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

Shallowest gradient at which a 100 mm foul drain will run correctly when it serves at least one WC.

Basis in regulation

Part H
Approved Document H
Read the source

How to check it on site

Check invert levels at each end of the run with a level and staff before backfill. Falls that are too steep separate solids just as effectively as falls that are too shallow.

Common failure

A flat section formed where the run passes an obstruction, leaving a low point that silts up.

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

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

exact

Drainage bed and manhole location

10 mm

Substructure drainage pipes must be laid on a suitable bed of granular material, such as 10mm pea gravel, to provide stable support and protect the pipes from ground movement