If you're installing or rebuilding a drainage field in Cornwall, the percolation test is the first technical step — and the one most DIY jobs skip. The result determines whether a standard drainage field is viable and how large it needs to be. Done properly (BS 6297:2007+A1:2008 method, on saturated ground, in multiple locations), it gives a defensible Vp value that satisfies Building Regulations Part H and the Environment Agency. Done badly, the field downstream of the bad test fails within years.
Why percolation matters
A drainage field works by letting septic-tank effluent slowly seep into the surrounding soil, where bacteria complete the breakdown over weeks. If the soil drains too fast (sandy), effluent passes through too quickly for proper treatment — risking groundwater contamination. If it drains too slowly (clay, peat), effluent backs up, the field saturates, and the system fails. The percolation rate — measured as Vp (seconds per mm of water drop) — tells you where on this spectrum your soil sits.
BS 6297 specifies the test method, the size formula, and the acceptable range of Vp values. Without a test, the installer is guessing — and the install can't be certified as compliant.
The proper BS 6297 method
- Choose 2-3 test locations across the proposed field area
- Dig test holes 300mm × 300mm × 300mm deep
- Soak overnight — 8-12 hours of pre-saturation
- Refill each hole to 300mm depth next morning
- Time the drop from 225mm (75%) to 75mm (25%) full — a 150mm drop
- Calculate Vp = time (sec) ÷ 150mm. A 4,500-second drop = Vp 30.
- Average across all test holes for the design value
What Vp values mean
| Vp range | Soil type | What it means |
|---|---|---|
| < 12 sec/mm | Sandy, gravelly | Too fast — drainage field not approved. Treatment plant needed. |
| 12-50 sec/mm | Sandy loam, light loam | Ideal range. Standard drainage field works well. |
| 50-100 sec/mm | Clay loam, silty clay | Viable but field needs to be larger. Cornwall mid-county clay often sits here. |
| > 100 sec/mm | Heavy clay, peat | Too slow — drainage field not viable. Alternatives needed. |
The size formula
Field area (m²) = Vp × P × 0.25
Where P = design population (typically bedrooms × 2). Example: 4-bedroom Cornwall property, Vp = 30: field area = 30 × 8 × 0.25 = 60m².
When to do the test yourself
- New install or significant rebuild: Get a specialist. The result must be documented to BS 6297 standards and signed off for Building Regulations Part H compliance. Specialist fee £200-£400 includes documentation.
- Preliminary self-assessment (e.g., considering a property): DIY gives a rough idea. Don't use as basis for a real install.
Cornwall soils — what to expect
- Mid-Cornwall clay loam: Typical Vp 40-80. Viable but field at the larger end.
- Bodmin Moor peat / acidic clay: Often Vp >100. Drainage field rarely viable.
- West Penwith granitic/shillet: Highly variable. Always test multiple locations.
- North coast sandy-loamy: Often Vp 15-40 (ideal). Standard fields work well.
- South coast estuarine: High water tables often a problem; test for water table too.
What the test costs in Cornwall
| Service | Cost |
|---|---|
| BS 6297 percolation test (specialist, multiple holes) | £200-£500 |
| Percolation test + drainage field design package | £300-£700 |
| Site visit and visual ground assessment only | £80-£150 |
Common mistakes
- Single test hole. Cornwall soils vary over short distances. Always do 2-3 minimum.
- Not soaking overnight. Dry-soil tests give wildly optimistic numbers.
- Testing after dry weather. Best done after the ground has been wet for at least a week.
- Ignoring seasonal groundwater. A site with dry-summer Vp 40 may be unviable in winter.
Considering a drainage field or rebuild in Cornwall? Submit your postcode and we'll match you with a specialist who'll do a proper BS 6297 test.
Frequently asked questions
What is a percolation test?
A test to measure how quickly soil absorbs water, used to size and locate septic-tank drainage fields. BS 6297:2007+A1:2008 specifies the methodology. The result (Vp, in seconds per millimetre of water drop) determines whether a standard drainage field is viable and how large it needs to be.
How long does a percolation test take?
2 days minimum. Day 1: dig test holes, fill with water, soak overnight (8-12 hours). Day 2: refill to 300mm, time the drop from 75% to 25% full (a 150mm drop), calculate Vp. Specialist tests include site assessment, multiple holes, and documentation — typically half-day to full day on site plus a written report.
How much does a percolation test cost in Cornwall?
£200-£500 for a specialist BS 6297 test with multiple holes and documentation. £300-£700 for test + drainage field design package. £80-£150 for a basic site visit and visual assessment (preliminary, not certifiable).
Can I do a percolation test myself?
For preliminary self-assessment, yes — the method is straightforward. For an actual install, no — the result must be documented to BS 6297 standards and signed off for Building Regulations Part H compliance.
What if my percolation test fails?
Too slow (Vp > 100): drainage field not viable. Options: sewage treatment plant with surface water discharge (EA permit required), mound soakaway built above natural ground, or in rare cases cesspit. Too fast (Vp < 12): treatment plant required for cleaner effluent before reaching groundwater.