so it is important to make sure that you firstly coat the pipe well with the solvent and, secondly, you give it enough time to set inside the socket. When the first half of the joint is solid, you can repeat the process with the pipe that will fit into the other side of the socket.

Compression Joints
The sockets for making compression joints come in several different parts and it is important that they are all fitted correctly and in the right order if you want to avoid leaking pipes. The socket comprises a central body section, two screw nuts and several rubber seals and O-rings. Unscrew the threaded nuts from the body of the socket and slide one over the end of each pipe you are joining. Slide slide a rubber seal onto each pipe next, about 10mm from the end, as well as any O-rings that are supplied. Slide each end into the body of the socket so that they meet in the middle. Push the threaded nuts up to the body and tighten them by hand. The threaded nuts will compress the rubber seals and create a (hopefully) watertight joint. It is important that the pipes are straight in the body of the socket. If they are at an angle, the seal may not be tight enough.


How a Waste Trap Works

waste-trap-diagramAll of the different types of trap work in basically the same way, using water as a barrier to stop smells penetrating back into the room through the outlet hole.

Water permanently collects at the bottom of the trap, blocking the pipe. When more water is flushed down the outlet hole, it pushes through the water already in the trap. The last bit of water to flow down the outlet is, in turn, held at the bottom of the trap and the seal is formed again. Traps should be cleaned out regularly using a plunger or auger, particularly if food waste is being washed down through the sink outlet. Shower traps can also become blocked with hair and soap residue.

Types of Waste Trap

The trap on a bath or sink is designed to create a seal of water which acts to stop smells rising up though the pipes from the drainage system. There are several different types of trap available, but which you choose depends on the location and several other factors. All of the different types of trap are available with push-fit, compression and solvent weld joints.

Bottle Trap
Bottle traps are only designed for sinks and washbasins, as they can not cope with a heavy outflow such as from a bath. The trap can be unscrewed and removed to allow it to be cleaned out, and can be a godsend if something valuable is dropped down the plughole.

Tubular Trap
Tubular traps come in two main types, either S-shaped or P-shaped. The s-shaped trap has an outlet which points down and a p-shaped trap has a side-facing outlet. Tubular traps are suitable for all types of waste pipe, but are usually found on baths. The whole of the trap can be removed for cleaning or replacement.

Washing Machine Trap
A P-shaped tubular trap with a long standpipe, into which the waste hose of the washing machine is fed. As with other tubular traps, the whole thing can be unscrewed and removed for cleaning.

Anti-Siphon Bottle Trap
This type of bottle trap allows air to enter, which stops the seal being lost by heavy water flow. Designed for use on sinks and wash basins where occasional heavy flow is a problem. It can also help to stop problems caused by a long steep pipe run.

Bath Trap with Overflow
A P-shaped tubular trap which features a flexible tube which connects to the overflow outlet on the bath. The overflow usually joins the fitting above the trap. Some bath traps also feature a cleaning cap which can be removed to give access to the trap without having to take the whole thing off. Bath traps often have a shallower U-bend than other traps to allow then to be fitted easily in the small space below the bath unit.

Double Inlet Trap
A single trap with inlets for two separate appliances such as a washing machine and the kitchen sink. Useful when two such appliances are plumbed in close to each other and there is limited room for pipework.

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