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

  • BRE Digest 365 — primary design method for domestic soakaways; published by Building Research Establishment
  • Percolation test — determines soil drainage rate; expressed as V_p (seconds per mm rise/fall of water level in test pit)
  • Feasibility limits — V_p 1–100 s/mm: soakaway feasible; V_p above 100 s/mm: soakaway impractical; V_p below 1 s/mm: drainage too fast for test reliability (exceptional)
  • Minimum distance from buildings — 5m from foundations; greater in some site conditions
  • Minimum distance from boundaries — 2.5m from property boundary
  • Design rainfall — UK lowlands: 10mm/hr (0.0028 L/s per m²) for 1-in-10-year storm; use local Climate Science data for accurate figures
  • Soakaway types — rubble-filled pit (traditional); ring soakaway (precast perforated concrete rings); geocellular crate soakaway (plastic crates in membrane)
  • Geocellular void ratio — typically 95% voids by volume; more efficient than rubble-filled (30–40% voids)
  • Inspection point — access for inspection and cleaning must be provided; typically a 150mm rodding eye or inspection chamber
  • Groundwater depth — base of soakaway must be minimum 1m above seasonal high groundwater level
  • Clay soil — if the site has significant clay content, a percolation test is essential — do not assume drainage will work

Quick Reference Table

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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:

  • Ground condition: probe the soil with a 25mm bar to at least 1.5m depth. If refusal is encountered before 1.5m, there may be an impermeable layer. Note the soil description at different depths.
  • Groundwater level: check at the intended soakaway location; if the ground is wet or waterlogged, groundwater may be within 1m of the surface — making soakaway installation impossible without deeper investigation.
  • Site history: made ground (fill), contaminated land, and former industrial land are not suitable for soakaways — percolate water could mobilise contamination into groundwater.
  • Proximity to Source Protection Zones (SPZs): the Environment Agency designates SPZs around water abstraction points. Soakaways within SPZ1 (innermost zone) are typically not permitted; SPZ2 and SPZ3 may have specific conditions.

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:

  • V = storage volume required (m³)
  • A_imp = total impermeable drainage area (m²) — the plan area of all roofs, driveways, etc. draining to this soakaway
  • D = design rainfall depth (mm) — for a 1-in-10 year, 1-hour storm in most of England: approximately 25–32mm (check UKCP18 data or Flood Estimation Handbook for site location)
  • f = storage factor, derived from V_p:
    • V_p 1–12 s/mm: f = 1.0
    • V_p 12–25 s/mm: f = 0.9
    • V_p 25–50 s/mm: f = 0.8
    • V_p 50–100 s/mm: f = 0.7

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

  • Roof area: 8m × 10m house = 80m² (plan area; use half for one side of a ridged roof)
  • Driveway: 5m × 6m = 30m²
  • Total A_imp = 80 + 30 = 110 m² (assuming all drains to one soakaway)
  • Design rainfall: 30mm (southeast England, 1-in-10 year)
  • Percolation test V_p = 20 s/mm → f = 0.9
  • V = 110 × (30/1000) × 0.9 = 110 × 0.03 × 0.9 = 2.97 m³

Required soakaway volume ≈ 3.0 m³

Worked Example 2: Small extension, fast-draining soil

  • Roof area: 3m × 4m extension = 12 m²
  • V_p = 5 s/mm (sandy soil) → f = 1.0
  • Design rainfall: 25mm
  • V = 12 × (25/1000) × 1.0 = 12 × 0.025 × 1.0 = 0.30 m³

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:

  • Approximate void ratio: 30–40% (30–40% of the pit volume is available for water storage)
  • Pit volume required = soakaway volume / 0.30 to 0.40
  • For 3.0 m³ storage: pit volume = 3.0 / 0.35 = 8.6 m³ (e.g., 2.5m × 2.0m × 1.7m deep)
  • Fill with 20–40mm clean angular gravel, compacted in 150mm layers

Ring soakaway (precast concrete rings):

  • Most common for domestic new build

  • 1.2m diameter rings with perforated sides (0.9m effective internal diameter)

  • Each 0.6m ring height provides approximately 0.38 m³ of storage (0.636 m² × 0.6m)

  • For 3.0 m³: 3.0 / 0.38 ≈ 8 rings (4.8m depth) — too deep for a single column; use wider diameter rings or multiple columns

  • 1.8m diameter rings provide more storage per ring — consult manufacturer's data tables

Geocellular (plastic crate) soakaway:

  • 95% void ratio — most volumetrically efficient
  • Standard units: 400mm × 800mm × 400mm high; each unit = 0.128 m³ void
  • For 3.0 m³: 3.0 / 0.95 = 3.16 m³ crate volume; 3.16 / 0.128 = 25 units
  • Wrapped in geotextile membrane (prevents fine soil particles entering and blocking crates)
  • Accessible via 150mm inspection pipe

Soakaway Location Requirements

  • Minimum 5m from any building foundation
  • Minimum 2.5m from site boundary
  • Minimum 1m above seasonal high water table
  • Not under areas of future vehicular traffic (unless structural design accommodates load)
  • Not in filled ground or made ground
  • Not in areas of known contamination

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:

  • Rainwater harvesting tank — stores rainwater for toilet flushing and garden use; requires a properly sized tank and controls
  • Attenuation tank with controlled flow discharge — holds peak rainfall and releases at a restricted rate to the sewer (requires agreement with the sewerage undertaker)
  • Connection to mains surface water drain — requires the drainage authority's consent; most urban properties can connect to the highway surface water drain
  • Green roof/permeable paving — reduces the runoff volume and peak flow rate

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

  • BRE Digest 365 — Soakaway Design (1991); Building Research Establishment; primary design methodology

  • Building Regulations Approved Document H — drainage; Part 4 covers soakaways

  • Environmental Permitting Regulations 2016 — for larger drainage volumes and sensitive locations

  • BS EN 752:2017 — Drain and sewer systems outside buildings; soakaway design guidance

  • CIRIA Report C753 — The SuDS Manual; sustainable drainage systems design

  • BRE Digest 365 — original soakaway design document; available from BRE Press

  • Approved Document H — MHCLG free download

  • Environment Agency Source Protection Zones — check if site is in an SPZ

  • CIRIA SuDS Manual (C753) — comprehensive sustainable drainage guidance

  • gutter downpipe sizing — gutter design feeding soakaways

  • drain testing — drainage system pressure testing before backfilling

  • beam and block floors — site drainage around extension footings

  • permeable paving — permeable paving as a soakaway alternative