1. What a percolation test measures
A percolation test answers a single question: how fast does water disperse through the soil at this spot? The answer is a number called Vp — the average time, in seconds, for the water level in a standardised test hole to fall by 1 mm.
Vp is what every drainage field design hangs on:
- Below 12 → soil drains too fast → effluent reaches groundwater untreated → drainage field not permitted
- Between 12 and 100 → drainage field viable, sized in proportion to Vp
- Above 100 → soil too slow → drainage field won't work → need treatment plant + watercourse, or raised mound
📘 Where the test fits in claims work
You'll meet percolation tests in three places:
- Pre-installation design — the surveyor or engineer running a fresh test before a new system goes in.
- Failure investigation — when a field has surcharged and we're trying to diagnose whether the soil or the design was at fault.
- Conveyancing / claim disputes — when a buyer's solicitor or insurer challenges whether the original installation was compliant.
2. Where the rules come from
Building Regs
Part H2
Part H2
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Approved Document H2, paragraphs §1.34–1.40. Sets the test method, the 12–100 Vp range, and the sizing formulas. Legal basis for any installation needing Building Regs approval.
BS 6297:2007
+A1:2008
+A1:2008
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Code of practice for drainage fields. Annex H mirrors Part H2 method. Adds 0.20 sizing factor for treatment plant effluent (vs 0.25 for septic tank).
EA General
Binding Rules
Binding Rules
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Permits ≤ 2 m³/day to ground or ≤ 5 m³/day to surface water without an Environmental Permit. Above these thresholds, a Permit is required.
BS EN 12566-3
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European standard for small treatment plants up to 50 PT. Performance basis for the 0.20 sizing factor.
Key paragraphs (you should know these by reference)
| Reference | What it covers |
|---|---|
| H2 §1.34 | Test hole geometry — 300 mm square, 300 mm below pipe invert. |
| H2 §1.35 | Pre-soak — fill, allow to seep away overnight. |
| H2 §1.36 | Test method — time the drop from 75% to 25% full, divide by 150 mm = Vp. |
| H2 §1.37 | 3 tests × 2 holes minimum. No abnormal weather. |
| H2 §1.38 | Vp must be 12–100 for a drainage field. |
| H2 §1.39 | Sizing: At = P × Vp × 0.25 (septic) or × 0.20 (treatment plant). |
| H2 §1.40 | Minimum distances — 50 m to drinking water, 10 m to watercourse, 1.2 m above HWL, etc. |
3. The method — step-by-step
This is what a competent surveyor does on site. Allow 2–3 days from start to finish.
Step 1 — Excavate at least 2 trial holes
- Each hole 300 mm square, dug to 300 mm below the proposed invert of the effluent distribution pipe.
- Hole bottom must be that exact size; the hole above can be enlarged for safe access.
- For deep installations, a 300 mm earth auger may be used.
- Remove all loose debris before testing.
- Two holes is the absolute minimum. Soil varies across a site; one hole tells you nothing.
Step 2 — Pre-soak overnight
- Fill the test section to at least 300 mm with water.
- Let it drain away overnight.
- Why: the surrounding soil saturates so the test result reflects long-term performance, not artificially fast initial absorption.
Step 3 — Run the test (3 times per hole)
- Refill the test section to at least 300 mm.
- Time, in seconds, the drop from 75% full to 25% full — i.e. a 150 mm drop.
- Repeat at least 3 times in each hole.
- Don't test in heavy rain, severe frost or drought. §1.37.
Step 4 — Calculate Vp
Vp = average time (seconds) ÷ 150 mm
If Vp is between 12 and 100, you have a viable site (subject to other checks). Outside that range, a drainage field is off the table.
Step 5 — Size the drainage field
At = P × Vp × k where k = 0.25 (septic) or 0.20 (treatment plant)
P is persons served. At is the total infiltrative area in m². Trench length = At ÷ trench width.
4. Live calculator
Plug in your test results — the calculator runs the maths and tells you whether you've passed.
Catalyst BS 6297 / Part H2 Percolation Calculator
Test results — minutes from 75% to 25%
Hole 1
Hole 2
Hole 3
Test 1 (min)
Test 2 (min)
Test 3 (min)
Site avg time
— s
Vp
— s/mm
Required area
— m²
Trench length @ 0.6m
— m
Trench lengths at other widths: 0.30 m → divide area by 0.3 ; 0.90 m → divide area by 0.9.
5. Decision tree — what does my Vp mean?
Click through to see what action your Vp result drives.
Suitability checks beyond Vp
A passing Vp is necessary but not sufficient. Even with Vp = 50, the site fails if any of these are missed:
| Check | Minimum | What goes wrong if breached |
|---|---|---|
| Distance to drinking-water well/borehole | 50 m | Public Health Act / Water Resources Act offence; potential serious health risk. |
| Distance to watercourse | 10 m | Pollution offence; EA enforcement. |
| Distance to building | 15 m | Odour complaints; foundation undermining. |
| Distance to property boundary | 5 m | Neighbour disputes; access issues for emptying. |
| Distance to water supply pipe | 10 m | Cross-contamination risk. |
| Vertical clearance above HWL | 1.2 m | Field floods in winter; effluent surfaces and runs off. |
| Slope of ground | ≤ 15° | Trenches surcharge at downhill end; uneven distribution. |
⚠ Highest known water table — assess in winter
Test holes dug in summer can be misleading. The 1.2 m clearance must be assessed against the WORST-CASE water table — look for tide marks, vegetation indicators, neighbouring borehole logs, or run a winter trial pit observation.
Common pitfalls in claims
❌ No overnight pre-soak
Test on dry soil → artificially fast Vp → field undersized → surcharges within 6–12 months. We see this on at least one claim a month.
❌ Single trial hole
The surveyor has cut corners and you can't validate the design. Always question a design with one hole only.
❌ Wrong sizing factor
Using 0.20 for a septic tank effluent gives a 20% undersized field. Septic and treatment plant effluents are not interchangeable.
❌ Surface water cross-connected to foul
Rainwater into a foul drainage field overloads it instantly. Always check downpipes during a failure investigation.
❌ Ignoring General Binding Rules
A property serving more than ~13 occupants exceeds the 2 m³/day GBR limit and needs an Environmental Permit — regardless of whether the percolation test passes.
Resources
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