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 rubble / gravel soakaway is
A rubble or gravel soakaway is an excavated pit filled with clean, uniformly graded stone. Water is stored in the voids between the stones — typically only about a third of the pit's volume — and soaks away through the sides and base.
When to use one
It is the simplest, most traditional soakaway, and fine for smaller roof or paved catchments. The trade-off is the low void ratio: because only around 30% of the pit stores water, you need roughly three times the excavated volume of a crate soakaway for the same storage.
How it's sized to BRE 365
BRE Digest 365 applies the granular void ratio, around 0.3, to work out how big the pit must be. SoakMonkey uses your percolation rate, catchment and rainfall to return the pit dimensions that pass — no guesswork about how big to dig.
Use clean, single-sized stone rather than fines-heavy hardcore, and wrap the pit in a geotextile so surrounding soil cannot wash in and clog the voids over time.
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 rubble / gravel soakaway?
A rubble or gravel soakaway is an excavated pit filled with clean, uniformly graded stone. Water is stored in the voids between the stones — typically only about a third of the pit's volume — and soaks away through the sides and base. It is the simplest, most traditional soakaway, and fine for smaller roof or paved catchments. The trade-off is the low void ratio: because only around 30% of the pit stores water, you need roughly three times the excavated volume of a crate soakaway for the same storage.
How is a rubble / gravel soakaway sized to BRE Digest 365?
BRE Digest 365 applies the granular void ratio, around 0.3, to work out how big the pit must be. SoakMonkey uses your percolation rate, catchment and rainfall to return the pit dimensions that pass — no guesswork about how big to dig.
Is the rubble / gravel 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?
Use clean, single-sized stone rather than fines-heavy hardcore, and wrap the pit in a geotextile so surrounding soil cannot wash in and clog the voids over time.
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.