Quality.
    Minimum
    England

    Minimum WC connections per pipe diameter

    100 mm

    For peak flows exceeding 1.0 litre per second, a 100mm diameter pipe serving multiple WCs should have a minimum of one WC connection

    For peak flows exceeding 1.0 litre per second, a 100mm diameter pipe serving multiple WCs should have a minimum of one WC connection. A 150mm diameter pipe for such flows is typically suitable for a minimum of five WC connections.

    Basis in regulation

    How to check it on site

    Verify that the installed pipe diameters are sufficient for the number of WCs and other appliances connected, based on flow calculations.

    Common failure

    Using pipe sizes that are too small for the connected sanitary appliances, especially WCs, can lead to inadequate flushing and blockages.

    Last updated 23 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.

    Related dimensions at this stage

    Minimum

    Gradient of a 100 mm foul drain

    1 in 80 (steeper for runs with fewer appliances)

    100 mm foul drain serving at least one WC

    Minimum

    Depth of cover to a buried drain

    600 mm under gardens and other areas not subject to vehicles; 900 mm under roads and driveways

    Buried foul or surface water drain outside the building footprint

    Minimum

    Distance from a soakaway to a building

    5 m

    Surface water soakaway serving a dwelling

    Minimum

    Height of a ventilating stack terminal above an opening

    900 mm above the top of any opening within 3 m horizontally

    Open ventilating stack terminating at roof level or on an external wall

    Maximum

    Spacing between drainage access points

    45 m

    Straight run of buried drain between access points

    exact

    Backfill compaction protocols

    300 mm

    Trench backfill must be well compacted in layers no deeper than 300mm to achieve stable ground