Soakaway Crates UK: Sizes, Prices and Installation

Worker in hi-vis kneeling to place black soakaway crates in a geotextile-lined garden pit

Soakaway Crates: Sizes, Cost and How They Are Installed

Written by the Urgent Drainage Solutions teamPublished 16 October 20268 min read

Quick answer

Soakaway crates are plastic boxes, usually 1 m × 0.5 m × 0.4 m, that are about 95% empty space, so each holds roughly 190 litres of rainwater. They are wrapped in geotextile, buried at least 5 m from buildings and sized from a BRE 365 soakage test. Supply-only crates cost from about £38 + VAT each at Drainfast.

A standard 1 m × 0.5 m × 0.4 m crate with a 95% void ratio stores about 190 litres; a rubble pit stores about 30% of its volume.
The number of crates comes from the area drained, the design storm and your ground's infiltration rate, measured with a BRE 365 trial pit.
A crate soakaway must also half-empty within 24 hours, which a long, narrow layout helps on slower ground.
Cover depth and crate strength depend on load: Drainfast gives 150 mm for gardens and 350 mm for light traffic as minimums.
Approved Document H says infiltration devices should not be built within 5 m of a building or road.

Soakaway crates are plastic boxes, most commonly 1 m × 0.5 m × 0.4 m, that are about 95% empty space. Each one stores roughly 190 litres of rainwater while it soaks into the ground, around three times what the same hole filled with rubble would hold. They must be wrapped in geotextile, sized from a soakage test and kept at least 5 m from buildings and roads. If you need one designed and built, our soakaway installation service starts every job with a percolation test.

This guide goes deeper than our general guide to what a soakaway is. It covers crate sizes, a worked sizing example, load ratings, prices, installation and upkeep.

What sizes do soakaway crates come in?

Most domestic crates are 1 m long, 0.5 m wide and 0.4 m high, a gross volume of 0.2 m³. Larger units of 1.2 m × 0.6 m are common on bigger systems. Crates clip together side by side and can be stacked in layers, so the soakaway can be shaped to fit the site.

Crate (manufacturer data)	Size	Storage per unit	Void ratio	Strength
Polypipe Polystorm-R (PSM1A)	1.0 m × 0.5 m × 0.4 m	190 litres	95%	Light/medium duty
Wavin AquaCell NG, standard	1.2 m × 0.6 m × 0.4 m (425 mm with base plate)	0.306 m³, then 0.288 m³ per extra layer	94.6% minimum	438 kN/m² vertical, short term
Wavin AquaCell NG, high strength	1.2 m × 0.6 m (460 mm high)	0.331 m³	92.5%	621 kN/m² vertical, short term
Drainfast Storm Water Crate (SWC20)	1 m × 0.5 m × 0.4 m	Not stated	Not stated	20 tonne loading

Figures come from JDP's Polystorm-R listing, Wavin's AquaCell NG technical data sheet and Drainfast's product page. Always use the data sheet for the exact crate you buy, because void ratio and strength vary between products.

How do crates compare with a rubble-filled soakaway?

Crates hold far more water for the same size of hole. A crate with a 95% void ratio stores 0.95 m³ in every cubic metre, while CNC Building Control's guidance assumes clean rubble leaves only about 30% voids. To store the same water, a rubble pit has to be roughly three times larger.

That matters in a small garden where the 5 m rule already limits where a soakaway can go. Rubble also has a second weakness: silt settles in the gaps between stones and cannot be cleaned out, which is why old rubble pits often end up rebuilt rather than repaired. Our guide to a blocked soakaway covers that in detail.

How many soakaway crates do I need?

The number comes from three things: the area draining to the soakaway, the design storm, and how fast your ground absorbs water. Approved Document H (para 3.29) says the storage should hold the difference between what flows in during the storm and what soaks out over the same time. BRE Digest 365 sets out how to measure the infiltration rate with a trial pit and how to design from it.

A worked example (illustrative only)

The numbers below are assumptions chosen to show the method. They are not a design for your property, which needs a site test and local rainfall data.

Step	Assumption or calculation	Result
Area drained	Roof of 60 m²	60 m²
Design storm	Assume 15 mm of rain in 15 minutes	0.015 m
Inflow	60 m² × 0.015 m	0.90 m³
Layout	6 crates, 1 m × 0.5 m × 0.4 m, in one layer, 3 m × 1 m	1.14 m³ storage
Side area at half depth	Perimeter 8 m × 0.2 m (base ignored, as a cautious assumption)	1.6 m²
Infiltration rate	Assume 1 × 10⁻⁵ m/s from a trial pit	0.00001 m/s
Outflow during storm	1.6 m² × 0.00001 m/s × 900 seconds	0.01 m³
Storage needed	0.90 − 0.01 m³	0.89 m³, about 5 crates
Half-emptying time	Half of 1.14 m³ ÷ (1.6 m² × 0.00001 m/s)	About 9.9 hours

Five crates would just hold the storm, so six gives a margin. The second check matters as much as the first: BRE 365 expects a soakaway to fall from full to half-full within 24 hours, ready for the next storm. If the ground drained ten times more slowly, the same six crates would take about 99 hours to half-empty and fail that test. The usual answer is a longer, narrower layout with more side area, or accepting that a soakaway will not work there.

A real design repeats the calculation for storms of different lengths, because Approved Document H (para 3.27) says the duration that gives the largest storage volume should be used, with a return period of once in ten years. For areas of 25 m² or less, it allows a simple design rainfall of 10 mm in 5 minutes.

What load rating and cover depth do crates need?

It depends on what will be above them. A crate under a lawn only carries soil and people, while one under a drive carries vehicles, so stronger crates and deeper cover are needed.

Location	Minimum cover (Drainfast guide)	What to check
Garden or lawn	150 mm	Light-duty crate is usually enough
Light traffic	350 mm	Crate rated for vehicles, deeper cover
Vehicles up to 9 tonnes gross	1,000 mm	Heavy-duty crate and a structural check

These figures come from Drainfast's installation guide; other makers set their own, so follow the instructions for your crate. JDP's listing for the Polystorm-R says installation depths vary with application and ground, and recommends a structural design check in line with CIRIA C680 before installation. A deeper soakaway also adds load from the soil above, which is another reason to check the maximum depth on the data sheet.

How is a crate soakaway installed?

A crate soakaway is installed in a lined pit below the incoming pipe, with the crates fully wrapped before backfilling. The sequence below follows Approved Document H, BRE 365 and Drainfast's installation guide:

Cross-section of a crate soakaway: downpipe, silt trap and geotextile-wrapped crates away from the house — A crate soakaway: a silt trap catches debris before the geotextile-wrapped crates. Not to scale.

Test the ground with a BRE 365 trial pit at the planned depth, and check the winter water table.
Set out the position at least 5 m from any building or road, as Approved Document H (para 3.25) requires, and clear of buried services. Drainfast advises a greater distance if the ground slopes towards the building.
Excavate 150 mm larger than the crates on every side, supporting the trench sides where needed.
Lay the base with 150 mm of 20 mm pea shingle or sharp sand, levelled and compacted.
Line the pit with non-woven geotextile, place and clip the crates, then fold the membrane over the top with no gaps.
Connect the inlet through a silt trap so leaves and grit are caught before they reach the crates.
Backfill the sides with 150 mm of sharp sand or pea shingle, compacted in layers.
Cover to at least the depth the crate maker specifies for the load above, then reinstate the surface.

New drainage work usually needs building control approval, so check with your council or an approved inspector before starting.

How much do soakaway crates cost?

Supply-only crates cost tens of pounds each, but the crates are only a small part of the job. On 16 October 2026, Drainfast listed these prices:

Item (Drainfast)	Price ex VAT	Price inc VAT
Storm Water Crate, 20 tonne, 1 m × 0.5 m × 0.4 m	£38.16	£45.79
Hydrocell Storm Water Crate, heavy duty, 1.0 m × 0.5 m × 0.4 m	£50.63	£60.76
Kit of 4 × 20 tonne crates with geotextile and tape	£250.38	£300.46
Kit of 6 × 20 tonne crates with geotextile and tape	£318.84	£382.61
Kit of 8 × 60 tonne crates with geotextile and tape	£499.18	£599.02

For a fitted soakaway, MyJobQuote's guide, last updated 6 October 2026, gives £660 for a 190-litre crate system and £1,200 for 950 litres, with a typical domestic soakaway around £850. It lists a percolation test at £150 to £300 and a building control application at £200 to £400. These are national figures from other companies, not our prices. We do not publish a price for soakaway installation, because the size depends on the soakage test, so we quote after a site visit and test.

How do you maintain and inspect a crate soakaway?

Most maintenance happens upstream of the crates. Drainfast's guide says the silt trap's mesh bucket needs periodic emptying, and keeping gutters and gully grates clear stops leaves reaching it. Our guide on how often gutters should be cleaned covers the roof side.

Check whether the crate you choose can be inspected. Wavin's data sheet lists open channels for inspection and maintenance on AquaCell NG; with other crates, the silt trap or an inspection chamber may be the only access point. Watch for water backing up at gullies in heavy rain or a damp patch over the soakaway, both early signs of a problem.

How do crates compare with other options?

Crates suit most domestic soakaways, but they are not the only choice, and on poor ground none of the infiltration options will work.

Option	Best for	Main limitation
Crate soakaway	Most homes with free-draining ground	Needs correct wrap, cover and load rating
Rubble-filled pit	Small areas where space is not tight	About 30% void, cannot be cleaned once silted
Perforated ring or chamber	Sites where access for cleaning matters	Heavier and slower to install
Sealed attenuation tank	Ground that will not drain	Needs an approved outlet with controlled discharge
French drain or trench	Spreading water along a garden	Stores less water than a crate soakaway

For wet lawns rather than roof water, see our guide to French drains.

When to call a professional

Call a drainage contractor if you are adding an extension, garage or drive with nowhere for the rain to go, if your trial pit barely drains, or if the soakaway has to sit under a driveway. Those are the cases where sizing, load rating and position matter most. Through our soakaway installation service we run a BRE 365 percolation test, size the crates from the result, and excavate, install and reinstate the ground. If the test shows a soakaway will not work, we will tell you and set out the alternatives.

We cover Nottinghamshire, Derbyshire, Leicestershire and Lincolnshire, with no call-out fee. Get a quote or call 07300 600 950.

Sources

Frequently Asked Questions

A common 1 m × 0.5 m × 0.4 m crate has a gross volume of 0.2 m³. With a 95% void ratio, which is the figure given for the Polypipe Polystorm-R, it stores about 190 litres. Larger units hold more: Wavin's 1.2 m × 0.6 m AquaCell NG gives 0.306 m³ for a single layer with its base plate.

It depends on the roof area, the design storm and how fast your ground drains, so it should be worked out from a BRE 365 trial pit. In our illustrative example a 60 m² roof needs about 0.89 m³ of storage, or five to six standard crates, but slow-draining ground can need many more or rule a soakaway out.

Deep enough for the incoming pipe to fall into the top of the crates, with the cover the crate maker specifies above them. Drainfast's guide gives minimum cover of 150 mm in gardens, 350 mm under light traffic and 1,000 mm for vehicles up to 9 tonnes gross. The base must stay above the winter water table.

Yes. Crates are wrapped in a non-woven geotextile membrane so that soil and silt cannot wash into the open cells while water can still pass out into the ground. Without it the crates gradually fill with fines and the soakaway loses capacity. Check there are no gaps in the wrap before backfilling.

Only if the crates are rated for it and buried with enough cover. Drainfast sells 20 tonne and 60 tonne crates, and Wavin gives a short-term vertical strength of 438 kN/m² for AquaCell NG. Under a driveway, a structural check in line with CIRIA C680 is recommended before installation.

Cite this guide: “Soakaway Crates: Sizes, Cost and How They Are Installed”, Urgent Drainage Solutions, 16 October 2026. https://urgentdrainagesolutions.co.uk/blog/soakaway-crates