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 box, usually 1.0 × 0.5 × 0.4 m, with about 95% void space; a rubble pit manages 30%. Same hole, three times the storage.
- Retail 2026: £37.50–£54.90 per crate including VAT (Naylor, Polypipe, Hydrocell, Brett Martin, EnviroCrate). Installed, a small domestic soakaway is quoted at £900 on a forum and £1,500–£2,500 by a drainage firm.
- Approved Document H: not within 5 m of a building or road, never where the water table reaches the base, designed for a once in ten years storm. The 2.5 m boundary rule is not in the document.
- They are for rainwater. England's general binding rules exclude “a soakaway (designed for draining rainwater) installed after December 2007” for sewage, and the industry guidance says it in capitals: NOT soakaway crates.
The arithmetic that sells soakaway crates is the void ratio. Polypipe's Polystorm literature puts a traditional rubble-filled soakaway at a 30% void ratio against 95% for the modular cell; JDP quotes 96% for its Rainbox 3S; a Reddit groundworker did the sum for a buyer, a rubble-filled 1 m³ pit is designed as 30% voids, 0.3 m³ effective, while the same hole with crates gives 0.95 m³. The standard domestic crate is 1.0 × 0.5 × 0.4 m, 0.2 m³ each, so four crates in two layers fill a cubic-metre hole and store three times what hardcore would. That is why no reputable installer builds a rubble soakaway any more, and why the crate itself is the cheapest line on the quote.
Prices in September 2026, per crate including VAT at Drainage Superstore: Naylor AquaVoid Home 20 t £37.50, Polypipe Polystorm PSM1A £42.00, Hydrocell 62 t £42.00, Brett Martin 55 t cellular crate 1200 × 600 × 347 mm £52.20, EnviroCrate 20 t £54.90. EasyMerchant lists the Naylor at £28.96, the Hydrocell at £31.67 and the Polystorm at £41.58 excluding VAT, with wrapped kits including geotextile and tape from £175. Screwfix does not stock a modular soakaway crate at all; its search returns gully grids, a condensate soakaway and tool crates. Squote's trade guide gives £45–£90 per 0.4–0.5 m³ crate and a mini digger with operator at £250–400 a day. What the crate does behind a house is explained in the drainage field guide, which is the thing it is not.
Reading a crate price list is easier than reading a quote. Every product in the table does the same job at the same volume; the load rating is the only spec that matters, and 20 t is more than any domestic drive will ever ask of it. What moves the bill is the wrapping, the silt trap, the skip and the digger, none of which appear on the product page, and the difference between a £37.50 crate and a £54.90 one is one bag of pea shingle. Buy on availability and delivery, not on brand, and put the saved money into the geotextile and the trap, which are the two items that decide whether the crates still store anything in ten years.
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.
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.
For comparison a rubble filled 1m3 soakaway is designed as 30% voids (0.3m3 effective volume) but a crate soakaway is 95% void (0.95m3 for the same hole). Crates are usually around 0.5m wide, 1.0m long, 0.4m high. In that hole you could 2 in 1 layer high, or 4 in 2 layers high. Either would give you more effective volume than 1m3 pit filled with rubble.
r/DIYUK, on a soakaway quotePolypipe’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.
| Crate | Load rating | Drainage Superstore, inc VAT | EasyMerchant, ex VAT |
|---|---|---|---|
| Naylor AquaVoid Home | 20 t | £37.50 | £28.96 |
| Polypipe Polystorm PSM1A | — | £42.00 | £41.58 |
| Hydrocell | 62 t | £42.00 | £31.67 |
| Brett Martin cellular, 1200 × 600 × 347 mm | 55 t | £52.20 | — |
| EnviroCrate | 20 t | £54.90 | — |
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:
Best part of 1k is skip and materials, 4 brett crates and wrap is most probably around £250 to £300, skip £350, drainage £100, boarding and sundries £50 to £100 soon adds up.
r/DIYUK, groundworkerThat 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.
| Product | Load | Inc VAT | Ex VAT (EasyMerchant) |
|---|---|---|---|
| Naylor AquaVoid Home | 20 t | £37.50 | £28.96 |
| Polypipe Polystorm PSM1A | — | £42.00 | £41.58 |
| Hydrocell | 62 t | £42.00 | £31.67 |
| Brett Martin cellular | 55 t | £52.20 | — |
| EnviroCrate | 20 t | £54.90 | — |
Inc-VAT figures from Drainage Superstore; wrapped kits with geotextile and tape from £175 ex VAT at EasyMerchant. Squote's trade band is £45–£90 per 0.4–0.5 m³ crate. Screwfix's search for soakaway crate returns no modular crate at all.
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.
General yardstick is 1 cube of soakaway for 30 sqm of roof.
Screwfix Community, Builders' TalkKalsi 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.
When I called him he confessed that it was still solid clay when he installed the soakaway crates.
mig-welding.co.uk forum, homeowner after a winter of floodingSiting: 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.
5m away from any building, the hole just needs to fit the crates in and the top of the crates needs to be a certain distance from the surface. Also, the ground WILL subside there and become obvious there's one there so when you fill it, put a hump on it so it naturally flattens
r/DIYUKThe 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.
Mate you got nowhere enough soil on top of them crates. Your grass is never going to grow properly and you will always see the outline of the soak away. You want at least 300mm of topsoil.
YouTube · Skill Builder, Build a DIY Soakaway in a DayThe 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.
Jason, I like all your vids, I’m reading confusing info on soak away crates for septic tank, are they a legal or legal ?
YouTube · Land Claimer, what is a soakaway crateIn 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.
We are installing a treatment plant now and no installer will put crates in for treatment plant!
YouTube · The Restoration Couple, installing a soakawayWales 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.
Typically 1.0 m long, 0.5 m wide and 0.4 m high, 0.2 m³ of storage, rated 20 t to 62 t depending on brand; Brett Martin's is 1200 × 600 × 347 mm.
How much does one cost?£37.50 to £54.90 including VAT at Drainage Superstore in September 2026; £28.96 to £41.58 ex VAT at EasyMerchant.
Why crates instead of rubble?95% void against about 30%: the same hole stores three times as much water.
Does Screwfix sell them?A search for soakaway crate returns gully grids and tool crates, not a modular soakaway crate; builders' merchants and drainage specialists do.
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.
Soakaway crates are one of the few genuinely good cheap products in this trade: forty pounds of plastic that does the work of a cubic metre of rubble and a second day's digging. The whole problem is the name. Because the drainage trenches behind a septic tank were called soakaways for fifty years, and still are in Scotland, a box marketed as a soakaway crate reads as a shortcut for sewage, and every third video treats it as one. Buy them for the roof and the drive without hesitation. For the tank, the only crate-shaped thing you should be buying is a percolation test.
Frequently asked questions
How many soakaway crates do I need?
Rule of thumb from the trade: about 1 m³ of storage per 30–50 m² of roof, so a typical house roof of 50 m² wants one to two 1.0 × 0.5 × 0.4 m crates (0.2 m³ each) stacked or side by side — Kalsi Plastics calculates 1.13 m³ for 50 m², Squote about 1.5 m³. Approved Document H does not give a table; it says design for a once-in-ten-years storm using a percolation test, and for roofs of 25 m² or less assume 10 mm of rain in 5 minutes. Clay changes everything: if the test fails, no number of crates helps.
How much do soakaway crates cost?
Per crate, including VAT, at Drainage Superstore in September 2026: Naylor AquaVoid Home 20 t £37.50, Polypipe Polystorm PSM1A £42.00, Hydrocell 62 t £42.00, Brett Martin 55 t £52.20, EnviroCrate 20 t £54.90. EasyMerchant lists the Naylor at £28.96 and the Hydrocell at £31.67 excluding VAT, and wrapped kits with geotextile from £175. Add membrane, a silt trap, pea shingle, a mini digger at £250–400 a day and a skip; a forum quote for a complete small job was £900, Clark Drainage says £1,500–£2,500.
Can I use soakaway crates for a septic tank?
No. In England the general binding rules cannot be met with “a soakaway (designed for draining rainwater) installed after December 2007”; septic effluent must go to a drainage field built to BS 6297, and the Artesia report for WWF-UK spells it out: a correctly designed drainage field, NOT a soakaway pit, soakaway crates, tunnels or Ezy Drain. Graf, a tank manufacturer, says the same: crates concentrate the effluent in one spot instead of spreading it through aerated soil. Wales requires BS 6297 through Natural Resources Wales; Scotland registers with SEPA.
How deep do soakaway crates need to be buried?
Deep enough for the load above them. JDP's Rainbox figures: at least 0.30 m of cover for pedestrians, 0.50 m for vehicles up to 3 t, 1.20 m for vehicles up to 12 t; Polypipe quotes 0.5 m minimum for Polystorm Lite. Less than that and the grass dies in the outline of the crate, as more than one YouTube commenter has found. The base must sit above the highest winter water table — SEPA says more than one metre above it.
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.