Plumbing – The Waste System

Plumbing - The Waste System

In modern plumbing systems, the pipes which carry used water away from baths, basins, WCs, bidets and showers have traps often called U-bends full of water. The water in these traps prevents smells from the sewers getting into the house – in Victorian times, this ‘drain air’ was thought to be directly responsible for a number of diseases. Even if this is not the case, smells from sewers are at least unpleasant. On a WC, the water trap is part of the fitting; in other cases, it is part of the outlet waste pipe.

Single-stack systems

Most houses built since about I960 have a single-stack waste system. The branch pipes from the U-bend traps, attached to baths, basins, WCs and so on in the upper storeys of the house, connect into a single pipe usually 100mm in diameter – called a discharge pipe, soil pipe or soil-stack -which runs vertically down the side of (or through) the house. The top of this pipe should terminate outside the building, not less than 900mm above the top of any opening windows (unless a relief valve is fitted). The bottom is connected directly into the house drainage system – it has no trap in it.

When designing a waste system, care has to be taken to ensure that the water in the traps cannot be sucked out so breaking the seal against smells. This can happen if waste water rushes through the branch pipe leading from the trap (or through other pipes connected to this branch) quickly enough to create sufficient suction to pull the water out of the trap.

To guard against unsealing, the top of the soil-stack is left open. It should, however, be fitted with a cage to stop birds nesting in it and stopping up the open end. (Technically, the length of pipe above the highest branch connection to it is called a vent pipe.) In the single-stack waste system, there are other design constraints – the slope, length and diameter of branch pipes, the position of their connections to the soil-stack, and the radius of the bend at the foot of the soil-stack all have to be worked out carefully in order to meet the requirements of the Building Regulations.

WCs at ground-floor level may also be connected to the soil-stack but are more usually connected directly to the drain. Other ground-floor waste pipes will prob¬ably discharge into a back-inlet gully or through the grid of an open gully. A gully is basically a water trap with the top open to the air at ground level and an outlet connected to the house drains.

The gully should be fitted with a grid to prevent leaves and other things blocking it. The waste pipes enter the gully below the level of the grid but above the level of the water in the gully trap either by simply passing through a hole cut in the top of the grid, or by being connected to an inlet forming part of the gully. When this inlet is at the back of the gully (the front of the grid is where the outlet is) it is called a back-inlet gully; when the inlet is at the side, it is called, not surprisingly, a side-inlet gully.

Extending a single-stack waste system will mean joining into the main soil pipe. This is usually fairly simple, provided the pipe is plastic.

Two-pipe system

Many older houses have a two-pipe waste system with WCs connected into one vertical soil pipe, and other wastes (baths, basin and bidets) connected into a separate vertical waste pipe. This system calls for less careful design of slopes and connections, but the vertical pipes still need to be vented to the air.

An existing two-pipe system can be extended by allowing extra waste pipes from upstairs rooms to discharge into the hopper head and ground-floor wastes to be led to the gully. This is clearly much simpler than having to cut into the side of the vertical soil or waste pipe – particularly a cast-iron one.

In the two-pipe system, the soil pipe is connected directly to the drains, and the waste pipe is connected via a trapped gully, which usually takes the waste pipe from the kitchen sink.


Most plumbing and waste work is covered by regulations designed to ensure that the results of the work are not a danger to health and do not lead to undue consumption, misuse, contamination or waste of water.

Building Regulations

The Building Regulations (and their equivalents in Northern Ireland and Scotland) control the way waste systems are designed. You should give notice of your plans to do anything to the waste system in your house (apart from straightforward repair or replacement). Ask the local authority for information about the person to contact-in England and Wales it is the Building Control Officers of your Borough or District Council.

The general requirement of the Regulations is that the systems carrying foul water (WC waste and water which has been used for cooking and wash¬ing) and for carrying rain-water away from the house shall be ‘adequate’. What this means is explained in the Approved Document for part H; most manufacturers of soil/waste equipment give guidance in their literature.

Water By-laws

Because there are differences in the type of water supplied to different areas of the country, each local water supply undertaking can issue its own water by¬laws – though these are normally based on the Model Water By-laws. You should give notice of your plans for certain plumbing work before it is started, including installing a bidet, flushing cistern or a tap for a hose or making a connection to the rising main.

The Water By-laws cover such things as: the size of storage cisterns and the position of inlets, outlets and over¬flows: the provision of stop taps and drain taps; the protection of pipes against frost damage, corrosion and vibration; and, most importantly, the design of the system so that there is no possibility of the supply becoming contaminated, particularly by back-siphonage. To make sure that any work complies fully, it is essential to read (and follow) the local water undertaking’s by-laws, which are normally sup¬plied free.

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