Soakaway Design & Installation: Part H Building Regs UK

Quick Answer: A soakaway is an underground structure that discharges surface water to ground via infiltration. Building Regulations Part H3 require soakaways to be at least 5m from any building, sized according to a percolation test (BRE Digest 365 method), and not used for foul drainage. A typical domestic soakaway for a 50m² roof is a 1m³ crate or rubble pit at least 1m below ground but above the seasonal water table, located in suitable permeable ground.

Summary

A correctly installed soakaway disposes of rainwater from roofs, drives or paving without overloading combined sewers. In the UK they are the first option in the surface water disposal hierarchy under Building Regulations Approved Document H3 — only if soakaway is impractical may a watercourse or sewer be used.

Soakaways are not suitable everywhere. Clay ground, high water tables, contaminated land, very small plots and sites with known shallow service runs all make a soakaway impractical. The BRE Digest 365 percolation test is the standard method of confirming suitability and sizing the structure.

The two main forms are the traditional rubble-filled pit (clean broken hardcore, geotextile-wrapped) and the modular plastic crate (proprietary structures such as Aquacell, Polystorm, Stormcrate). Plastic crates have higher void ratio (95%+ vs ~30% for rubble), faster install, and a known design void volume — they have largely replaced rubble pits on new build.

Key Facts

Quick Reference Table

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Soil Type Typical f Value (m/s) Suitability for Soakaway
Coarse sand / gravel 10⁻³ to 10⁻⁴ Excellent
Fine sand 10⁻⁴ to 10⁻⁵ Good
Sandy loam 10⁻⁵ Acceptable
Silt 10⁻⁵ to 10⁻⁶ Marginal
Clay-silt 10⁻⁶ to 10⁻⁷ Poor — usually not viable
Pure clay <10⁻⁷ Not viable
Made ground / fill Variable Test required

Detailed Guidance

Percolation test (BRE Digest 365)

The standard UK method for sizing a soakaway:

1. Excavate trial pit: 300mm square × 1m deep
   (or to the full depth of the proposed soakaway base)
2. Fill with water and allow to drain overnight (saturate ground)
3. Refill pit with water to 300mm above the base
4. Record time for water to fall from 75% (225mm) to 25% (75mm) full
5. Repeat test three times consecutively
6. Average the three readings to get the test time Tp
7. Calculate f = Vp/(50 × Ap × Tp) where:
     Vp = volume of water (Vp75 to Vp25)
     Ap = average internal surface area between 75% and 25%
     Tp = time in seconds
8. Required storage volume Vs = Cr × A × i × D
     Cr = runoff coefficient (0.95 paved, 0.50 lawn)
     A  = catchment area (m²)
     i  = design rainfall intensity (mm/hr)
     D  = design storm duration (hr)

If three tests give wildly different results (a factor of 2+ apart), retest — surface water tracking may be skewing the result.

Sizing the soakaway (worked example)

A typical UK domestic roof: 50m² catchment, design 10-year storm.

Use one 1m³ plastic crate or equivalent rubble pit of 3m³ (allowing for 30% void ratio).

Choosing a location

Excavation

For a 1m³ plastic crate (typically 1000 × 500 × 400mm or 1200 × 600 × 420mm depending on brand):

  1. Excavate the pit to allow 150mm bedding all round and a 300mm gravel cap above the crate
  2. Hand-trim the base level — never sit a crate on uneven or loose ground
  3. Place 50mm pea shingle bedding, levelled with a spirit level
  4. Lay non-woven geotextile membrane in the pit, allowing 1m overlap on all sides
  5. Position the crate centrally on the bedding
  6. Connect the inlet pipe (typically 110mm) to the rodding/inlet socket with the correct adaptor
  7. Fold geotextile up and over the crate, lapping the edges minimum 300mm
  8. Backfill carefully — 10/20mm clean angular stone all round; do not compact directly on the crate
  9. Cap with 300mm of clean stone, then non-woven geotextile, then 150mm topsoil

Inlet and rodding access

Building Control will look for a rodding eye or inspection chamber upstream of the soakaway. This allows the inlet pipe to be cleared if leaves or sediment build up. Many crate manufacturers supply purpose-made inspection turrets that bolt to the crate and bring access to ground level.

Roof vs paved catchments

Roofs are clean catchments — rainwater carries leaves and grit but no oils. Driveways and paved areas carry diesel, oil and tyre dust. Where surface water from a driveway discharges to a soakaway, an oil interceptor or trapped gully is required upstream. For domestic driveways, a trapped gully with regular silt clean-out is usually sufficient.

When a soakaway is not viable

Alternative discharge route hierarchy (Part H3):

  1. Soakaway / infiltration
  2. Watercourse (with EA consent if abstraction zone)
  3. Surface water sewer (with sewerage undertaker permission)
  4. Combined sewer (last resort, sewerage undertaker approval required)

Frequently Asked Questions

Do I need Building Control approval for a soakaway?

Yes for any soakaway forming part of new build, an extension, or work that changes the surface water disposal point. Soakaways replacing existing ones at the same point may not need approval, but check with your local authority Building Control or Approved Inspector.

How long does a soakaway last?

A well-installed plastic crate soakaway should last 50+ years. The structure itself is inert; failure is usually caused by silt and leaf litter blocking the void over time. Regular cleaning of upstream gullies and an inspection access on the soakaway itself extends life dramatically. Rubble pits often silt up faster because fines from surrounding ground migrate into the void if no membrane was used.

Can a soakaway take foul water?

No. A soakaway is for clean rainwater only. Foul water disposal off-mains requires a septic tank with drainage field (a different structure to British Standard BS 6297) or a packaged treatment plant. Connecting foul to a soakaway breaches Part H1 and is illegal under the Environment Agency's General Binding Rules for small sewage discharges.

Can I share a soakaway between two houses?

Possible in principle but the soakaway must be sized for combined catchment, ownership must be clearly recorded in title deeds (or by easement), and maintenance responsibility shared. Most new builds run one soakaway per dwelling for this reason.

What if my soakaway floods?

Three usual causes:

  1. Inadequate sizing — recalculate using BRE 365, larger storage may be needed
  2. Silt-up — open the inspection access and clean the inlet pipe; replace the soakaway if the crate is choked
  3. Rising water table — soakaway base is below winter water table and ground cannot accept more water

In all cases, do not assume "soakaways flood from time to time". A correctly designed and installed soakaway holds the design storm event. Frequent overflow indicates undersized storage or unsuitable ground.

Regulations & Standards