Soakaway crates: what they cost, how many you need for a roof, and why no installer will put them behind a septic tank

A soakaway crate is a plastic milk crate the size of a suitcase, and the whole product category rests on one number: it is roughly 95% empty. Fill a hole with broken bricks and about 30% of it is air the water can occupy; stack it with crates and 95% is. That is the entire case for them, and it is a good one, for rainwater. The trouble starts when the same box gets sold, or found on YouTube, as a cheap way to get rid of what comes out of a septic tank.

👉 For your own ground, size the drainage field from your percolation test.

What the box actually is

The standard domestic crate is 1.0 m long, 0.5 m wide and 0.4 m high, which is 0.2 m³ of storage each, rated by how much can drive over it. Naylor’s AquaVoid Home and EnviroCrate are 20-tonne crates, Brett Martin’s cellular crate 55 tonnes at 1200 × 600 × 347 mm, Hydrocell 62 tonnes. Polypipe’s Polystorm PSM1A is the one most builders’ merchants stock. You wrap the stack in a non-woven geotextile so silt stays out and water gets through, pipe the downpipe or the drive gully into the top crate through a silt trap, and backfill with pea shingle before the topsoil.

Polypipe’s own literature puts it the same way: only a 30% void ratio with traditional rubble-filled soakaways against 95% with Polystorm, and JDP quotes 96% for its Rainbox. A one-cubic-metre hole with four crates in it stores three times what the same hole full of hardcore does, which is why nobody reputable does rubble any more, and why a crate that costs forty pounds is cheaper than the digger time it saves.

What they cost in September 2026

Prices are public and they do not vary much between the products; what varies is whether VAT is shown.

EasyMerchant sells wrapped kits with the geotextile and tape from £175 excluding VAT. Screwfix, which most people search first, does not carry a modular soakaway crate at all: a search returns gully grids, a condensate soakaway and tool crates. Squote’s trade guide gives the same band as the retailers, £45–£90 per 0.4–0.5 m³ crate, and puts a mini digger with operator at £250–400 a day.

The crates are a fifth of the bill. A Reddit groundworker broke a small job down for a buyer who thought the quote was high:

That matches the forum quote of £900 for a patio-and-garage soakaway including materials and micro digger hire, and sits under Clark Drainage’s £1,500–£2,500 for a typical crate installation. MyJobQuote’s £660–£1,200 for a domestic replacement is the DIY-plus-digger end. Anyone quoting the crates alone as the job is leaving out the part that takes the day.

How many, and the rule that decides it

The forums have a yardstick and the trade has two calculations, and they agree closely enough to plan with.

Kalsi Plastics calculates 1.13 m³ of storage for a 50 m² roof; Squote gets about 1.5 m³ for the same 50 m² and recommends one 1 m³ plastic crate as the typical domestic answer. Approved Document H does not give a table. Section 3.27 says soakaways should be designed to a return period of once in ten years, that the design should be run with storms of differing durations to find the one needing the most storage, and that for small soakaways serving 25 m² or less a design rainfall of 10 mm in 5 minutes may be assumed as the worst case. Above 100 m² of roof it points you to BRE Digest 365, the infiltration test, and above that to BS EN 752-4.

The number that actually decides it is not the roof. It is the ground. Section 3.28 sends you to the same percolation test as a drainage field, dig a hole, fill it, time the fall, and the percolation calculator turns the seconds into a value. Squote’s threshold: a soil infiltration rate below 1 × 10⁻⁶ m/s and the soakaway is not viable; Kalsi’s: a percolation value over 300 seconds and stop. Clay does not care how many crates you buy. The rules that a sewage drainage field has to meet, and why crates are not one, are in septic tank soakaway regulations 2020.

Siting: the 5 m that is law and the 2.5 m that is not

Approved Document H section 3.25 lists where an infiltration device may not go: within 5 m of a building or road or in areas of unstable land; in ground where the water table reaches the bottom of the device at any time of the year; too close to any drainage field, mound or other soakaway so that the soakage capacity of the ground is exceeded; and anywhere contaminated runoff could pollute groundwater. Four rules, all quotable.

The 2.5 m from a boundary that Clear Stream and Squote attribute to the same document is not in it. Some councils do set a boundary distance, and they do not agree with each other: Forest of Dean asks 2 m, CNC 4.5 m, Scottish Borders 5 m. Ask yours; do not assume the number from a contractor’s website is statutory.

The certain distance from the surface has numbers. JDP’s Rainbox data: a minimum 0.30 m of cover under pedestrian areas, 0.50 m under vehicles up to 3 t, 1.20 m under vehicles up to 12 t, and no compactor run directly over the crates. Polypipe asks 0.5 m minimum over Polystorm Lite. Under that, two things happen, and both are visible from the kitchen window.

The other is silt. Kalsi warns that within 5–10 years a crate soakaway without geotextile can lose significant storage volume, and the fix is a silt trap on the inlet, a chamber that costs less than one crate and is the item most DIY installs skip.

The one thing crates cannot legally do

Search results, and a surprising number of videos, blur two products that the law keeps apart. One YouTube channel films crates going in as a temporary outlet for a treatment plant; the comments under it are the honest part.

In England the answer is in the general binding rules: you cannot meet them if you are using “a soakaway (designed for draining rainwater) installed after December 2007” for a sewage discharge. The industry guidance written for WWF-UK by Artesia Consulting removes any ambiguity: settle the solids, then discharge the liquid to ground via a correctly designed and constructed drainage field, NOT a soakaway pit, soakaway crates, tunnels or Ezy Drain. Graf, which makes the tanks, says crates aren’t legal for the job because the water would dissipate into the ground in a more concentrated area. The point of a drainage field is the opposite of a crate: it spreads effluent thinly through aerated soil so the soil treats it, at least 15 m from a building, 10 m from a watercourse and 50 m from a groundwater abstraction. A crate concentrates water in one void as fast as possible.

Wales says the same through Natural Resources Wales: where a septic tank or small sewage treatment plant discharges to ground, the wastewater must pass through a suitable drainage field designed and installed in line with BS 6297. Scotland registers every discharge with SEPA and, confusingly, calls the drainage trenches a soakaway, defined as a sealed system of gravel-filled trenches, not a crate, with the highest winter water table more than one metre below its base and a minimum area of 15 m². The one place Approved Document H itself uses the word for sewage is §1.38: where the ground is outside the Vp band and a proper secondary treatment stage exists, it may still be possible to discharge the treated effluent to a soakaway. Note the order: treatment first, and only then ground that failed the drainage-field test. That is a design decision for an engineer with a treatment plant, not a reason to put four crates behind a septic tank.

Doing it once

Dig the test hole first, in winter if you can, because a soakaway that works in August and floods in February is the usual story. Keep 5 m from the house and the road, more from a neighbour if the council asks. Size to the roof with the 1 m³ per 30–50 m² yardstick, then check it against a ten-year storm. Wrap the crates, fit the silt trap, cover them with at least 0.3 m, half a metre if anything drives there, and mound the backfill because it will settle. Then leave the septic tank out of it entirely.