Soakaway Sizing Calculator: BRE Digest 365 Percolation Test Method, Volume Calculation Formula and Worked Examples

Quick Answer: Soakaway sizing follows BRE Digest 365 (1991). A percolation test establishes how quickly water drains from the soil (expressed as the percolation value V_p, in seconds per mm). The required soakaway volume is calculated as: V = A_imp × D × (1/f) where A_imp is the impermeable area draining to the soakaway (m²), D is design rainfall depth (mm), and f is an empirical factor derived from V_p. For most UK lowland soils with V_p between 12–100 s/mm, soakaway design is feasible; V_p above 100 s/mm (clay soils) typically means a soakaway won't work.

Summary

Soakaways dispose of surface water (from roofs, driveways, and impermeable surfaces) into the ground where mains drainage is unavailable. They are required by Building Regulations Approved Document H and are subject to the Environment Agency's requirements under the Environmental Permitting Regulations 2016 (for certain drainage volumes). Getting the sizing right avoids the most common failure mode — a soakaway that fills faster than it drains, causing the ground to saturate and the soakaway to overflow.

BRE Digest 365 (1991) is the industry-standard UK method for domestic soakaway design. Despite being over 30 years old, it remains the primary reference in Approved Document H and is accepted by local authorities and building control across the UK. The method is empirical — it is based on field percolation tests rather than complex hydraulic modelling — which makes it accessible and practical for site use.

The most common mistake in soakaway design is skipping the percolation test and guessing the volume. Clay-dominant soils can appear to drain initially (surface percolation) while the subsoil is impermeable — resulting in a soakaway that fails in its first wet winter. A proper percolation test on the actual site is non-negotiable for any soakaway larger than a small garden feature drain.

Key Facts

Quick Reference Table

Got your quantities? squote builds the full quote with labour, materials and markup.

Try squote free →
V_p (s/mm) Approximate Drainage Rate Soil Type Soakaway Feasibility
1–5 Very fast Gravel, coarse sand Feasible; small soakaway adequate
5–12 Fast Sandy soil, sandy loam Feasible
12–50 Moderate Sandy clay, loam Feasible; standard design
50–100 Slow Clay-loam, silty soil Feasible but large soakaway required
100–200 Very slow Clay Marginal; may not sustain drainage
Over 200 Near-impermeable Heavy clay Not feasible — alternative drainage needed

Detailed Guidance

Step 1: Site Assessment

Before any calculation, assess site suitability:

Step 2: Percolation Test (BRE Digest 365 Method)

The percolation test measures how quickly water levels fall in a pit filled with water.

Prepare the test pit:

  1. Excavate a pit approximately 300mm × 300mm in plan at the intended soakaway location, to the proposed invert level (typically 1.0–1.5m depth)
  2. Fill the pit with water and allow to drain — this pre-saturates the soil to reflect the worst-case condition (the test should not be carried out in an artificially dry soil)
  3. Repeat saturation at least twice; allow to drain between soakings

Conduct the test:

  1. Refill the pit to 300mm depth
  2. Record the time (in seconds) for the water level to fall from 75% depth to 25% depth (i.e., fall by 50% of the 300mm fill = 150mm fall)
  3. Express the result as: V_p = time (seconds) / 150 (mm fall)

Example: water level falls 150mm in 450 seconds → V_p = 450/150 = 3 s/mm

Run the test three times and use the average V_p value.

Seasonal variation: run the test in autumn/winter where possible. Spring or summer tests may give optimistically fast drainage rates that don't reflect wet season performance.

Step 3: Calculate Required Soakaway Volume

Using BRE Digest 365 empirical formula:

For a pit/crate soakaway:

V (m³) = A_imp × (D / 1000) × f

Where:

Note: these f values are simplified. BRE Digest 365 provides charts for exact values. The simplified values give conservative (larger) results — appropriate for a safety margin.

Worked Example 1: Standard domestic house, average soil

Required soakaway volume ≈ 3.0 m³

Worked Example 2: Small extension, fast-draining soil

Required soakaway volume ≈ 0.3 m³ — a standard 1m³ rubble pit is more than adequate.

Step 4: Choose Soakaway Type and Size

Rubble-filled pit:

Ring soakaway (precast concrete rings):

Geocellular (plastic crate) soakaway:

Soakaway Location Requirements

Frequently Asked Questions

Can I combine roof water and driveway water in the same soakaway?

Yes, for residential properties this is common. Use the combined area (roof + driveway) in the calculation. However, driveway runoff may contain oil and debris — consider a petrol interceptor or silt trap before the soakaway, especially for commercial driveways or properties with several vehicles.

The percolation test shows V_p over 100 — what are the alternatives?

If the soil is too impermeable for a soakaway, options include:

Does a soakaway need Building Regulations approval?

Yes, for drainage serving a new building or extension. Approved Document H Part 4 applies. Submit the soakaway design (percolation test results, calculation, and layout drawing) as part of the building regulations application. Building control will inspect before backfilling.

Regulations & Standards