Maximum lengths and gradients for 32 mm, 40 mm, and 100 mm unvented waste branch runs.
Under Approved Document H Section 1, sanitary appliances connect to the soil and waste stack via unvented branch pipes. To prevent siphonage (where flowing water draws water out of the trap, breaking the seal), unvented branch lengths and slopes are strictly limited:
- Washbasins (32 mm Pipe): The maximum permitted length of an unvented 32 mm waste pipe is 1.7 metres. The recommended gradient is 1 in 60 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): The maximum permitted length of an unvented 100 mm branch pipe connecting a WC is 6.0 metres. The gradient must be at least 1 in 110.
If these lengths must be exceeded, the branch must be fitted with an air admittance valve (AAV) or connected to a dedicated ventilating pipe to safeguard the trap seal.
*Original revision guidance; confirm sizing tables against Approved Document H Table 2.*
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