Free to size. Building Control-ready PDF £25 +VAT (£30 inc VAT).
Runs in your browser. Enter your percolation test, choose the construction, read off the size.
What a crate soakaway is
A crate soakaway is built from modular plastic geocellular units — soakaway crates — stacked in an excavation and wrapped in a geotextile membrane. The crates are almost entirely hollow, with a void ratio around 95%, so they store far more water per cubic metre of excavation than loose stone.
When to use one
Crates are the go-to where space is tight or the storage volume is large — driveways, extensions and new builds where a gravel pit would mean an enormous hole. Because roughly 95% of a crate is usable storage, you dig the smallest excavation for a given design volume.
How it's sized to BRE 365
BRE Digest 365 sizes a soakaway by the storage volume it must hold in the design storm. With crates at around 95% void, the volume of crates you need is close to that raw storage figure — far less than a filled pit. SoakMonkey takes your percolation rate and catchment, runs the storm-duration sweep, and returns the crate volume that passes.
Crates need adequate cover for any traffic above, a silt trap or catchpit to keep fines out, and enough clearance above the water table. Wrap the whole assembly in a non-woven geotextile.
How many soakaway crates do I need?
Start from the storage volume BRE 365 gives you (SoakMonkey reports it for your soil and catchment), enter the crate you plan to use, and read off the number of units and the plan size.
The counter rounds up to whole crates and assumes the crates are the only storage. Excavation size adds the geotextile and the granular surround the manufacturer specifies — typically 100 mm or more on each side — plus cover for the load above. Get the storage volume first: size the soakaway free.
Free to size — pay only for the report
SoakMonkey is free to use: enter your percolation test, add the area draining to the soakaway, and read the size on screen for nothing. You only pay £25 +VAT (£30 inc VAT), once, when you export the Building Control-ready PDF — with the percolation analysis, the BRE Digest 365 storm table and the dimensioned result.
Frequently asked questions
What is a crate soakaway?
A crate soakaway is built from modular plastic geocellular units — soakaway crates — stacked in an excavation and wrapped in a geotextile membrane. The crates are almost entirely hollow, with a void ratio around 95%, so they store far more water per cubic metre of excavation than loose stone. Crates are the go-to where space is tight or the storage volume is large — driveways, extensions and new builds where a gravel pit would mean an enormous hole. Because roughly 95% of a crate is usable storage, you dig the smallest excavation for a given design volume.
How is a crate soakaway sized to BRE Digest 365?
BRE Digest 365 sizes a soakaway by the storage volume it must hold in the design storm. With crates at around 95% void, the volume of crates you need is close to that raw storage figure — far less than a filled pit. SoakMonkey takes your percolation rate and catchment, runs the storm-duration sweep, and returns the crate volume that passes.
How many soakaway crates do I need?
Divide the storage volume BRE Digest 365 says the soakaway must hold by the usable volume of one crate — its external volume times its void ratio, about 0.19 m³ for a 1000 × 500 × 400 mm crate at 95% — and round up. A 3.2 m³ design storage is 17 crates; a 6 m³ one is 32. The storage volume itself has to come from the BRE 365 calculation for your soil, catchment and rainfall, which SoakMonkey does free; the crate counter below turns that volume into a number of units and a plan size.
Is the crate soakaway calculator free?
Yes — entering your percolation test and sizing the soakaway on screen is free. You only pay £25 +VAT (£30 inc VAT) when you export the Building Control-ready PDF.
Anything to watch on site?
Crates need adequate cover for any traffic above, a silt trap or catchpit to keep fines out, and enough clearance above the water table. Wrap the whole assembly in a non-woven geotextile.
Other soakaway types
SoakMonkey sizes every common construction. Compare another:
Or back to the main soakaway calculator, size one for a house extension, or work out what size you need.