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 soakaway trench is
A soakaway trench, or infiltration trench, is a long, relatively narrow excavation filled with stone or crates rather than a compact pit. Spreading the storage along a trench gives a much larger area of soil contact for the water to infiltrate through.
When to use one
Trenches suit shallow or rocky ground, a high water table, or a long thin plot. The extra side area can make infiltration work in slower-draining soil, where a compact pit would struggle to empty in time.
How it's sized to BRE 365
BRE 365 sizes a trench on both storage volume and infiltration area, so the greater soil contact of a long run is a real advantage in poorer soil. SoakMonkey solves the trench geometry for your percolation rate and catchment.
Keep a soakaway trench clear of foundations and boundaries — typically at least 5 m from any building — and check it stays above the water table along its length.
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 soakaway trench?
A soakaway trench, or infiltration trench, is a long, relatively narrow excavation filled with stone or crates rather than a compact pit. Spreading the storage along a trench gives a much larger area of soil contact for the water to infiltrate through. Trenches suit shallow or rocky ground, a high water table, or a long thin plot. The extra side area can make infiltration work in slower-draining soil, where a compact pit would struggle to empty in time.
How is a soakaway trench sized to BRE Digest 365?
BRE 365 sizes a trench on both storage volume and infiltration area, so the greater soil contact of a long run is a real advantage in poorer soil. SoakMonkey solves the trench geometry for your percolation rate and catchment.
Is the soakaway trench 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?
Keep a soakaway trench clear of foundations and boundaries — typically at least 5 m from any building — and check it stays above the water table along its length.
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.