Soakaway Crates: The Plastic Cubes Your Extension Roof Drains Into

You picked the wrong corner of the garden. The trench is dug, the crates are wrapped and lowered in, the pea gravel's piling up beside the spoil heap, and the building control officer arrives, takes one look, and tells you the soakaway is 4.2 metres from the back wall of the extension, not the 5 metres Approved Document H demands. Out it comes. Pay the groundworker again. Pay for another mini-digger day. Pay for replacement gravel because half of it's now mixed with clay. Soakaway crates themselves are cheap. Putting them in the wrong place is not.

What it is and what it's for

A soakaway crate is a hollow plastic cube, typically 1,000mm × 500mm × 400mm, made from recycled polypropylene and engineered with internal pillars that give it a 95% void ratio (95% of its volume is empty space for water). You bury several crates underground, wrap them in geotextile membrane (a permeable fabric that lets water through but blocks soil), connect a drainage pipe from your gutters to one side, and rainwater from your roof flows into the void and seeps slowly out into the surrounding soil. That's it. A modular underground sponge.

The job a soakaway does is called infiltration drainage. Building Regulations Approved Document H, Section H3, governs the disposal of surface water (rainwater) from any new roof. The hierarchy is fixed in law: infiltration (a soakaway) first, then a watercourse (stream or river), then a public sewer. You don't get to pick. You can only move down the hierarchy if you can demonstrate that the option above is not viable. For a new extension on a typical UK plot, infiltration is what your building control officer expects to see on the drawings.

Soakaway crates replaced the older approach of rubble-filled pits. Rubble has a void ratio of around 30%, so a rubble soakaway needs roughly three times the volume of a crate soakaway for the same water storage. Crates also have engineered flow channels that resist silt blockage, where rubble fills with sediment over time and gradually loses capacity. Modern crate systems have a 50-year design life when installed correctly. A rubble soakaway might give you 15-20 years before it stops working.

The applicable design standard for any soakaway serving more than 25m² of roof area is BRE Digest 365 (BRE = Building Research Establishment). Smaller installations can use the simplified rule of thumb. The two methods give different results and your building control officer may ask which you've used.

Where the regulations actually draw the line

The distances in Approved Document H and NHBC Standards 5.3.11 are not flexible suggestions. Get them wrong and the soakaway either causes the very problem it's meant to solve, or building control rejects the installation.

The 5-metre rule exists because water leaving the soakaway has to go somewhere. If the soakaway is too close to a building, the surrounding soil becomes saturated and water tracks back toward the foundation. Over years, that water destabilises the bearing soil, washes out fines, and in clay soils causes seasonal heave. Foundation movement caused by an undersized or wrongly-sited soakaway is the kind of defect that surfaces on a buyer's homebuyer survey and kills a sale.

Some building control officers have accepted 3 metres in genuinely constrained situations (small roof area, oak tree on one side, boundary on another). Treat that as an exception you negotiate in writing before excavation begins, not a default. If you have the space for 5 metres, use 5 metres.

The other rule that catches people: the soakaway must be on ground lower than the building, or sloping away from it. A soakaway uphill of the foundation creates a clear path for saturated soil water to migrate downhill toward the wall. NHBC will fail an installation that ignores this even if the 5-metre distance is met.

Will your ground actually drain? The percolation test

Soakaways only work in soil that lets water through. Sand, gravel, and chalk drain quickly. Clay barely drains at all. Before you order a single crate, you have to test the ground where the soakaway will go. This is non-negotiable for a building control submission and NHBC will require the result on file.

The test method is BRE Digest 365, also referenced as the test in BS 6297 (the simplified version used for stormwater soakaways). The procedure:

1. Dig a test hole 300mm square and 300mm deep below the proposed invert level (the bottom of the soakaway).
2. Pre-saturate the hole by filling it with water and letting it drain overnight.
3. Refill the hole to the 300mm mark.
4. Time how long it takes for the water to fall from the 75% mark (225mm) to the 25% mark (75mm). Record this in seconds.
5. Divide that time (in seconds) by 150 to get Vp, the soil percolation rate in seconds per mm.
6. Repeat at least three times in the same hole, then in two more holes at different points across the proposed soakaway footprint. Average the results.

Vp is the soil's percolation rate. Lower numbers mean faster drainage. A Vp of 30 means it takes 30 seconds for the water level to drop one millimetre. A Vp of 200 means clay-like ground that won't support a soakaway at all.

A professional percolation test from a drainage specialist or groundworker costs £150 – £300. The DIY route costs the price of a hosepipe and your weekend. If your building control officer will accept the result either way (most will, because the methodology is fixed), the DIY test is genuine money saved.

What if the test fails? If your Vp is above about 100 seconds per mm, infiltration drainage isn't going to work no matter how many crates you buy. Clay soils across much of central and southern England fall in this range. The fallback path is a controlled-discharge attenuation tank (similar crates but with a flow-restrictor outlet to a watercourse or sewer), permeable paving with sub-base storage, or a connection to the existing surface water drain. All three are more involved than a soakaway. Find this out before excavation, not after.

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Sizing: how many crates do you actually need?

For a typical kitchen extension (roof area under 100m² draining to one soakaway), the simplified BRE method works. The full BRE Digest 365 calculation is required for larger areas and incorporates a 1-in-100-year storm event plus a 40% climate-change uplift, but it's the same logic at higher precision.

The simplified rule, used by most council building control offices for domestic extensions:

Volume required (m³) = Roof area (m²) × (50mm/hr ÷ 3000)

The 50mm/hr is the design rainfall intensity. The 3000 is a safety factor. For a 50m² extension roof in good-percolating ground, that gives 0.83m³. Round up to 1m³.

That 1m³ figure is where the rule of thumb "1m³ per 50m²" comes from. It assumes good ground (Vp under 50). In poorer ground, you scale up. Wavin's manufacturer guidance for clay-edge soils is one crate per square metre of roof area, which is roughly five times the standard sizing. Your percolation test result tells you which end of the scale you're on.

A standard 190-litre crate (1m × 0.5m × 0.4m) is 0.19m³. So:

• 25m² roof → 0.42m³ → 3 crates
• 50m² roof → 0.83m³ → 5 crates (one 1m³ kit)
• 75m² roof → 1.25m³ → 7 crates
• 100m² roof → 1.67m³ → 9 crates
• 150m² roof → 2.5m³ → 14 crates

For pitched roofs, use the plan area (the footprint), not the developed surface area. Rain hits the plan area. A steep pitch has more roof surface than plan area, but the rain volume is determined by what falls vertically from the sky onto the building's footprint.

Add 10-20% to the calculated volume if your test came back marginal (Vp 60-100), or if the soakaway is sized at a single point rather than spread across multiple smaller pits. Multiple smaller soakaways outperform a single large one when ground permeability varies across the site, which it usually does.

The sizing test that matters for building control: the soakaway must discharge from full to half-volume within 24 hours. If your sizing calculation gives a volume that won't empty in time given your Vp, you need either more volume or better-draining ground. The BRE Digest 365 spreadsheet (or any of the free online calculators built on it) does this check automatically.

Picking a crate: the three products you'll see

Three soakaway crates dominate the UK market. Their dimensions are nearly identical. The differences are load rating, void ratio, and use case.

Per-crate pricing splits along load-rating lines: budget domestic (AquaVoid Home tier) at £28 – £40 per crate, and the heavier-duty Polystorm and Hydrocell range at £35 – £80 per crate. Polystorm sits at the lower end of the heavy-duty range; Hydrocell 62T at the upper end.

The Polystorm is the safe default for almost every domestic extension where the soakaway sits in a garden. It has the regulatory paperwork (BBA approval) that some building control officers ask for, and it's stocked at every major drainage merchant. AquaVoid is the budget pick if your soakaway is genuinely garden-only with no risk of future vehicle access; the lower load rating is fine for foot traffic and topsoil cover.

If the soakaway is going under a driveway or any area where a car might park, you want a 40-tonne crate minimum. The 62-tonne Hydrocell or Wavin Q-Bic is for installations under heavily-trafficked paving or where cover depth is more than 1.5 metres.

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A common practical buying unit is the 1m³ kit (5 crates plus 10m² of geotextile membrane). At £240 – £270, the kit is cheaper than buying components separately and the geotextile is sized correctly for the crates. For most extensions you'll need either one kit (50m² roof) or two kits (100m² roof). A pallet delivery surcharge typically applies; if you're ordering bricks or aggregate at the same time, get the merchant to combine deliveries to absorb it.

Online drainage specialists (Drainage Superstore, Cotterill Civils, BP Plastics, EasyMerchant) consistently beat builders' merchant prices on crates by 20-30%. Travis Perkins, Jewson, and MKM stock Polypipe but rarely the Wavin or Hydrocell ranges.

How to install crates the way building control wants to see them

The installation sequence is fixed. Get any step wrong and you either fail inspection or shorten the soakaway's life.

Step 1: Excavate to the calculated dimensions. A 5-crate (1m³) installation needs a pit roughly 1.2m × 1.2m × 0.8m to accommodate the crate stack plus 100mm of pea gravel surround on all sides and the base. Larger installations are wider and deeper but follow the same proportional surround. Keep the spoil separate from the topsoil; you'll backfill with topsoil at the very end.

Step 2: Lay 150mm of pea gravel or washed crushed stone in the base. Level it. The crates sit on this bed. Pea gravel is right; MOT Type 1 is wrong.

Step 3: Wrap the crates fully in non-woven geotextile membrane. Every face. Including the top. The membrane is what stops soil washing into the crates and silting them up. It must be non-woven (Draintex, Terram, Lotrak), not woven (Groundtex, weed barrier from Toolstation). Non-woven has a higher flow rate and filters soil better. Woven membrane is cheaper but blocks more easily. The 1m³ kit includes correctly-spec'd membrane. If buying separately, look for 100gsm minimum non-woven, at £1 – £3.

Step 4: Assemble the crates using the supplied clips and shear connectors. Modern crates ship flat-packed; you press them together on site. Connectors lock crates together so the assembly behaves as one structural unit under load. Don't skip them.

Step 5: Lower the assembled, wrapped crates into the pit. For a 5-crate stack you can do this by hand. Larger assemblies need two people or a strap and a digger.

Step 6: Connect the inlet pipe. Your 110mm surface water drain comes in through one face of the crate stack. Cut an X in the geotextile at the entry point, fold the four flaps back, push the pipe approximately 300mm into the crate, then tape or cable-tie the membrane back around the pipe entry to stop soil ingress at that join. Some crates have moulded knock-out plates; others let you butt the pipe flush. Check the manufacturer's instructions.

Step 7: Install a silt trap upstream of the soakaway. This is non-negotiable on any soakaway taking roof water. Leaves, moss, grit, and roof debris accumulate over time. A silt trap is a small inline pre-filter chamber that drops sediment out of the water before it enters the crates. Without one, the void inside the crates fills with silt over the years and the soakaway loses capacity. Cost: £35 – £90. Compared to digging out and replacing a silted-up soakaway, it's the best money you'll spend on the system.

Step 8: Backfill with pea gravel to all four sides and 150mm above the top of the crates. Keep the gravel inside the geotextile envelope; the membrane should still be folded over the top of the gravel layer.

Step 9: Close the geotextile envelope over the top. Overlap by at least 300mm. The whole assembly is now wrapped on every face.

Step 10: Backfill above the geotextile with topsoil for garden use, or sub-base and surface for trafficked areas. Minimum cover above the geotextile: 150mm for garden, 350mm for occasional vehicle, 1,000mm for sustained vehicle loading.

Step 11: Do not backfill above the crates until building control has inspected.

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What it costs in total

For a typical single-storey kitchen extension with around 30m² of new roof draining to one soakaway, the materials side breaks down like this:

Item	Quantity	Unit cost	Subtotal
1m³ soakaway kit (5 crates + 10m² geotextile)	1	£240 – £270	£240 – £270
Inline silt trap	1	£35 – £90	£35 – £90
Pea gravel (10mm or 20mm)	1 bulk bag (~0.5m³)	£60 – £90	£60 – £90
110mm underground pipe and fittings	3-5m run	typical	budget per £14 – £16 + couplers
Pallet/delivery surcharge	1	typical	per supplier

That's the parts. The labour to excavate, install, and reinstate the lawn or paving is where most of the budget sits. £700–2,500 covers the typical professional installation range from drainage specialists; the bottom of that range buys a 190-litre system in easy ground, the top buys a 1m³ system with awkward access or longer pipe runs. A drainage specialist day rate sits well above the hourly figure a plumber would quote, but for a one-day install in good ground, a groundworker is the cheaper trade.

Building control inspection of drainage is normally bundled into your main extension building regs application, so you don't pay separately for the soakaway sign-off. The percolation test, if you pay a professional rather than DIY, is £150 – £300.

When a soakaway isn't the answer

Clay soils defeat soakaways. Forum threads and BCO casework are full of installations in clay where the system functions as a holding tank that empties over days rather than hours. After a heavy rain, the garden floods. The 24-hour empty-to-half rule fails. The system is technically non-compliant. There's no fix involving more crates; the ground itself is the problem.

If your percolation test gives Vp above 100, or if your test fills with groundwater rather than draining (high water table), infiltration is not viable. The Approved Document H hierarchy lets you move down to the next option. The fallback path:

1. Watercourse discharge. Connect to an open ditch, stream, or culvert with the landowner's permission. Often unavailable on urban plots.
2. Attenuation tank with controlled release. Same crates, but with a flow-restrictor outlet that releases stored water slowly to a watercourse or surface water sewer at a controlled rate (typically 5 litres/sec/hectare). The water company or local highways agency must agree the discharge.
3. Permeable paving with sub-base storage. For driveways, the paving itself becomes the soakaway. The sub-base is a deep crushed-stone reservoir that infiltrates over time. Works in marginal soils that won't support a buried soakaway.
4. Connection to the existing surface water sewer. Last resort under Part H. Requires water company approval and sometimes a surface-water drainage charge. If the existing house already discharges to the surface water sewer, this is often the easiest route for an extension.

A traditional rubble-filled soakaway is technically still legal under Part H for small areas. It's not worth doing. The void ratio is poor, it silts up faster, and it costs the same to install. Crates have replaced rubble for a reason.

Where you'll need this

Soakaway crates and the surface water drainage system around them appear in several places on the build:

• Drainage - the main task covering both foul and surface water drainage for the new extension. Soakaway crates are the standard surface water solution where ground permits.
• Foundations and footings - the percolation test and soakaway location are decided alongside foundations because both depend on the same ground conditions. Test before you dig the footings if possible.
• Building control inspection: foundations - the BCO will check soakaway location and depth at the same visit they sign off the footings, if the timing aligns.
• Garden and external works - garden reinstatement above the soakaway needs to allow for the geotextile cover and the 150mm minimum topsoil depth. Don't plant trees over a soakaway; roots find geotextile and break it.

Soakaways apply to any extension or renovation project that adds new roof area or hard standing, anywhere on a UK plot.

The mistakes that catch people out

Installing without a percolation test. Buying crates based on roof area alone, then finding the ground won't drain. The crates aren't refundable once they've been on site. Test first.

Soakaway too close to the building. Five metres is the rule. From the foundation, not the wall face. Building control will measure with a tape and they don't accept "close enough."

Wrong geotextile. Woven weed barrier from a garden centre is not the same as non-woven drainage geotextile. The cheap stuff blocks fast and the soakaway silts up within years. Buy from a drainage merchant, not a garden centre. Specify non-woven, 100gsm minimum.

MOT Type 1 as backfill. Type 1 compacts to nearly solid. It belongs under a driveway sub-base, not around a soakaway. Pea gravel or washed crushed stone, every time.

No silt trap. Roof water carries leaf, moss, and grit. Without a silt trap, that debris ends up inside the crates and slowly fills the void. A modest £35 – £90 spend at install saves the far larger cost of digging the whole system out in 15 years.

Backfilling before the building control inspection. The BCO must see the system before it disappears. Take photos anyway, but call them out before you cover anything.

Ignoring the 1m³/50m² rule in clay. Clay soils need 4-5 times the standard volume, sometimes more. The simplified rule of thumb assumes good ground. Your Vp result tells you whether you can use the rule or whether you need the full BRE Digest 365 calculation.

Connecting foul drainage to a soakaway. Crate soakaways are surface water only. Connecting a toilet or sink waste to one is illegal under the Environment Agency rules and an obvious health hazard. Foul drainage goes to the public sewer, a septic tank, or a treatment plant. Never to a crate soakaway.