What size soakaway do I need for my house?

Soakaway size depends on your roof area, any paved areas draining into it, and your soil type. A typical 3-bed semi with 50 m² of roof draining to one soakaway in loam soil typically needs a soakaway volume of around 1-2 m³. Clay soils require much larger soakaways — sometimes making them impractical. This soakaway volume calculator estimates the size you need based on BRE DG 365.

What are the UK Building Regulations for soakaways?

Building Regulations Approved Document Part H requires surface water to be disposed of via soakaways where ground conditions allow. Soakaways must be at least 5 metres from any building and not in an area of fill or made ground. BRE Digest 365 provides the design methodology for sizing soakaways. All drainage pipes connecting to the soakaway need the correct gradient — use our Drainage Fall Calculator (/landscaping/drainage-fall-calculator).

Are soakaway crates better than rubble fill?

Soakaway crates are significantly more efficient than rubble fill. Crates have a void ratio of approximately 95% (meaning 95% of the volume stores water), while rubble fill only has about 30% void ratio. This means a crate soakaway can be roughly one-third the size of a rubble fill soakaway for the same storage capacity.

Can I install a soakaway in clay soil?

Clay soil has very poor drainage (infiltration rate of roughly 0.36 mm/hr), which means soakaways are often impractical in heavy clay. Building Control may not approve a soakaway in clay soil. In these cases, you may need to connect surface water to a public sewer or watercourse instead, or consider a French drain — see our French Drain Calculator (/landscaping/french-drain-calculator). A percolation test (BRE DG 365) is essential before designing a soakaway in clay.

How do I do a percolation test?

Dig a hole 300mm x 300mm to below the proposed soakaway invert level. Fill with water to 300mm deep and let it drain. Refill to 300mm and time how long it takes to drain from 75% full (225mm) to 25% full (75mm). The result is used in BRE DG 365 calculations. Tests should be done in winter when the water table is highest. The gravel needed to fill a soakaway can be calculated with our Gravel Calculator (/landscaping/gravel-calculator).

Soakaway Sizing Calculator — BRE DG 365 & UK Building Regs

How We Calculate This

Our soakaway calculator uses the methodology from BRE Digest 365 and UK Building Regulations Approved Document Part H to estimate the required soakaway volume for your property.

The design method

The soakaway must store the volume of rainwater that falls during a design storm (30 minutes at 50mm/hr — the UK standard) minus what drains away through the soil during that time.

Required Volume = (Drainage Area × Rainfall Rate × Storm Duration) - Infiltration

Design parameters

Design rainfall rate: 50 mm/hr (UK standard for soakaway design)
Storm duration: 30 minutes
Sand/gravel infiltration: 36 mm/hr
Loam infiltration: 3.6 mm/hr
Clay infiltration: 0.36 mm/hr

Important notes

Soakaways must be at least 5m from any building (Part H requirement)
Crate soakaways have ~95% void ratio; rubble fill has only ~30%
A site-specific percolation test is essential for accurate results
This calculator provides estimates — always consult Building Control

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Soakaway Sizing Calculator — UK Building Regs Part H