Drainage

Soakaway sizing, channel-drain falls, where rainwater is allowed to go.

All guides

Stub — growing

Last reviewed 20 May 2026

2 sources

Regulations

Surface water must not be connected to the foul sewer. Soakaway, watercourse or surface-water sewer only, in that order of preference under the SuDS hierarchy. ^[1]
Soakaway design and percolation testing follow BRE Digest 365. ^[2]

Hints & tips

Before trenching a single metre — CAT & Genny the line (HSE HSG47). Drain runs invariably cross incoming gas, water, electric and BT. A CAT alone won't find plastic mains; the Genny in Induction mode will. Full method in the Safety guide.
Channel-drain (ACO) fall: minimum 1:200 to the outlet. Confirm with manufacturer for shallow runs.
Soakaway rule of thumb (London clay): 1 m³ of void per 50 m² of impermeable surface — but always percolation-test before specifying.
On sandy / loam soils budget ~1 m³ of void per 80 m² and still percolation-test.
Cover soakaways with geotextile to stop fines clogging the void.
If existing house drains take run-off, confirm capacity with the water authority before adding more area — overloading is illegal under Building Reg H3.
Drain-to-lawn or border counts as a permeable area under GPDO Class F — keep at least 1 m² of receiving ground per 5 m² of paving.
Land drains behind retaining walls: 100 mm perforated pipe in a permeable backfill, exiting to a free outfall.

This app provides general UK guidance and material estimates only. It is not legal, planning, engineering or building-control advice. Always confirm requirements with your local planning authority, building control, utility providers, manufacturers or qualified professionals.