Soakaway Installation Guide: Design, Construction and Building Regulations

Quick Answer: A soakaway must be designed to BRE Digest 365 using a percolation test, positioned at least 5 metres from any building and 2.5 metres from a boundary, and must not be installed where groundwater is within 1 metre of the base. Soakaways for surface water from impermeable areas over 100 m² require Building Control consent in England and Wales under Approved Document H. Plastic crate systems have largely replaced rubble-filled soakaways for new construction.

Summary

Soakaways are the first-choice solution for surface water disposal where mains connection isn't available — and often the only permitted option in areas with SuDS planning requirements. A properly designed soakaway discharges storm water into the ground, reducing run-off peaks and recharging groundwater. An undersized or badly positioned one floods, undermines foundations, and creates neighbour disputes.

The design is governed by BRE Digest 365, which sets out a percolation test method to determine whether the ground is suitable and to size the storage volume. The test is straightforward, but skipping it is a common mistake that leads to soakaways that fill and never drain. Some ground conditions — heavy clay, shallow bedrock, high water table — are unsuitable for soakaways entirely.

For UK tradespeople, understanding soakaway design is increasingly valuable as planners push SuDS requirements and clients want surface water solutions that won't involve ongoing drainage bills.

Key Facts

Quick Reference Table — BRE Digest 365 Vp Values

Spending too long on quotes? squote turns a 2-minute voice recording into a professional quote.

Try squote free →
Vp (s/m) f (m/s) Soil Type Suitability
< 1 > 0.001 Gravel, coarse sand Suitable — check for subsidence risk
1–3 0.001–0.00033 Sandy gravel, coarse gravel Very suitable
3–25 0.00033–0.00004 Fine sand, sandy loam Suitable
25–50 0.00004–0.00002 Silty sand, sandy silt Marginal — larger soakaway needed
50–100 0.00002–0.00001 Silty loam Poor — very large storage required
> 100 < 0.00001 Clay, heavy silt Generally unsuitable
> 3000 < 0.00000033 Heavy clay Unsuitable — soakaway not viable

Detailed Guidance

Step 1 — Site Assessment

Before designing or pricing a soakaway, assess:

  1. Available positions — mark the 5m exclusion from buildings and 2.5m exclusion from boundaries. If the available area is too small, soakaway may not be viable.
  2. Ground type — look at the geological map (British Geological Survey OneGeology portal), existing neighbours' experience, and inspect excavated soil during trial pit.
  3. Water table — local knowledge, BGS borehole data, or winter-level site inspection. In clay-heavy areas such as the Thames Valley or the Midlands, winter water tables can be within 0.5m of surface.
  4. Trees — soakaways can affect the moisture content of clay soils; a soakaway within root zone of established trees (typically 1.5× tree height) can cause soil shrinkage and structural movement. Check with structural engineer where relevant.

Step 2 — BRE Digest 365 Percolation Test

The percolation test establishes Vp, the time in seconds for water level to fall 1 mm in a test pit.

Test procedure:

  1. Excavate three test pits at the proposed soakaway location, each 300×300mm plan and 300mm deep below the proposed soakaway invert level
  2. Auger or bore 300mm further into the base of each pit
  3. Fill each pit with water to the 300mm mark and allow to drain completely (this saturates the soil — do not record this drain time)
  4. Refill to 300mm and measure time for water level to fall from 75% depth (225mm) to 25% depth (75mm) — this is a fall of 150mm over time T seconds
  5. Calculate Vp = T / 150 (seconds per metre)
  6. Average the three test results; use the highest Vp for design (most conservative)

If the pit remains full after 24 hours, the ground is unsuitable for soakaway drainage.

Step 3 — Soakaway Volume Calculation

BRE Digest 365 provides storage volume equations based on storm return period and percolation rate. For domestic installations, the 1-in-10-year storm event is the minimum design standard.

Simplified method for domestic soakaways:

Required storage volume (m³) = A × D / (1000 × Vp × 2)

Where:

This is a simplified version — the full BRE Digest 365 method uses more complex routing calculations for larger catchments. For catchments over 100 m², use the full method or engage a drainage engineer.

Example calculation:

This soakaway needs at least 2.16 m³ of effective storage volume.

Step 4 — Sizing Crate or Rubble Systems

Plastic crate systems are now standard for new construction. Common UK products (Polypipe, Clark-Drain, Wavin, Brett Martin) have void ratios of 93–95%. Calculate:

Number of crate units = Required volume / (unit void volume per crate)

For a typical 1m × 0.6m × 0.4m crate with 420 litres void capacity: 2160 litres required ÷ 420 litres per unit = 6 units (minimum).

Arrange crates in a block (e.g. 2 wide × 3 long, or 2 high × 3 wide) wrapped in geotextile. The geotextile prevents soil ingress while allowing water to pass.

Rubble-fill soakaways use broken brick, clean stone, or coarse gravel. The effective void ratio of rubble is approximately 30% — making the required excavation volume about 3× the calculated storage volume. Rubble soakaways are harder to design consistently and are no longer the preferred approach for new work.

Step 5 — Installation

  1. Excavate to the required depth and plan dimensions, plus 150mm all round for geotextile wrap
  2. Line the excavation with geotextile (150 g/m² minimum, non-woven), leaving enough to fold over the top
  3. Install inlet pipe at correct level — ideally with a 900mm inspection pit/chamber before the soakaway for access and de-silting
  4. Place crate units, connecting as per manufacturer's instructions
  5. Install access inspection point on the soakaway (recommended even if not required by Building Control)
  6. Wrap geotextile over the top of the crate block
  7. Backfill with pea gravel or MOT Type 2 to 300mm depth, then topsoil to finish

Pipe connection — connect the downpipe or surface water drain to the soakaway via a trapped gully (to prevent smells and vermin access through a perforated soakaway inlet). Install a 100mm or 150mm inlet pipe set 150mm above the soakaway top of crates.

Soakaway for Driveway/Permeable Paving

Where a driveway is being re-laid and the area exceeds 5m², Building Regulations require that surface water does not discharge to the highway. Options include permeable paving (which infiltrates in-situ) or a soakaway/drain. For block paving with permeable drainage the sub-base itself provides some attenuation.

A separate soakaway for a new driveway may not be required if permeable paving is laid over a permeable or semi-permeable sub-base — check with the local planning and Building Control authorities.

Frequently Asked Questions

Can I connect a soakaway to a rainwater or surface water sewer?

A soakaway is a ground infiltration device — by definition it cannot connect to a sewer. If the ground is unsuitable for soakaway drainage, you need to connect to an existing surface water sewer (requires consent from the sewerage undertaker), a watercourse (requires consent from the Environment Agency or Lead Local Flood Authority), or design an alternative SuDS feature.

Do I need Building Regulations approval for a soakaway?

In England and Wales, under Approved Document H: a soakaway for a new drainage system serving an impermeable area over 100 m² requires Building Control approval. For smaller domestic installations (e.g. a garage or garden shed roof), notification is not usually required but the installation must still comply with H3 standards. Always check with your local Building Control office, as requirements vary.

How deep should a soakaway be?

Depth is dictated by: the invert level of the incoming pipe, the groundwater level (minimum 1m clearance to the base), and the required storage volume. Most domestic soakaways are 1.0–2.0m deep. The base of the soakaway must be in permeable ground — if a clay layer is encountered, the soakaway depth may need adjusting or the location reconsidered.

What maintenance does a soakaway need?

A properly installed crate soakaway with a geotextile wrap requires minimal maintenance. An inspection chamber before the soakaway should be inspected annually and cleared of silt and debris. If the soakaway performance degrades (slow drainage), the inlet chamber or gully should be cleaned first — most soakaway failures are actually inlet blockages, not soakaway saturation.

Regulations & Standards