Soakaway Design & Installation: BRE Digest 365 Calculations

Summary

A soakaway is the most common method of disposing of surface water (rainwater) where connection to a surface water sewer is impractical or unavailable. Done properly, a soakaway can last decades with minimal maintenance. Done incorrectly — undersized, placed in unsuitable ground, or positioned too close to a building — and it can cause foundation movement, damp problems, or outright flooding.

BRE Digest 365 (published by the Building Research Establishment) is the definitive UK design method. It uses an on-site percolation test to establish the soil's infiltration rate, then sizes the soakaway storage volume to accommodate a design storm of specified return period. Most domestic soakaways are designed for a 1-in-10-year storm event, though in flood-risk areas or where downstream consequences are serious, a 1-in-100-year event may be required.

Many soakaways fail because they were never properly sized — installers simply dig a hole and fill it with rubble, using intuition rather than calculation. This may work for years in well-draining sandy soils but will fail rapidly in clay. The only reliable approach is to carry out the percolation test first, before committing to the soakaway design and before any other drainage options are ruled out.

Key Facts

• Primary design standard — BRE Digest 365 (Soakaway Design, 2016 edition)
• Minimum separation from building — 5 metres from any foundation
• Minimum separation from boundary — 2.5 metres from the site boundary
• Minimum separation from water mains — 5 metres
• Not suitable for clay soils — percolation test value Vp > 1:6 (takes more than 6 seconds per mm) means a soakaway is unlikely to work
• Not suitable in flood zones — Environment Agency Flood Zone 2 or 3 generally prohibits soakaways
• Not suitable near contaminated ground — soakaways can mobilise contaminants toward groundwater
• Test holes — minimum 2 tests per proposed location; use 300mm × 300mm × 300mm holes
• Design storm — typically 1-in-10-year return period for domestic; 1-in-100-year for higher-risk applications
• Crate soakaways — modular plastic crates (e.g. Infilta, Ecobloc) now preferred over rubble-filled pits
• Filter membrane — geotextile membrane required around all soakaways to prevent silt migration
• Inspection access — access pipe required for maintenance and de-silting
• Roof catchment coefficient — 0.9 (impermeable); driveways 0.75–0.9; gravel 0.25–0.35
• Approved Document H references soakaways under H3 (rainwater drainage)
• Separate from foul drainage — soakaways must only receive clean surface water, never foul water

Quick Reference Table

Detailed Guidance

Carrying Out the Percolation Test

The BRE Digest 365 percolation test is the foundation of the design. It must be carried out in representative soil at the proposed soakaway depth.

Method:

1. Excavate a test hole 300mm × 300mm × 300mm at the proposed soakaway base depth
2. Pre-soak the hole — fill with water and allow to drain completely; repeat twice to saturate the surrounding soil
3. Fill the hole to the brim with water (300mm depth)
4. Time how long it takes for the water level to drop from 75% full to 25% full (i.e., 150mm to fall)
5. Record time in seconds; repeat three times and average

Calculating Vp (percolation value):

Vp = average time (seconds) ÷ 150mm (the drop in water level)

A Vp of 1.0 means 1 second per mm — excellent drainage. A Vp of 500 means 500 seconds per mm — clay, effectively impermeable.

Carry out tests at a minimum of two locations. If results differ significantly, use the higher (worse) value for design.

BRE Digest 365 Volume Calculation

The storage volume required is calculated as:

As = Vp × D × r × A / 2

Where:

• As = required storage area of soakaway walls (m²)
• Vp = percolation value (s/mm)
• D = design storm duration (seconds) — typically 2 × time of concentration
• r = rainfall intensity (m/s) from UK design rainfall maps (FEH or Wallingford)
• A = catchment area (m²)

For a practical domestic example: a 50m² roof, Vp of 50 sec/mm, 1-in-10-year storm in the South East (approximately 35mm/hr):

1. Inflow volume = 50m² × 0.035m = 1.75m³
2. Outflow during storm = calculated from infiltration rate and surface area
3. Storage required = inflow minus outflow

Many online calculators (including the Environment Agency's drainage calculator) automate this. For complex sites, engage a drainage engineer.

Soakaway Construction Options

Rubble-filled pit (traditional):

• Dig to required volume, fill with clean hardcore or rubble
• Wrap in geotextile membrane
• Effective volume is approximately 30% of excavated volume (void ratio)
• Difficult to inspect or clean; lifespan can be short if silt not managed
• No longer recommended for new installations

Modular crate system (preferred):

• Plastic crates (typically 400mm × 600mm × 800mm) interlocked to required volume
• Wrapped entirely in geotextile membrane
• Effective volume is typically 95% of crate volume
• Access pipe for inspection and jetting
• Can be extended in future if catchment increases
• Brands: Infilta, Ecobloc Inspect, Wavin Aquacell

Ring soakaway (circular precast):

• Precast concrete rings (typically 1.2m or 1.8m diameter) with perforated walls
• Suitable for smaller residential applications
• Access via chamber cover at top
• Base must be open to ground; do not concrete the base

Surface Water Design Rainfall

UK design rainfall varies significantly by location and return period. Use the following approach:

1. Access the SEPA/Environment Agency's UK Design Rainfall tool or the Wallingford HydroCal software
2. Enter your OS grid reference and the required storm duration and return period
3. The tool returns rainfall depth (mm) for the specified event
4. Divide by storm duration (hours) to get rainfall intensity (mm/hr)

For quick estimates in England, use 35mm/hr for 1-in-10-year events in most lowland areas; increase to 50mm/hr for upland areas or if designing for 1-in-30-year.

Frequently Asked Questions

Does a soakaway need building control approval?

Under Part H, surface water drainage is subject to Building Regulations. A soakaway serving a new extension or dwelling requires building control sign-off. The inspector will want to see evidence of the percolation test and size calculation. For a like-for-like replacement of an existing soakaway with no change in catchment area, many inspectors take a pragmatic view, but it is always better to notify.

Can I use a soakaway for a patio or driveway?

Yes, for driveways particularly. Under the SUDS requirements in England, any driveway over 5m² must be permeable or drain to a soakaway — you cannot now direct driveway water to the foul sewer or to the road. A purpose-designed driveway soakaway must be sized for the driveway catchment and should incorporate a silt trap upstream to prevent premature blocking.

What happens if my soil fails the percolation test?

If Vp exceeds 500 sec/mm (effectively clay), a soakaway will not work reliably. Alternatives are:

• Connection to a surface water sewer (if within reasonable distance)
• A drainage ditch or French drain discharging to a watercourse (EA consent may be required)
• A detention basin or attenuation tank that throttles discharge to a sewer or watercourse
• In exceptional cases, a pumped system

Do not install a soakaway in clay in the hope it will work — it is a waste of money and will cause problems when it eventually fails.

How far down should a soakaway be?

The base of the soakaway should be at least 1.0 metre above the maximum groundwater table. In areas with seasonally high water tables (lowland areas, near rivers), carry out the percolation test in winter or spring when the water table is highest. A soakaway installed above a seasonally high water table will back up during wet winters.

How do I maintain a soakaway?

1. Inspect the inlet pipe and upstream gully/trap annually — remove silt and debris
2. If performance degrades, attempt jetting from the access point inside the crate
3. Check that the geotextile membrane has not been breached
4. A well-maintained crate soakaway can last 30–50 years; a rubble pit 10–20 years

Regulations & Standards

• BRE Digest 365 (Soakaway Design) — the definitive UK design method for sizing soakaways

• Building Regulations Approved Document H (H3: Rainwater Drainage) — statutory requirements for surface water disposal

• BS EN 752 — drain and sewer systems outside buildings; covers surface water management

• CIRIA Report C753 (The SuDS Manual) — comprehensive guidance on sustainable drainage systems

• Environment Agency Groundwater Protection Policy — restrictions on soakaways near water features and in sensitive zones

• The Water Environment (Water Framework Directive) Regulations 2017 — controls on discharge to ground and watercourses

• BRE Digest 365: Soakaway Design — primary design reference (available from BRE Bookshop)

• Approved Document H — Building Regulations H3 rainwater drainage

• CIRIA SuDS Manual C753 — free download, comprehensive SUDS guidance

• Environment Agency: Soakaways — groundwater protection zones

• underground drainage — foul drainage gradients, pipe sizes, and connections

• rainwater harvesting — alternative use for collected rainwater before disposal

• manhole construction — inspection chambers for drainage systems