Drainage Field Replacement

In short: a failed drainage field usually shows up as wet or boggy ground, bad smells, or slow drainage from the house — and once a field has genuinely failed, rehabilitation is rarely effective; replacement is the standard fix.

Signs your drainage field has failed

• Persistently wet, boggy or lush green ground above the field, even in dry weather
• Bad smells near the drainage field area
• Sewage-coloured water pooling on the surface
• Slow-draining sinks, baths or toilets in the house, with no other obvious cause
• The septic tank needing emptying more frequently than usual

Why drainage fields fail

• Age — most fields have a working life of roughly 15–25 years
• Overloading — more occupants or water use than the field was designed for
• Poor original design — undersized for the ground conditions, or sited on unsuitable soil
• Neglected septic tank — solids escaping into the field clog the soil’s ability to absorb water
• Surface damage — vehicles or heavy structures compacting the ground above the field

What replacement typically involves

1. Assessment — confirming the field has genuinely failed rather than the septic tank itself.
2. Percolation test — required to size the new field correctly; see our percolation test guide.
3. Design — following BS 6297, sized to occupancy and soil performance.
4. Installation — usually sited in a different location from the failed field to allow the old ground to recover.
5. Reinstatement — backfilling and restoring the surface.

Preventing it happening again

• Have the septic tank emptied on a regular schedule rather than waiting for problems
• Avoid parking vehicles or building over the drainage field
• Divert surface water (like guttering) away from the field area
• Be mindful of what goes down the drain — fats, wipes and harsh chemicals all shorten a field’s life

Cost

Drainage field replacement cost depends heavily on ground conditions and the size of field required. See our drainage field cost guide for a fuller breakdown.