Quality.

    Drainage and below ground — critical dimensions

    Gradients, cover depths, access spacing and trap seals for foul and surface water.

    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

    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

    Range

    Gradient of a branch discharge pipe

    18 to 90 mm per metre for a 75 mm or 100 mm branch

    Measured along the branch from the appliance to the stack

    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

    Maximum length of a WC branch to the stack

    6 m

    For a 100 mm branch serving a single WC, with no more than 90 mm/m fall

    Maximum

    Spacing between drainage access points

    45 m

    Straight run of buried drain between access points

    Minimum

    Water seal depth at appliance traps

    75 mm for baths, basins, sinks and showers on a branch; 50 mm for a WC trap

    Appliance trap on a branch discharge pipe

    All critical dimensions · All topics

    These entries are our own editorial summaries of publicly available regulation. They are not building control approval, a warranty determination, or a substitute for the full source document.