Plumbing problems

Soil pipework

Major problems can arise from unsatisfactory installation of soil pipework under suspended timber floors.

Installing pipework under a suspended floor is a difficult task due to the lack of working space. Achieving a constant gradient is a challenge – with the common method to hang straps from the floor joists.

If there is a belly in the pipe run and the drain blocks, the weight can pull apart push-fit joints and cause foul water and contaminants to pour out into the subfloor – a major damp and health problem to address. There were few clips supporting the soil pipe you see in figure 1, and the pipework seemed to have an irregular gradient. You would require at least a 1:60 fall for such a soil run – and many more support clips than you see. One manufacturer advises that such a pipe should be clipped every metre. Even if the tube is aligned carefully to the required gradient and secured by sufficient clips, there may still be a lack of sideways rigidity.

Figure 1: Soil pipe under a suspended floor. A 100mm PVCU soil pipe heads off generally downwards towards the drain connection at the back of the property. Note the metal strap tacked haphazardly at an angle to the side of a floor joist

Blockages, leaks and seepages can occur when an original cast-iron stack has been replaced above ground. Old stacks often suffer severe rusting and are too dangerous to ignore – they must be renewed. Cast iron work is expensive so is rarely used as the replacement material, except perhaps on listed buildings where original external drainwork is to be retained.

Adding a waste branch to an existing soil stack requires careful thought. You first need to decide whether you can replace the soil stack above the proposed branch position, allowing you to easily fit a straight coupling with bosses already attached as manufactured. Failing this, you may need to attach branches to the existing soil stack.

To do this, a hole cutter cuts the required diameter of the entry hole in the SVP (soil vent pipe), and either a 2-part boss or a strap-on boss is fitted. A 2-part boss is probably the most reliable option, but you need to use a special clamping tool to hold the 2 components of the connector tightly together as the solvent weld solution cures.

Figure 2: A 2-part plastic boss assembly has just been clamped on a 100mm soil pipe. The wing nut clamps the assembly tightly in place for a few minutes while the adhesives cure

Figure 3: A 38mm strap-on boss has just been fitted. A hole is cut in the SVP. The outside of the pipe is sanded where the strap and boss will make contact. The boss is liberally coated with solvent weld solution on its inside face, and clamped by the strap and its tightening nut onto the pipe. Such a strap looks a little cumbersome, but if fitted carefully there will be no leaking. Note the additional adaptor glued into the 38mm boss to accept a 32mm basin waste pipe

For larger buildings pipework may need to be designed to accommodate expansion and contraction forces. Special couplings may be required, and will need to be supported nearby by brackets. Failure to take expansion and contraction into account could mean your soil or waste system will fail and leak.

Regarding plastic pipe jointing, while 32mm or 38mm diameter waste pipework is quite easy to join using solvent weld connections, the same cannot be said of the larger diameter tubes. It requires great care and experience to joint 100mm soil vent pipework. You need to work swiftly: the welding solution cures fast once applied and the mating surfaces need to be quickly pushed together with a slight twist to make sure the 'glue' covers the whole joint zone. It is difficult to manipulate larger-sized tubes as the welding begins to take hold. Ample solvent solution is needed for the larger tubes. If not carefully welded, joints may leak.

Figure 4: Solvent weld solution is brushed around the finely sanded pipe end, marked with a 'V' to ensure satisfactory pipe end insertion into the connector. The author has seen cases of leaking joints – where such a pipe joint had been without the necessary jointing solution. A patent cleaning agent can be applied to clean the pipe end prior to making the joint

Figure 5: A 38mm diameter waste pipe is about to be push fitted into a straight connector. The cut end of the pipe should be chamfered by sanding or fine filing. Note the black rubber sealing O-ring, which you might smear with a trace of lubricant to assist the operation. Failure to lubricate the seal or chamfer the pipe end can mean a successful joint is not achieved – a raw cut end of a pipe can displace the 'O' ring. Manufacturers advise withdrawing the pipe by 5mm to leave an allowance for expansion. Such pipework is speedy to push together, but, as with all plumbing, careful setting up of pipe brackets is necessary to achieve satisfactory falls and a neat job. If the plumbing is altered at a future time, fittings can be pulled apart and reused

Waste pipework

In London, solvent-weld soil and waste systems seem to proliferate. With care, extremely reliable joints can be achieved in plastic pipework using this method of jointing. Pipes must be clean, lightly sandpapered and generously smeared with solvent-weld solution. As with all plumbing jointing, the pipe end must be pushed fully home into the receiving socket.

What is often omitted is the commissioning of the completed system. All pipework needs to be tested. This job is often neglected – the plumber will be tired at the end of the working day and will leave. So many pipe leaks occur because of joints that were just hand-tightened, or worse still, copper plumbing joints that were prepared, wire-wooled and fluxed, but not soldered.