Drainage needs planning during plumbing installation because waste water has to be carried away by gravity, so the route, fall and connection point of each waste pipe can decide where new fixtures can go. Supply pipes can usually be extended more easily than drainage, so drainage often sets the limits of a layout.
Thinking about drainage first can help avoid a design that looks good on paper but is hard to plumb. It can also help avoid problems such as slow drainage, smells or blockages once the work is finished.
Waste Needs a Route and a Fall
Water supply pipes are under pressure, so they can run up, down and around obstacles. Waste pipes rely on gravity and need to fall steadily towards the soil pipe or drain. The further a fixture is from the drainage point, the harder this can be.
A waste that does not fall properly may drain slowly or let waste settle, while one that runs too far or has too many bends can be more likely to block.
The right fall and pipe size depend on the fixture and the design, so they are best worked out for the specific installation rather than guessed.
Toilets and Soil Pipes
Toilets usually connect to the soil pipe, which carries waste to the drain and is often larger than other wastes. Because of this, toilets can be the hardest fixtures to place in a new position. Some installations use pumped systems where gravity drainage is not possible, but these have their own requirements and are not suitable for every situation.
Access, Venting and Regulations
Drainage needs to be accessible for clearing if it blocks, so access points should be planned rather than boxed in. Drainage systems also need venting so traps are not emptied when water flows, which can cause smells.
Building regulations cover drainage, and new connections to drains or soil stacks may need building control approval. Work that changes underground drainage may also involve the sewerage company. It is worth checking what applies before work starts.
Planning Drainage Early
When planning a new bathroom, kitchen, utility room or extension, find out where the existing soil pipe and drains run and how deep they are. This can help decide where fixtures should go and how much work will be needed.
Adding new fixtures can also put more water through the existing drainage, so its condition and capacity are worth considering.
If fixtures already drain slowly, it may be worth looking into that before adding new ones to the same system. Our blocked drain page covers problems with existing drains.
Our page on new plumbing installation explains how supply and drainage are planned together for new work.
If you are planning new fixtures in your Poole property, a plumber can look at the existing drainage and explain what may be possible.