Possible failure signs include slow drainage, high tank levels, sewage odour, wet or unusually lush ground and effluent surfacing over the field. The cause may be hydraulic overload, solids carry-over, compaction, unsuitable ground or end-of-life pipework. Diagnosis should review the tank, loading, land and discharge route together.
When this guide fits
Clues worth recording
- Wet or black ground near distribution pipes
- The tank level remains high after desludging
- Problems worsen after rain
- Vehicles or buildings have affected the field area
A drainage field is treatment infrastructure
Perforated pipes distribute settled effluent through suitable soil, where further treatment occurs. In England, a discharge through a soakaway designed for rainwater cannot meet the general binding rules if the soakaway was installed after December 2007, and a well or borehole cannot meet them at all: the effluent needs a drainage field built to the British Standard, or a permit.
Investigate before excavating
Review system capacity, desludging history, baffles, flows, groundwater, site slope and records. Trial holes, percolation testing or a designed site assessment may be needed for replacement; improvised tests should not expose people to effluent.
Protect the field
Keep vehicles, hardstanding, buildings, livestock congregation and deep-rooted planting off the area. Divert roof and surface water away from the foul system and repair leaking fixtures.
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Replacement planning
A new field needs suitable land, separation from sensitive receptors and building-control and environmental consideration. Approved Document H suggests siting a drainage field at least 10 metres from a watercourse, 50 metres from a groundwater abstraction point and 15 metres from any building. Switching between ground and surface-water discharge, or moving the discharge more than 10 metres, makes it a new discharge under England’s rules.
Do not create an overflow pipe to a ditch. It transfers untreated effluent and pollution rather than repairing the system.
Questions homeowners ask
What people usually want to know
Can a failed drainage field be cleaned?
Some blockages or upstream defects can be repaired, but a biologically clogged, damaged or unsuitable field may require replacement. Beware guaranteed “rejuvenation” claims without site evidence.
Can I build over the drainage field?
That can damage access, ventilation and soil function. Seek building-control and drainage design advice before any development.
Is a drainage field the same as a soakaway?
No. Approved Document H distinguishes drainage fields for treated foul effluent from soakaways used for surface water.
Sources & review
Evidence behind this guide
Reviewed 13 September 2026. We use primary guidance where available and label trade-body advice clearly.