Fixing and Fitting Domestic Drainage Systems

The drains — the system of pipework, hoppers, gullies and underground pipes that carries waste water of all types away from the house — rarely get the regular checking and maintenance they deserve; too often, the only attention they get is when there is a blockage, yet a regular check-up would help to avoid these problems.

The importance of traps

Every water-using appliance — bath, basin, sink, wc, washing machine and so on — has a trap on the run of pipework that carries the waste water to the drains The purpose of this is to provide a water seal in the pipe run that will prevent smells from the drains entering the house. On old lead pipework, the trap will be a U-shape bend with a small metal plug at the bottom; this can be unscrewed to allow a blockage to be cleared. You must take care when undoing it not to deform the soft pipework; you can then insert a length of wire to poke the blockage clear.

On modern plastic pipework, you will find a plastic trap — either a U-shape section joined to the appliance and the pipework with a screwed fitting, or else a one-piece bottle trap. In the former case, you can unscrew the fittings to remove the trap, which can then be unblocked, flushed through and replaced. In the case of the bottle trap, the base of the trap can be unscrewed to clear a blockage. Both types are sealed with rubber 0-rings; make sure that these are replaced carefully when the trap is reassembled, or you will have leaks.

Before attempting to clear a blocked trap beneath a bath, basin or sink, put the plug in and have a bucket handy to catch the water in the trap itself.

Blockages in WCs can often be cleared by using a rubber or plastic cup plunger. If the blockage persists, you may have a blocked drain further down the system, and this will have to be tackled from one of the inspection chambers or manholes outside your house. To locate the blockage, lift the cover of the manhole nearest to the house; if it is empty, the blockage is between it and the house, whereas if it is full the blockage is further down still. Move on down the drain run until you find an empty chamber; the blockage will be between it and the last full chamber.

To clear a blocked drain you will need a set of drain rods — lengths of cane which can be screwed together like a chimney sweep’s brush, and which can be fitted with an assortment of plungers and brushes to dislodge the blockage. At the full manhole, screw a couple of canes together, attach the plunger and feed it down the drain, adding more canes as you do so. Rotate the rods clockwise as you feed them in the drain, to prevent the screwed ferrules from undoing; if you do not do this you will lose the rods irretrievably in the drain. Place a board across the outlet from the next (empty) manhole, to catch the debris as you dislodge it and prevent it causing a blockage further down. When you have cleared the blockage, scrub through the drain run with the brush attachment, using plenty of clean water from a hose.

When outside gullies become blocked, lift the grating and scoop out the debris from the gully trap by hand (wearing rubber gloves) or by using an old tin as a scoop. Hose the gully through with clean water, scrub the grating and replace it. Fitting a gully cover made from exterior-quality plywood will prevent debris from blowing into the trap and causing another blockage.

Repairing manholes

If a manhole cover is cracked or broken, it should be replaced as a matter of urgency. You will probably have to buy a new rim as well as a new lid, since getting exactly the right size is likely to prove difficult. Chip away the concrete holding the old rim, using a club hammer and cold chisel. Clean up the surround, and bed the new rim in a 1:3 cement to sand mortar, checking with a spirit level that it is sitting level and is not twisted. Leave to harden for 24 hours before laying the new cover in place.

If the benching — the sloped mortar in the base of the manhole that supports the half-pipes crossing the chamber — is cracked, chip it away carefully taking care not to damage the pipes and replace it with a 1:4 cement to sharp sand mortar, finished with a steel trowel.