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Waste water traps explained

All appliance using water and installed within a house needs a form of water trap in the waste pipework to give a water seal to the drains or outside atmosphere; a water trap prevents smells, bacteria and insects entering the property.

Modern plastic waste plumbing, (apart from WC's), normally use either 32mm (1.25 inch) (for hand basins) or 40mm (1.5 inch) (for sinks, baths, showers, dishwashers, washing machines) pipe and fittings (including traps); runs of waster pipe more than 2m (6 feet) long may use 50mm (2 inch) diameter pipe - this avoids the syphon effect when the pipe is used - the syphon effect could empty the water from a trap at the start of the run so making it ineffective.

The sizes quoted for pipework is the internal pipe sizes - likewise the size quoted for the waste outlets from the basin/sinks/bath etc), also refers to the internal size of the connecting pipe. The external diameter of pipework does vary between different manufacturers - this means that pipes and fitting from different manufacturers may not be interchangeable.

The main point of a trap is to provide a water seal in a 'U' bend (or other arrangement). The important aspect of any water traps is the depth of seal (see right).

Current regulations specify that where waste pipes discharge into a single stack (as is the current 'new build' requirement), the depth of water seal must be at least 75mm (3 inches).

Where a bath or basin is being replaced in an older property where the existing waste pipes feed into an open hopper or drain gully, this arrangement may be retained and the depth of water seal may be only 50mm (2 inches).

P trap depth of seal

Bottle trap depth of seal

Typically, waste pipes fits to modern water trap by either a screw coupling (with a compression seal ring and washer) or a push fit into an 'O' ring seal coupling. From an installation or use point of view, there is not much to choose between the two types although screw couplings can be easier to fit where there is restricted access.

The following are the common types of trap which may be found.

The 'P' trap - normally these have a screw joint half way along so that: For installation, the outlet can be moved through about 270° in the horizontal plane so that it will mate with a horizontal waste pipe coming in from an angle. After installation, the trap can be easily removed for clearing blockages etc without disturbing the rest of the pipe run. One disadvantage of the 'P' trap is that it requires a fair amount of room around the waste outlet.
The 'S' trap - normally these have a screw joint half way along so that: For installation, the outlet can be moved through about 270° in the horizontal plane so that it will mate with a vertical waste pipe coming up 'off centre'. After installation, the trap can be easily removed for clearing blockages etc without disturbing the rest of the pipe run. Like the 'P' trap, a disadvantage of the 'S' trap is that it requires a fair amount of room around the waste outlet.
The 'Bottle' trap - normally the bottom unscrews to allow the clearing of blockages etc. The output for the waste pipe is always horizontal. The 'Bottle' trap takes up less radial space under a waste outlet and is ideal for pedestal mounted basins where space is usually very limited. They can however be deeper that the 'P' and 'S' traps.
Various 'special' traps are available for specific uses, such as the waste water from: Twin bowl sinks (as illustrated right); Washing machines (see below); Dish washing machines (effectively, the same as for a washing machine).
The 'Shallow' trap is normally fitted to baths and showers where there is limited space above the floor. Where fitted to a bath, a banjo connector is normally used above it for the overflow from the bath. This type of waste trap may not meet the depth of water seal required by the water regulations. Where there is no other alternative to fitting a shallow trap, it may be necessary to fit another (deeper) trap further along the pipe run where space allows, and definitely before the pipework enters a stack pipe, or discharges.
The 'Shallow' trap with a connector for the bath overflow. Similar to a shallow trap described above but with an addition inlet in the bend. Usually this type of trap is supplied with a blanking plug fitted in the second inlet and a separate connector for this inlet must be purchased. The point of the second inlet is connecting the hose from the bath overflow. This removes the need for a banjo connector above the trap, allowing these traps to fit where space is very limited. As with ordinary shallow traps, these traps may not meet the water regulations regarding the depth of water seal. To overcome this problem, another (deeper) trap may need to be fitted further along the pipe run where space allows.
The Washing Machine 'S' trap (usually suitable also for Dish Washing Machines) incorporates a vertical standpipe above the water trap - with the outlet of the water trap facing downwards. The waste pipe from the washing machine fits into the top of the vertical pipe. The individual Washing Machine installation instructions usually specify the required height required (normally an minimum and maximum) for the standpipe to of the vertical pipe in relation to the floor level of the machine. The only difference with a Washing Machine 'P' trap is that the outlet from the suits a horizontal waste pipe.