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Discharges and vents

Discharges and vents should comply with the size, material and performance requirements in Acceptable Solution G13/AS1.

On this page:

  • water traps with single discharge outlet
  • water traps with multiple discharge outlets
  • sizing discharge pipes
  • access points
  • venting discharge pipes
  • vent pipes
  • WC pans – discharge pipes and venting
  • air admittance valves
  • floor wastes
  • pipe materials

Water traps with single discharge outlet

Every fixture that discharges foul water must incorporate a water trap to prevent foul air from entering the building.

The type of fixture determines the size of the discharge pipe and trap, and the size of the discharge pipe determines the minimum gradient required for the pipe:

  • 1:20 for 32 mm pipes
  • 1:40 for pipes 65 mm diameter and under
  • 1:60 for pipes 100 mm diameter and under

Water traps must be:

  • located as close to the sanitary fixture as possible
  • removable or fitted with a cleaning eye.

Easy-clean traps are acceptable and are used mainly in showers, urinals and trapped floor wastes

The developed length of a fixture discharge pipe between the sanitary fixture outlet or a sanitary appliance (e.g. a washing machine) discharge and the water seal must not be greater than 1.2 m.

Water traps with multiple discharge outlets

A trap may serve multiple fixtures such as:

  • two domestic sinks and one dishwasher machine (note that if one sink has a waste disposal unit installed, it must be trapped separately)
  • two laundry tubs
  • one laundry tub and a clothes washing machine
  • two hand basins.

Sizing discharge pipes

The minimum diameter for a discharge pipe is given in G13/AS1:Table 2 Fixture discharge pipe sizes and discharge units.

A discharge pipe that receives discharge from two or more fixtures must have a minimum diameter calculated from G13/AS1:Table 4 Discharge unit loading for stacks and graded discharge pipes.

Discharge pipes must not decrease in size in the direction of flow.

Access points

Access points must be provided so that blockages can be cleared where:

  • a soil discharge pipe connects to a stack
  • a soil stack connects to a drain
  • there are bends and/or junctions likely to cause a blockage
  • access to a pipe is otherwise restricted.

Venting discharge pipes

All discharge pipes must be vented in accordance with G13/AS1:Table 5 Venting requirements.

A vent pipe may be connected to a relief vent, a discharge stack vent, or a branch vent.

A vent pipe must:

  • have a continual rise from the point of connection with the discharge pipe to the open air
  • terminate in the open air
  • terminate no less than 50 mm above the overflow level of the highest fixture that it serves
  • be fitted with a bird exclusion device
  • have a gradient no less than 1:80.

Vent pipes must have a minimum diameter as given in G13/AS1:Table 6 Vent pipe sizes.

Discharge stack vents must have:

  • a minimum diameter of 80 mm where they are also acting as a drain vent pipe, or
  • a minimum diameter as given in G13/AS1:Table 6 Vent pipe sizes.

WC pans – discharge pipes and venting

WC pans may connect directly to a drain or a discharge stack. When discharging to a drain, WC pans may be:

  • vented by connection to a drain vent as long as the developed length does not exceed 1.5 m for an 80 mm diameter pipe, or 6 m for a 100 mm diameter pipe
  • connected directly to a drain without venting if both the main drain and discharge pipe gradients are 1:60 or more, and the developed length does not exceed 1.5 m for an 80 mm diameter pipe, or 6 m for a 100 mm diameter pipe.

When discharging to a stack, WC pans must be vented.

Air admittance valves

Air admittance valves (AAV) provide an alternative to running vent pipes to the outside of the building. When flowing water causes a reduction of air pressure within the system, the air admittance valve will open automatically, admitting air into the system. It will close again when the pressure in the system is equal to or greater than the external pressure.

Air admittance valves must be:

  • used in accordance with manufacturer’s instructions
  • a minimum of 100 mm above the weir of any trap that they serve
  • installed in an accessible, ventilated space where they are protected from damage, sun exposure and freezing
  • fitted in an upright position.

They must not be used:

  • on a stack vent that is also a drain vent
  • as main or branch drain vents – these must be vented to the open air.

Floor wastes

Dry floor wastes are provided to drain away accidental water spillage. They:

  • do not need to have a water trap
  • may discharge directly outdoors, or into a gully trap
  • must not be connected to a foul drainage system – water in the trap may be lost through evaporation admitting unpleasant odours
  • must have a minimum diameter of 40 mm
  • must have a grating flush with the floor so they do not create a hazard.
Floor wastes



Floor drainage may also be provided by a trapped floor waste or a floor waste gully, both of which require a charged water trap.

Pipe materials

Acceptable materials for sanitary plumbing include:

  • pipes and fittings – copper, PVC
  • traps – PVC
  • expansion joints – PVC.

Sanitary plumbing pipework must allow for thermal expansion to prevent damage to pipework and fixtures.