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Joining plastic piping

Most waste pipes installed today are made of plastic, which is cheap, lightweight and easy to work with. A little practice and careful measuring will enable you to replace all parts of your system.

Waste systems draining baths, basins and sinks used to be made of lead, heavy galvanised steel with screwed joints, or copper. Soil pipes from WCs were traditionally cast iron, as was all the outside pipework for both waste and soil disposal. Nowadays waste and soil pipes are made of one or other of a variety of plastic materials, which may be used for repairs, extension work or complete replacement of an existing system. These plastic pipes are lightweight and easily cut, handled and joined.

They are made of materials usually known by the initials of their chemical names – UPVC (unplasticised polyvinyl chloride), MPVC (modified polyvinyl chloride), ABS (acrylonitrile butadiene styrene) and PP (polypropylene). CPVC (chlorinated polyvinyl chloride) is usually used for hot and cold water supply pipes. Pipes and fittings are available in white, grey or a copper colour, depending on type and manufacture. All these materials are satisfactory for domestic waste systems and – with one exception – can all be joined in the same way: either by push-fit (ring-seal) jointing or by solvent welding.

The exception is PP pipe. This was first developed because of its good resistance to very hot water and chemical wastes, and was therefore extensively used in industry. Nowadays, however, it is frequently used in the home for waste or rainwater drainage. The big difference between PP and other plastic pipes used in waste drainage is that it cannot be solvent-welded. All joints must be push-fit. In most situations this is no great disadvantage but it does make it important to be able to distinguish PP from other plastics. It has a slightly greasy feel and, when cut with a fine toothed saw, leaves fine strands of fibrous material round the cut edges.

Sizes of the waste pipes

When buying plastic pipe and components it is wise to stick to one brand only. Pipes and fittings from different makers, though of the same size, are not necessarily interchangeable. Most suppliers stock the systems of only one manufacturer, although the same manufacturer may make both PP and either PVC or ABS systems. It is worth asking the supplier if there is an instruction leaflet supplied by the maker. There are slight variations in the methods of using each particular make of pipe and fitting.

The manufacturer’s instructions, if available, should be followed to the letter. Buying new pipe Existing waste pipe is likely to be imperial in size – 11/2in internal diameter for a sink or bath and 11/4in internal diameter for a wash basin. Metric sized plastic pipes are normally described – like thin-walled copper tubes – by their external diameter, though at least one well-known manufacturer adds to the confusion by using the internal diameter. Both internal and external diameters may vary slightly – usually by less than one millimetre between makes. This is yet another reason for sticking to one make of pipe for any single project. The outside diameter of a plastic tube that is the equivalent of an imperial sized metal tube is likely to be 36mm and the inside diameter 32mm. The outside diameter of the equivalent of a 11/2in pipe is likely to be 43mm and the inside diameter 39mm.

If in doubt, it is usually sufficient to ask the supplier for waste pipe fittings for a basin waste or – as the case may be – a bath or sink waste. Plain-ended plastic pipe is usually supplied in 3m (10ft) lengths, though a supplier will probably cut you off a shorter piece. Joining solvent-weld types Solvent-weld fittings are neater and less obtrusive than push-fit ones and they offer the facility of pre-fabrication before installation. However, making them does demand a little more skill and care and – unlike push-fit joints – they cannot accommodate the expansion (thermal movement) that takes place as hot wastes run through the pipe.

A 4m length of PVC pipe will expand by about 13mm (just over Vfein) when its temperature is raised above 20°C (70°F). For this reason, where a straight length of waste pipe exceeds 1.8m (6ft) in length, expansion couplings must be introduced at 1.8m intervals if other joints are to be solventwelded. This rarely occurs in domestic design, however, and use of push-fit or solvent-weld is a matter of personal preference. Although the instructions given by the different manufacturers vary slightly, the steps to making solvent-weld joints follow very similar lines. Of course, the first rule is to measure up all your pipe lengths carefully. Remember to allow for the end of the pipe overlapping the joint. When you’ve worked out pipe lengths cutting can start.

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