Waste water is discharged from the sanitary appliances in your home into large vertical pipe or pipes, and from there into the main underground drainage system. Newer homes have a single pipe (a soil stack), while older ones can have separate pipes for the soil and waste water.
Waste water from your appliances either flows to the main soil stack, and from there directly into the underground drain, or (in the case of a ground-floor sink, basin or appliance) it can flow into a trapped gully. Linked to the underground drain, this is fitted with a metal or plastic grid to stop leaves and other debris from falling into the trap and blocking it. The drain itself might run to the main sewer or, in rural areas, to a cess pit or septic tank buried in your garden.
Rainwater from gutters should be drained separately - as heavy, prolonged downpour could overwhelm the system. Downpipes can discharge over the grid of an open gully or be connected directly to an underground drain pipe. From there, the water might run into a storm drain beneath the road or be piped to a soakaway somewhere in your garden, from where it gradually percolates back into the soil.
You're responsible for your home's drainage system up to the point where it enters the main sewer. Therefore, you'll need to fix any blockages or damage in this area at your own expense. Because of the potential health hazards from a drainage system, the rules governing any changes are quite strict. Unlike the supply network (which is governed by the Water Regulations), the drainage system comes under the Building Regulations - and these are enforced by your local council, not the water company. If you're thinking about modifying any part of the drainage system, make sure you talk to your council's Building Control Officer first (see page 378).
Modern domestic drainage systems are based on the single-pipe arrangement. In this, one large-diameter vertical pipe (known as the 'soil stack') provides the essential connection with the underground drain. The waste pipes of all your upstairs basins, baths, showers and toilets will all run here.
Downstairs waste pipes can also run into the soil stack, but, in some situations, it can be more convenient for a sink or basin to discharge directly into a gully. If this is the case, the waste pipe should terminate below the gully grid (unless the gully incorporates a special pipe connector behind the grid) and above the water level in the trap.
A downstairs toilet will almost certainly be connected directly to the underground drain. In modern homes, the soil stack usually runs up inside the house. At the top, it emerges through the roof and extends above it - this expels any drain smells well clear of nearby windows and stops any suction in the system that could empty traps of their water seals.
Older systems and conversions from two-pipe systems usually have the soil pipe on the outside of the house.
Traditionally, domestic drainage was provided by a two-pipe arrangement, where the waste and soil water were kept separate until they reached the underground drain. Although many older properties have been converted to single-pipe drainage, separate systems are still quite common.
Toilets are connected to a large vertical soil pipe that's attached to an outside wall and extends above the roof. A second, smaller-diameter vertical pipe drains waste water. This ends at about the level of the first floor in a funnel-like fitting called a hopper head, and all the waste pipes from the upstairs basins and baths discharge into this.
At ground level, the waste pipe ends just above the grid of a gully. In most cases, downstairs fittings flow into gullies, but their pipes end above the grids. This is one of the major disadvantages of the system - as the grids may get blocked with sink waste, while the hopper isn't self-cleaning and might suffer a build-up of soap. All this can cause nasty odours and even overflows.
If you need to run a waste pipe through your outside wall, you'll need a power hammer drill or a diamond-tipped core drill. Use an electronic pipe and cable detector to check there are no hidden pipes or cables behind the inside wall. Then drill from both inside and out to avoid a messy exit hole.
With a core drill, you'll need to drill a few centimetres at a time, then stop to empty the drill core and clean out the hole in the masonry with a club hammer and cold chisel.
Make sure your hole runs downhill slightly towards the outside. There might be a spare boss on the soil stack that's suitable for connecting a new waste pipe. Otherwise, you can drill a hole and use a strap-on boss and solvent-weld fitting. Or you can feed your new waste pipes into an existing network of hoppers and gullies.
However, there are strict rules about where you can connect pipes into a soil stack, so make sure you get in touch with your local Building Control Officer before you make any modifications.
'Grey water' is the term for water that's been used for showering, bathing, handwashing, low-intensity laundry and dishwashing. It can be reused for flushing toilets and irrigating the garden (as long as it's not contaminated with excessive detergents or bleach). You can simply collect it in a bucket or ask your water company for information on installing a domestic grey water unit.