Understanding your drainage system


The sanitary appliances in your home send your waste water into large pipes, and then into the main drainage system underground. New homes have a single pipe, knows as a soil stack, while older homes can have separate pipes for soil and waste water.

The waste water flows to either the soil stack, and then directly into the underground drain or, it can flow into a trapped gully linked to the underground drain (for ground-floor sink and basin appliances). Gullies have a metal or plastic grid to stop eaves and other debris from causing blockages. The drain itself will run to the main sewer, or in rural areas, to a cess pit or septic tank buried near your house.

Rainwater drainage

Rainwater should be drained separately to waste water, as prolonged downpours could overwhelm the system and cause problems. Downpipes can be connected directly to an underground drainpipe, and from there rainwater often runs into a storm drain beneath. Alternatively, the rainwater may drain into a soak away, from where it gradually percolates back into the ground.

Who’s responsible?

You! Yes that’s right, you are responsible for your home’s drainage system, but only up until the point it enters the main sewer. You’ll need to fix any problems in this area yourself. The government are quite strict regarding any changes to the drainage systems, because of the potential health hazards involved. If you are thinking about changing/modifying any part of the drainage system, be sure to talk to your council’s Building Control Officer first!

Single pipe drainage systems

Modern domestic drainage systems are based on a single pipe arrangement. One larger vertical pipe (the ‘soil stack’) provides the all important connection to the main underground drain. The waste pipes from all your upstairs basins, baths, showers and toilets will run here.

Waste pipes that are downstairs can also run into the oil stack, however it is often more convenient for a sink or basin to drain into a gully. In this situation the waste pipes should terminate below the gully grid (unless there is a special pipe connector behind the grid!), and above the water level in the trap.

Downstairs toilet will almost certainly connect to the underground drain directly. Modern homes usually have a soil stack which runs up through the inside of the house and emerges through the roof and extends above – this stops any drainage smells and any suction in the system which could empty traps of their water seals. Older systems often have the soil pipe on the outside of the house.

Domestic drainage traditionally involved a two pipe arrangement. The waste water and soil water were kept apart until they reached the main underground drain. Many older properties have been converted to single pipe drainage, however separate systems are still quite common.

Separate drainage systems

Toilets are connected to a vertical soil pipe, then attached to an outside wall and protrudes out of the roof. Another smaller vertical pipe drains waste water. This ends at the first floor level in a fitting called a hopper head. All the waste pipes from upstairs appliances drain into this.

The waste pipe at ground level ends just above the grid of a gully. Usually, downstairs fittings drain into gullies, but the pipe ends above the grid. This is unfortunately a major disadvantage of the system, and the grids often get blocked with sink waste, while the hopper may suffer a build-up of soap – which often results in nasty smells and overflows!


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