How to Size a Soakaway: A Crate Calculator Guide

By Josh Rickard July 11, 2026blog

TL;DR: Sizing a soakaway comes down to three numbers: the area of hard surface draining into it, how quickly your ground absorbs water, and the design rainfall it has to cope with. Work out the storage volume you need in cubic metres, then divide by the usable volume of one crate (roughly 0.19m3). Percolation test first. Building control will ask for the figures.

Getting the size of a soakaway right is the difference between a driveway that drains and a garden that sits under water every winter. Most undersized soakaways we dig up in Cornwall were never calculated at all. Someone guessed, dropped in four crates, backfilled, and moved on. Two winters later the crates are full, the water has nowhere to go, and it backs up towards the house.

The calculation itself is not difficult. It uses three inputs, one on-site test, and some straightforward arithmetic. This guide walks through each step, shows you how to convert a storage volume into a crate count, and explains the ground conditions that change the answer in Cornwall.

What Size Soakaway Do I Need?

Soakaway size is set by three figures: the impermeable area draining into it (in square metres), the soil infiltration rate measured by a percolation test on site, and the design rainfall for the storm it has to hold. The storage volume you need is the rainwater that arrives during that storm, minus the water that soaks away while it is raining.

There is no single correct size for a given roof. Two identical bungalows, one on free-draining sand near Hayle and one on clay outside Bodmin, can need soakaways that differ by a factor of three. The ground decides.

What the standards say:

• For soakaways serving 25m2 or less, Approved Document H allows a design rainfall of 10mm in five minutes to be assumed as the worst case.
• Anything larger than that is designed to BRE Digest 365, which tests a range of storm durations and takes the worst result.
• Local authority building control teams will expect to see percolation test data behind the numbers, not an assumed rate. LABC guidance on drainage of paved areas sets out the same approach.

So before any arithmetic, you need to know what you are draining and what you are draining into.

The Three Numbers That Decide Soakaway Size

Every soakaway calculation rests on three inputs: catchment area, soil infiltration rate, and design rainfall. Get any one of them wrong and the finished soakaway will be the wrong size, no matter how carefully you do the maths that follows.

1. The Catchment Area

The catchment area is the total impermeable surface that drains into the soakaway, measured in plan. Roofs are measured on plan (footprint), not along the slope. A 9m by 6m roof is 54m2 whether it is a shallow pitch or a steep one.

Do not stop at the roof. Extensions, patios, block paving, and a resin driveway all count if their water ends up in the same pipe. This is where holiday-let conversions catch people out. Turning a front garden into two parking spaces can add 40m2 of hard surface to a system that was sized for a roof alone.

2. The Soil Infiltration Rate

The soil infiltration rate, written as f, is the speed at which water passes from the soakaway into the surrounding ground, measured in metres per second. It comes from a percolation test, and it is the single figure that decides whether a soakaway is viable at all.

The BRE Digest 365 test is done at the depth the soakaway will sit, not at the surface:

1. Dig a trial pit down to the proposed base level of the soakaway.
2. Fill it with water and let it drain away completely. Repeat, so the surrounding soil is properly wetted.
3. On the final fill, time how long the water takes to fall from 75% to 25% of the effective depth.
4. Use that time and the pit dimensions to calculate f.

Ground below roughly 1 x 10-6 m/s will not accept water fast enough to support a soakaway, and an alternative disposal route is needed. A test done in the top 300mm of topsoil will always read fast and always mislead. Test at depth, and test in the wet months if you can.

3. The Design Rainfall

Design rainfall is the depth of rain, in millimetres, that the soakaway is designed to store from a single storm. Approved Document H permits 10mm in five minutes for small soakaways of 25m2 or less. BRE Digest 365 uses site rainfall data across several storm durations, applies a return period (commonly a 1 in 10 year storm for domestic work), and adds a climate change allowance.

Here is the part people miss: the worst storm for a fast-draining site is short and violent, while the worst storm for slow clay is long and steady. That is why BRE 365 checks several durations rather than one.

How Do You Calculate Soakaway Storage Volume?

Soakaway storage volume is the inflow from the design storm minus the water that infiltrates into the ground while the storm is falling. BRE Digest 365 expresses it as S = I – O, where I is the rainfall volume arriving from the drained area, O is the outflow through the soakaway sides during the storm, and S is the storage the structure must provide.

Broken down:

• I (inflow) = drained area in m2 x design rainfall depth in metres.
• O (outflow) = the side area of the soakaway to half its effective depth, x the infiltration rate f, x the storm duration in seconds.
• S (storage) = the answer, in cubic metres. This is what the crates have to hold.

Note what is excluded from the outflow: the base. BRE 365 assumes no infiltration through the bottom of the soakaway, because silt settles there and seals it over time. Only the sides count.

Worked example, 25m2 garage roof:

1. Inflow: 25m2 x 0.010m (10mm) = 0.25m3 of water.
2. Outflow during a five-minute storm on typical ground is a few litres, so it barely moves the figure.
3. Required storage: approximately 0.25m3.

Scale that roof up to 100m2 on slow clay and the storm that matters is longer and heavier, and the storage requirement climbs into several cubic metres. That is a BRE 365 calculation, not a rule of thumb. Once you have the storage figure, converting it into crates is simple.

How Many Soakaway Crates Do You Need?

To find the number of soakaway crates, divide the required storage volume by the usable volume of one crate, then round up. A standard modular crate measures 1000mm x 500mm x 400mm, which is 0.2m3 of gross volume. At a 95% void ratio, each crate holds roughly 0.19m3 of water.

Void ratio is the proportion of a structure that is empty space available to hold water. Plastic crates sit at approximately 95%. Clean stone rubble manages 30% at best, which is why a rubble pit needs an excavation three times larger for the same storage.

Always round up, never down. Crate dimensions vary between manufacturers, so check the stated void volume of the product you are buying rather than assuming 0.19m3.

Two rules govern how you stack them. Keep the arrangement compact enough to fit the excavation, and remember that the side area of the finished block feeds directly back into the outflow part of the calculation. A long, shallow run of crates has more side surface in contact with the soil than a tall, narrow block of the same volume, which is why trench-style soakaways work well on slow ground. Wrap the whole assembly in a non-woven geotextile membrane. Skip that step and fine soil migrates into the voids until the storage capacity quietly disappears.

Where a Soakaway Can Legally Go

Position is a compliance matter, not a preference. Building Regulations Approved Document H requires soakaways to be sited at least 5 metres from any building or road, and building control teams commonly ask for at least 2.5 metres from a boundary. Get this wrong and the soakaway will not be signed off.

The other siting rules that matter:

• The base of the soakaway must sit clear of the highest seasonal groundwater level. A pit dug in August on a coastal Cornish site can look perfect, then sit half full of groundwater in February.
• Surface water only. Foul water from a septic tank goes to a drainage field, which is a different structure designed to different rules. If your system involves foul water, our Septic Tank Maintenance team can advise on the correct disposal route.
• Never site a soakaway over a public sewer, and never within the protection zone of a drinking water borehole or spring.
• Crates under a driveway need a load-rated product. Standard domestic crates are not designed for vehicle loading.

Those distances also explain why so many Cornish properties struggle. Terraced cottages, narrow plots, and granite boundary walls leave very little ground that satisfies the 5 metre rule.

Why Cornwall Ground Conditions Change the Answer

Cornwall’s geology makes soakaway sizing harder than the textbook examples suggest. Clay subsoil, shillet, and granite bedrock all restrict infiltration, and coastal water tables rise sharply through the winter. The same crate count that works in the Home Counties will fail here.

Rainfall is the first factor. Camborne records an average of 1,071mm of rain a year across the 1991 to 2020 period, according to Met Office long-term averages. Much of it falls in concentrated Atlantic bands between October and January, exactly when the ground is already saturated.

Geology is the second. Dig 800mm on a lot of Cornish sites and you hit shillet or a dense clay horizon that will not take water at any useful rate. Percolation tests on those sites often come back marginal, which pushes the design towards a longer trench soakaway with more side contact area, or towards a different disposal route entirely.

Then there is the water table. On low-lying coastal plots near Falmouth, Penzance, and the Fal estuary, winter groundwater can rise to within a metre of the surface. A soakaway with its base sitting in groundwater has no storage at all when you need it most. If your existing soakaway floods every winter, the size is rarely the whole story: a CCTV Drain Survey tells you whether the pipe feeding it has collapsed, silted, or filled with roots before anyone starts digging.

What Goes Wrong When a Soakaway Is Undersized?

An undersized soakaway fills during a storm and stays full. Water then backs up the pipe, surcharges at the nearest gully, and finds the lowest point available, which is often a patio, a garage, or the base of a wall. Repeated saturation next to foundations is a structural problem, not a landscaping one.

The failures we see most often across Cornwall:

• The percolation test was done in topsoil, so the infiltration rate was three times too optimistic.
• The driveway and patio were left out of the catchment area calculation.
• No geotextile wrap, so the crates silted up within a few seasons.
• The soakaway was placed too close to the house because there was nowhere else to put it.
• The pipe feeding the soakaway had cracked or dropped, so water never reached the crates in the first place.

That last point matters more than most people expect. A soakaway that suddenly stops coping has often not shrunk. Something upstream has failed. Root ingress and cracked clay pipework are common in older Cornish properties, and both are repairable without excavation through Drain Pipe Relining. If water is already backing up at the gully, that is a blocked drain to clear before any sizing work begins.

Getting the Size Right First Time

Three things decide whether your soakaway works: an honest catchment area, a percolation test done at the right depth, and a crate count that follows the calculation rather than the budget. Round up, wrap it in geotextile, and keep it 5 metres from the building.

If you want the numbers checked before you dig, or you have an existing soakaway that floods every winter, Clear Stream Drainage Solutions can survey the site, run the percolation test, and give you a fixed-price quote for the work. No call-out fee, engineers on site within 1 to 2 hours anywhere in Cornwall, and a 5-year guarantee on every repair. Call us today on 01872 222555.

Frequently Asked Questions

What Size Soakaway Do I Need for a 50m2 Roof?

A 50m2 roof needs a soakaway sized from a percolation test, not a standard figure. As a first pass, 50m2 of roof collects 0.5m3 of water from a 10mm storm, which is around 3 modular crates. On slow-draining clay the designed figure will be considerably higher, because a longer storm delivers more water than the ground can absorb during it.

How Many Crates Make Up 1m3 of Soakaway Storage?

Six standard crates give roughly 1m3 of usable storage. A typical crate measures 1000mm x 500mm x 400mm, giving 0.2m3 of gross volume, and a 95% void ratio leaves approximately 0.19m3 of that available to hold water. Always check the void volume quoted by the manufacturer of the crates you buy.

How Far from the House Does a Soakaway Have to Be?

Approved Document H requires a soakaway to be at least 5 metres from any building or road. Most building control teams also want a minimum of 2.5 metres from a boundary. The distance protects foundations from saturated ground, which is why it is enforced rather than advisory.

Can You Put a Soakaway in Clay Soil?

Clay can support a soakaway only if the percolation test shows the ground accepts water fast enough. Where the infiltration rate falls below roughly 1 x 10-6 m/s, a soakaway will not empty between storms and a different solution is needed. A longer trench soakaway with greater side contact area sometimes works on marginal clay, which is common across mid Cornwall.

Do I Need Building Control Approval for a Soakaway?

Surface water drainage falls under Part H of the Building Regulations, so a soakaway installed as part of an extension, a new build, or a new driveway is subject to building control. Inspectors will ask for the percolation test result and the sizing calculation. Confirm the position with your local authority before starting work.

How Deep Should a Soakaway Be?

Soakaway depth is set by the invert level of the incoming pipe and the position of the water table. The crates sit below the inlet so they fill from the top, and the base must stay clear of the highest seasonal groundwater level. On coastal Cornish sites that constraint often forces a wider, shallower design rather than a deep pit.

Can a Soakaway Take Waste Water from a Septic Tank?

No. Soakaways handle rainwater only. Treated effluent from a septic tank must discharge to a drainage field, which is a separate structure with its own design rules and Environment Agency requirements. Connecting foul water to a surface water soakaway is a pollution offence.