| Before you attempt to unblock main drains, make | | | | Interceptors were once used to cut houses off from |
| absolutely certain what system you have and get to | | | | the main drains; this is no longer done, so they are no |
| know how it works. | | | | longer fitted. But you may find that you share an |
| Two pipe systems are still very common on house | | | | interceptor with one or more neighbouring properties. |
| built before the second world war. There are two | | | | Interceptors are easily distinguished from ordinary |
| separate waste stacks running down the outside of | | | | inspection chambers by an air inlet terminal nearby. |
| the house – one for waste water and | | | | The single stack system is the one now in common |
| one for soil. | | | | use. As its name implies, waste water and soil pipes |
| The waste pipes from your plumbing fittings run into | | | | all connect to the same stack. Until recently the stack |
| the waste stack either directly or via a hopper head | | | | had to be inside the house but the rules have since |
| (now obsolete but still very common). Pipes from | | | | been relaxed to allow outside stacks. Ground floor |
| ground floor fittings often connect to the stack | | | | appliances too far away from the stack to connect |
| under ground. But if they are far away from the | | | | to it have their own sub stack or run to a closed |
| stack they run instead into a separate gulley | | | | (back inlet) gulley. All underground pipes run in a |
| – a kind of underground U trap. This joins | | | | straight line to a meet at an inspection chamber. |
| the underground pipe from the waste stack at an | | | | There are as many variations in drainage as there is |
| inspection chamber, covered by a manhole. | | | | in plumbing. Houses with a two pipe system which |
| Soil from the WC always runs to the soil stack direct. | | | | have been modernized may also have an internal |
| The underground pipe from the stack joins the | | | | single stack or sub stack. Some larger houses have |
| waste water pipe at the inspection chamber. | | | | the one pipe system in which a single stack runs on |
| Rainwater may be collected at a gulley to join the | | | | the outside of the building. Some early single stack |
| waste water system. It may run from the gulley to | | | | systems have additional old style gullies. |
| the inspection chamber via a separate pipe. It may | | | | There may also be more than one inspection |
| be dispatched to a separate gravel filled pit or soak | | | | chamber: they must be installed where ever pipes |
| away. Or, in areas where water is in short supply, it | | | | join and where the gradient or direction of the drain |
| may run to a separate storm water drain. | | | | changes. |
| From the inspection chamber, the combined waste | | | | The only way to really be one hundred percent sure |
| and soil water flows towards the main drain, normally | | | | on how your drains are laid out is to piece them |
| in the road. Before it gets there it may well pass | | | | together on a sketch plan, using the above |
| through another chamber – the | | | | information as a guide. |
| interceptor – containing a large U trap. | | | | |