The Underground Drain System of a House

The underground drain system of a house takes the waste from soil pipes and gullies to the main public sewer (if the house is on mains drainage) or to a septic tank or cesspool (if mains drainage is not available).

The layout of underground drains is rather less standardised than that of above-ground soil and waste pipes, and because it is hidden it is a little more difficult to trace.

Here there are just two connections to the drain one at the foot of the soil pipe and one from the gully. There is no trap at the base of a single-stack waste pipe (or at the base of the WC soil pipe in a two-pipe system); the gully has a trap incorporated in it and so do pipes carrying waste from fittings other than the WC in a two-pipe system. To gain access to the drains in order to clear any blockages - there are inspection chambers. These are sited near the connection with the soil-stack, where the drain turns a right-angle at the side of the property, and at the boundary of the properly.

In most cases these drains do not carry rain-water - sewers and sewage plants would have to be made much bigger if they did. Instead the rain-water either from the gutters and rain¬water pipes or from the surface-water drains in a drive, say - is carried through a separate set of drains either to a public surface-water drain, or to a soakaway in the grounds of the house. A house may have a soakaway even if the foul drains are connected to a main sewer.

A different drain layout is often used when there is a group of houses together. There is only one connection to the sewer, and from this runs a communal drain through the back gardens of all the properties. The individual house drains are connected to this communal drain. This clearly saves money and effort when several houses are being built at the same time but because the responsibility for the drain is shared, there can be more headaches for the householders when something goes wrong.