Sanitary waste pipe sizes and length limits under Approved Document H

Approved Document H gives branch layouts that need no separate vent when their length and fall fit Table 2 or Diagram 3. A branch running full can pull water out of its own trap (self-siphonage), so read the appliance, pipe size, length and fall together:

• Washbasins (32 mm pipe): Keep an unvented branch at 1.7 metres or shorter. Read its fall from the design curve: the longer the run, the flatter it must be, so at 1.7 m it falls about 18 to 22 mm per metre (roughly 1 in 55 to 1 in 45).
• Sinks, Baths & Showers (40 mm Pipe): The maximum permitted length of an unvented 40 mm waste pipe is 3.0 metres. The recommended gradient is 1 in 55 to 1 in 11.
• Water closets (100 mm pipe): A single WC may have up to 6.0 metres of unvented branch to the stack. Give it at least 18 mm fall per metre (about 1 in 55). The flatter 9 mm per metre (about 1 in 110) provision is for a long branch serving more than one WC where space is tight.

These limits belong to the pipe sizes and falls stated above. For a longer basin run, 40 mm pipe can reach 3.0 m or 50 mm pipe 4.0 m without a separate vent, provided the fall stays between 18 and 44 mm per metre. For a sink or bath, 50 mm pipe can reach 4.0 m at a fall between 18 and 90 mm per metre. For a shared 100 mm branch, Table 2 allows up to eight WCs with outlets over 80 mm along 15 m, subject to its fall rules; do not apply that shared-branch figure to the single WC above.

If the planned run is outside the unventilated size-and-fall combination, choose a compliant layout or ventilate the branch to outside air, to a ventilating stack or with a suitable air admittance valve (AAV). A vent protects the trap seal; it does not make an excessively flat pipe drain properly.

Trap seal depths by appliance

Approved Document H also sets how much water seal each trap should start with, in Table 1. Greater seal depth provides more resistance to a pressure difference across the trap. This depends on the vertical water barrier, rather than simply the total volume stored in the trap; suitable branch design and ventilation are still needed.

• Washbasins, bidets, sinks, urinal bowls, food waste disposal units, washing machines and dishwashers: 75 mm seal.
• A washbasin with a flush-grated waste and no plug, or with spray taps, may use the Table 1 exception of 50 mm.
• Baths and showers: 50 mm seal.
• WC pans: 50 mm seal, whatever the outlet size.
• A bath, shower, washing machine or dishwasher that discharges directly to a gully may have a shallower seal, but never less than 38 mm.
• Under working and test conditions a water-sealed trap must still retain at least 25 mm of seal. The guidance also permits an equivalent barrier; an approved membrane waste valve must suit its particular application. The 25 mm retained figure is not a universal initial trap depth.