Two pipe systems are still very common on house built before the second world war. There are two separate waste stacks running down the outside of the house – one for waste water and one for soil.

The waste pipes from your plumbing fittings run into the waste stack either directly or via a hopper head (now obsolete but still very common). Pipes from ground floor fittings often connect to the stack under ground. But if they are far away from the stack they run instead into a separate gulley – a kind of underground U trap. This joins the underground pipe from the waste stack at an inspection chamber, covered by a manhole.

Soil from the WC always runs to the soil stack direct. The underground pipe from the stack joins the waste water pipe at the inspection chamber. Rainwater may be collected at a gulley to join the waste water system. It may run from the gulley to the inspection chamber via a separate pipe. It may be dispatched to a separate gravel filled pit or soak away. Or, in areas where water is in short supply, it may run to a separate storm water drain.

From the inspection chamber, the combined waste and soil water flows towards the main drain, normally in the road. Before it gets there it may well pass through another chamber – the interceptor – containing a large U trap. Interceptors were once used to cut houses off from the main drains; this is no longer done, so they are no longer fitted. But you may find that you share an interceptor with one or more neighbouring properties. Interceptors are easily distinguished from ordinary inspection chambers by an air inlet terminal nearby.

The single stack system is the one now in common use. As its name implies, waste water and soil pipes all connect to the same stack. Until recently the stack had to be inside the house but the rules have since been relaxed to allow outside stacks. Ground floor appliances too far away from the stack to connect to it have their own sub stack or run to a closed (back inlet) gulley. All underground pipes run in a straight line to a meet at an inspection chamber.

There are as many variations in drainage as there is in plumbing. Houses with a two pipe system which have been modernized may also have an internal single stack or sub stack. Some larger houses have the one pipe system in which a single stack runs on the outside of the building. Some early single stack systems have additional old style gullies.

There may also be more than one inspection chamber: they must be installed where ever pipes join and where the gradient or direction of the drain changes.

The only way to really be one hundred percent sure on how your drains are laid out is to piece them together on a sketch plan, using the above information as a guide.