Drainage field, not soakaway: the half of your septic system that actually treats the sewage
- A rainwater soakaway is not a drainage field, and the general binding rules exclude one “installed after December 2007”. The words get used interchangeably; the law does not.
- The drainage field is where treatment happens. The tank only settles — “Septic tanks should only be used in conjunction with a form of secondary treatment” (AD H §1.15).
- The size comes from the ground, not the household: At = p × Vp × 0.25. Four people on a Vp of 40 need 40 m²; on a Vp of 80 they need 80 m².
- Outside Vp 12–100 a drainage field is not allowed at all. Under 12 the effluent reaches groundwater too fast; over 100 the ground will not take it.
- Groundwater ends more fields than clay does: it “should not rise to within 1m of the invert level”. Test in summer and you are measuring the wrong season.
When a soakaway has to be rebuilt, the quote arrives as one number and it is worth seeing it disassembled. One published breakdown lists trenching and excavation at £1,500 to £4,000, perforated pipework and distribution at £800 to £2,500, gravel or crate systems at £1,000 to £3,000, and geotextile membranes and finishing at £300 to £1,000. Around that sit the items people forget: waste disposal at £300 to £1,500 and professional fees at £300 to £1,200. A new septic tank, if one is needed at the same time, is put at £1,200 to £3,500 for supply alone.
The same market gives the assembled figures. A full replacement is put at between £6,000 and £18,000, and installed cost is given as £6,000 to £12,000 for a septic tank system and £9,000 to £18,000 for a sewage treatment plant. The spread is not vagueness: it is the difference between reusing a serviceable tank and replacing the lot, and between easy ground and difficult ground.
Waste disposal at £300 to £1,500 and professional fees at £300 to £1,200 are the items that turn a builder's estimate into a final invoice. Neither is optional: the spoil from a trench has to go somewhere, and the design and sign-off of a drainage field is not a job a groundworker does for free. When a quote omits both, it is not cheaper — it is incomplete, and the difference reappears later as an extra.
Add the four groundwork lines together and they outweigh the tank in almost every scenario. That is the practical argument for protecting the field rather than the tank: the tank is a manufactured object with a price list, and the field is a piece of engineering priced by the metre in whatever ground you happen to own.
Start with a word problem, because it is the source of half the trouble on this subject. British people say soakaway for two different things:
👉 For your own ground, size the drainage field from your percolation test.
- a pit or crate system that swallows rainwater off a roof or drive;
- the drainage field that treats the effluent leaving a septic tank.
The regulations do not confuse them, and neither does the Environment Agency. The general binding rules exclude, in as many words, “a soakaway (designed for draining rainwater) installed after December 2007”. If someone connected your septic tank into the surface water soakaway, you do not have a drainage field. You have a pipe to nowhere with a legal problem attached.
So: this article is about the drainage field. gov.uk defines it plainly — “A drainage field, also known as an infiltration system, is a series of pipes with holes placed in trenches” — and Approved Document H tells you exactly how to build one. It is the half of the system that does the actual work, and the half nobody looks at until it fails.
Why it exists at all
What leaves the tank is settled sewage — clearer, still sewage. The tank’s own job ends at settlement. The treatment happens when that liquid trickles through unsaturated soil and the soil’s own biology takes the organic matter apart. The pipes do not treat anything either. The ground treats it. The pipes just spread it out thin enough for the ground to cope.
Once you see it that way, the whole design makes sense: the field’s size is a measure of how much ground you need to borrow, and that depends entirely on how fast your particular ground works.
Before you dig: the colour test
§1.31 hands you a free preliminary assessment, and almost nobody uses it:
“Well drained and well aerated subsoils are usually brown, yellow or reddish in colour. Examples of subsoils with good percolation characteristics are sand, gravel, chalk, sandy loam and clay loam. It is important that the percolation characteristics are suitable in both summer and winter conditions. Poorly drained or saturated subsoils are often grey or blue in colour. Brown and grey mottling usually indicates periodic saturation. Examples of subsoils with poor percolation characteristics are sandy clay, silty clay and clay.”
Brown, yellow, reddish: promising. Grey or blue: the ground is telling you it spends part of the year underwater. Mottling — brown and grey patches together — is the most useful sign of all, because it is a record of periodic saturation. The soil is showing you last winter.
§1.32 adds two more free indicators: consult the Environment Agency and the local authority early, and look at what grows there. Rushes and reeds are not a landscaping choice.
The percolation test, exactly as written
This is the bit that gets improvised, and improvising it produces a number that means nothing. Four paragraphs, in order:
§1.34 — the hole. “A hole 300mm square should be excavated to a depth 300mm below the proposed invert level of the effluent distribution pipe.” Deep drains get a concession: the hole “may be enlarged above the 300mm level to enable safe excavation”, or you use “a 300mm earth auger”.
§1.35 — the soak. “Fill the 300mm square section of the hole to a depth of at least 300mm with water and allow it to seep away overnight.”
That step is not optional and it is the one most often skipped. Dry ground drinks the first fill greedily and gives you a flattering Vp. The overnight soak brings the soil to the condition it will actually be in when your household is using it every day.
§1.36 — the measurement. “Next day, refill the test section with water to a depth of at least 300mm and observe the time, in seconds, for the water to seep away from 75% full to 25% full level (i.e. a depth of 150mm). Divide this time by 150. The answer gives the average time in seconds (Vp) required for the water to drop 1mm.”
So Vp is seconds per millimetre. If the water takes 6,000 seconds to fall 150mm, your Vp is 40.
§1.37 — the honesty clause. “The test should be carried out at least three times with at least two trial holes. The average figure from the tests should be taken. The test should not be carried out during abnormal weather conditions such as heavy rain, severe frost or drought.”
Three runs, two holes, averaged. One hole tested once in August is not a percolation test; it is an anecdote with a number attached.
Run your own figures through the Vp and drainage field calculator — it uses the regulation’s own formula and band.
The band: 12 to 100, and why both ends are hard limits
“1.38 Drainage field disposal should only be used when percolation tests indicate average values of Vp of between 12 and 100 and the preliminary site assessment report and trial hole tests have been favourable. This minimum value ensures that untreated effluent cannot percolate too rapidly into groundwater. Where Vp is outside these limits effective treatment is unlikely to take place in a drainage field.”
The top end surprises nobody: clay will not take the water. The bottom end surprises everybody. Gravel that drains instantly sounds like the perfect site and is disqualified, because the effluent reaches groundwater before the soil has had time to treat it. The document says why in one clause: the minimum “ensures that untreated effluent cannot percolate too rapidly into groundwater.”
Fast ground is not good ground. It is a short circuit to the aquifer.
Land outside the band at either end and a drainage field is off the table, whatever you spend on it. What remains is a treatment plant, which does the biology itself rather than borrowing your soil’s.
The formula, and what it tells you about your plot
“1.44 Drainage fields should be set out as a continuous loop fed from the inspection chamber (see Diagram 1). To calculate the floor area of the drainage field (At in m²), the following formula should be used: At = p x Vp x 0.25 where p is the number of persons served by the tank, Vp is the percolation value (secs/mm)“
| People | Vp 15 (sandy) | Vp 40 (loam) | Vp 80 (heavy) |
|---|---|---|---|
| 2 | 7.5 m² | 20 m² | 40 m² |
| 4 | 15 m² | 40 m² | 80 m² |
| 6 | 22.5 m² | 60 m² | 120 m² |
Read across a row and the point lands: the household is fixed, the ground is the variable. The same four people need 15 m² on sand and 80 m² on heavy soil. Which is why “how big is a drainage field?” has no answer until someone has dug a hole and timed water — and why sizing the tank is the easy half of the job. Bedrooms give you the tank in an afternoon; only the ground gives you the field.
The trenches, and the two metres between them
“1.42 Trenches should be filled to a level 50mm above the pipe and covered with a layer of geotextile to prevent the entry of silt. The remainder of the trench can be filled with soil; the distribution pipes should be laid at a minimum depth of 500mm below the surface. Drainage trenches should be from 300mm to 900mm wide, with areas of undisturbed ground 2m wide being maintained between parallel trenches (see Diagram 1).”
Three numbers worth holding on to:
- Geotextile above the stone. Silt from the soil above is what blinds a field from the top down. The membrane is not packaging, it is the filter that keeps your own garden out of the drainage stone.
- 500mm minimum cover. Deep enough to be undisturbed, shallow enough to stay in the biologically active, oxygenated layer. A field buried too deep suffocates.
- 2m of undisturbed ground between trenches. This is the one that catches people squeezing a field into a small plot. The gaps are not spare land — the effluent spreads sideways as well as down, and trenches too close together simply saturate each other.
“1.43 An inspection chamber should be installed between the septic tank and the drainage field.”
That chamber is the cheapest diagnostic you will ever own. It is where you can see what is leaving the tank before it enters the field — and if solids are showing up there, you have found your future field failure while it is still cheap.
Groundwater: the thing that actually kills fields
“1.33 A trial hole should be dug to determine the position of the standing groundwater table. The trial hole should be a minimum of 1m2 in area and 2m deep, or a minimum of 1.5m below the invert of the proposed drainage field pipework. The groundwater table should not rise to within 1m of the invert level of the proposed effluent distribution pipes.”
A drainage field works because effluent passes through unsaturated soil, where oxygen and biology live. Raise the water table to within a metre of the pipes and that zone disappears: the effluent is no longer trickling through soil, it is joining groundwater. Nothing gets treated, and the field behaves as if the soil had turned to clay overnight — because functionally it has.
What to do
- Establish which thing you have. Rainwater soakaway or drainage field. If the tank feeds a rainwater soakaway installed after December 2007, that is a compliance problem, not a plumbing preference.
- Look at the soil colour before you pay anyone. Grey, blue or mottled changes the conversation.
- Insist on the real test: overnight soak, 75% to 25%, ÷150, three runs, two holes, averaged, not in freak weather. A cheap test is worse than no test, because you build on it.
- Check the winter water table, not August’s.
- Size it with the formula, not with the space you happen to have. If At says 80 m² and you have 40, the answer is not a smaller field.
- Fit the inspection chamber and actually look in it once a year. It tells you the field’s future.
Published breakdowns put trenching at £1,500 to £4,000, pipework at £800 to £2,500 and gravel or crates at £1,000 to £3,000.
Membranes and finishing at £300 to £1,000, waste disposal at £300 to £1,500 and professional fees at £300 to £1,200.
Between £6,000 and £18,000, with installed cost given as £6,000 to £12,000 for a septic system.
Its installed cost is put at £9,000 to £18,000.
No: supply of a new tank is £1,200 to £3,500, less than the groundworks around it.
Protect the field. It is the half priced by the metre in your own ground.
Waste disposal and professional fees, at £300 to £1,500 and £300 to £1,200.
Not necessarily: it often means an incomplete list, and the difference returns as an extra.
Access, depth and the soil itself, which is why the same design costs differently on two sites.
Frequently asked questions
Is a soakaway the same as a drainage field?
No, and the distinction is legal, not pedantic. A drainage field is “a series of pipes with holes placed in trenches” designed to spread septic tank effluent through soil for treatment. A soakaway is built to swallow rainwater. The general binding rules explicitly exclude “a soakaway (designed for draining rainwater) installed after December 2007” — if your tank was plumbed into the surface water soakaway, that is not compliant, whatever it looks like from above.
How do I do a percolation test?
Approved Document H §1.34–1.37. Dig a hole 300mm square to 300mm below the proposed pipe invert. Fill it to at least 300mm and let it seep away overnight — that wets the ground so the real test is not flattered. Next day, refill to 300mm and time in seconds how long the water takes to fall from 75% full to 25% full (a drop of 150mm). Divide that time by 150: the answer is your Vp, the seconds it takes to fall 1mm. Do it “at least three times with at least two trial holes”, average them, and never in heavy rain, severe frost or drought.
How big does my drainage field need to be?
At = p × Vp × 0.25, in square metres, where p is people and Vp is your percolation value. Four people on a Vp of 40 need 40 m². The same four people on a Vp of 80 need 80 m². The household is fixed; the ground is what moves the number.
What Vp is acceptable?
Between 12 and 100. §1.38: drainage field disposal “should only be used when percolation tests indicate average values of Vp of between 12 and 100”. Below 12 the ground drains too fast for treatment to happen — the limit exists so “untreated effluent cannot percolate too rapidly into groundwater”. Above 100, “effective treatment is unlikely to take place in a drainage field”.
What soil is good for a drainage field?
§1.31 gives you a colour test before you dig anything: “Well drained and well aerated subsoils are usually brown, yellow or reddish in colour”, and good examples are “sand, gravel, chalk, sandy loam and clay loam”. Bad news looks like this: “Poorly drained or saturated subsoils are often grey or blue in colour. Brown and grey mottling usually indicates periodic saturation” — and the poor performers are “sandy clay, silty clay and clay”.
Are soakaways and drainage fields the same thing?
No, and the distinction matters legally. Rainwater soakaways take clean water from roofs and hard surfaces; a drainage field takes the effluent leaving a septic tank and treats it in the soil. The two are built differently, sized differently and governed by different rules, which is why sending septic effluent to a rainwater soakaway is not a shortcut but a breach. If somebody quotes you for soakaways when the job is a septic system, ask which of the two they mean.
How deep is the trial hole?
§1.33: “a minimum of 1m2 in area and 2m deep, or a minimum of 1.5m below the invert of the proposed drainage field pipework”. Its job is to find the standing groundwater table, which “should not rise to within 1m of the invert level of the proposed effluent distribution pipes”.
Researcher & editor, off-mains drainage
Writes independent guides on septic tanks, cesspits and sewage treatment plants for homes off the mains. Cross-checks the general binding rules and the Environment Agency, SEPA, NRW and NIEA against real prices, British Standards and what owners actually report on the forums.