Soakaway Design and Installation: Crate Soakaways, French Drains and BS EN 752 Sizing

Quick Answer: A domestic soakaway must sit at least 5m from any building and 2.5m from a boundary, with capacity calculated from a BRE Digest 365 percolation test on the actual ground. For an average UK 100m² roof on moderately permeable subsoil, expect 1.5–4m³ of crate or rubble storage. Approved Document H1 requires soakaways for surface water disposal where mains sewer connection is impractical, and the local authority must approve the design as part of Building Control.

Summary

A soakaway disposes of surface water by letting it percolate into the ground. Done well it is a permanent, maintenance-free solution that meets Building Control and SuDS requirements. Done badly it floods foundations, undermines patios, and triggers expensive remedial work years after the original installer has moved on.

Two failure modes account for almost every soakaway problem: undersized capacity (no percolation test was done, or the result was ignored), and ground that does not actually drain (clay subsoil, shallow water table, or a perched water layer over impermeable rock). The percolation test required by BRE Digest 365 takes a couple of hours and a 300mm auger, and it tells you both whether a soakaway is feasible at all and how big it needs to be. Skipping it is the most common, and most expensive, mistake.

This article covers the design calculation, the test method, construction options (crate vs rubble vs French drain), and the regulatory requirements under Approved Document H1 and the SuDS hierarchy. For the wider drainage picture see the underground drainage and gradient article and the existing soakaway overview.

Key Facts

Quick Reference Table

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Soil Type Typical f (m/s) Suitability for Soakaway
Clean sand/gravel 10⁻³ to 10⁻⁴ Excellent
Sandy loam 10⁻⁴ to 10⁻⁵ Good
Silty loam 10⁻⁵ to 10⁻⁶ Marginal — large soakaway required
Silty clay 10⁻⁶ to 10⁻⁷ Poor — soakaway often impractical
Clay <10⁻⁷ Unsuitable — alternative disposal required
Roof Area (impermeable) Typical Soakaway Volume (moderate soil, f ≈ 10⁻⁵)
50m² (small extension) 1.0–1.5m³
100m² (typical UK semi) 2.0–3.5m³
150m² (detached) 3.0–5.5m³
200m² (large detached) 4.0–7.5m³
300m² (small commercial) 6.0–12m³
Construction Type Void Ratio Excavation per m³ Storage Typical Lifespan
Crate soakaway ~95% 1.05m³ 50+ years
Clean stone (40–80mm) ~30% 3.3m³ 30–50 years (assumes geotextile)
Brick honeycomb ~30% 3.3m³ 30–50 years (legacy method)
Pre-cast concrete chamber varies varies 50+ years (commercial use mainly)

Detailed Guidance

When a Soakaway Is the Right Solution

Approved Document H1 sets out a SuDS-led drainage hierarchy. Surface water should be disposed of in this priority order:

  1. Adequate soakaway or other infiltration system
  2. Watercourse
  3. Surface water sewer
  4. Combined sewer

Building Control will not approve a connection to surface water sewer or combined sewer if a soakaway is feasible. So the question is rarely "should we use a soakaway?" but "can we use a soakaway here?". The answer depends on three things:

Eliminate any of these three and you are looking at watercourse discharge (with Environment Agency consent for non-trivial flows) or sewer connection (requires a build-over or connection agreement with the water company).

The BRE Digest 365 Percolation Test

Every domestic soakaway must be sized from an actual on-site test, not from a soil-type assumption. The BRE method:

Test pit:

Test procedure:

  1. Refill the pit to the top.
  2. Record the time taken for the water level to drop from 75% full to 25% full (i.e. across the middle 50% of the pit depth).
  3. Repeat at least three times. The infiltration rate is calculated from the slowest (worst) result.

Calculation:

The soil infiltration rate f is calculated as:

f = Vp75-25 / (a × tp75-25)

Where:

A typical result of 30 minutes for the level to drop from 75% to 25% gives f ≈ 4.2 × 10⁻⁵ m/s — moderate to good infiltration.

If the slowest test gives a tp75-25 longer than about 4 hours (roughly f < 10⁻⁶ m/s), a soakaway is not feasible.

Sizing the Soakaway — The BRE Method

The required storage volume V is calculated for a chosen design storm (typically the 10-year return period, 30-minute or 60-minute duration depending on local rainfall data):

V = (i × A × D) − (f × A50% × D)

Where:

The calculation is iterated for several storm durations (15, 30, 60, 120 minutes) and the largest required volume is the design size.

Worked example — 100m² roof, moderate soil:

Assume soakaway 2m × 1m × 1.5m (3m³ external volume, ~2.85m³ storage with 95% void crate). Wetted side area when half full: 2 × (2+1) × 0.75 = 4.5m². Outflow over 30 min: 4.2 × 10⁻⁵ × 4.5 × 1800 = 0.34m³. Net storage required: 3.0 − 0.34 = 2.66m³ — within the 2.85m³ capacity, so the design is adequate.

For a tradesperson-level shortcut, the rule of thumb on moderate soil is to provide storage equal to 30–40mm of rainfall over the catchment area — 3.0–4.0 m³ per 100m² of roof. Always confirm with the percolation result.

Construction Methods Compared

Crate soakaway (modern standard for domestic):

Faster to install than rubble, smaller excavation footprint, easier to specify and certify. Cost is higher (£150–£400/m³ storage in materials versus £50–£100/m³ for stone) but labour is dramatically lower.

Stone-filled (rubble) soakaway:

Lower material cost but more excavation, more spoil to remove, more compaction risk to surrounding ground. Common on rural and self-build sites where excavator and stone are cheap but crates are not.

French drain (linear soakaway):

See the garden drainage article for French drain detail in landscape contexts.

What to avoid:

Building Control and Approval

Soakaway design forms part of any application that involves new surface water drainage (extensions, conservatories, new buildings, replacement driveways over 5m² of impermeable surface). Submit:

The Building Control surveyor will inspect the excavation before backfill — book the inspection. Post-completion, retain the test result and design for the building file (it will be asked for at sale time on the property information form).

For sites where mains drainage is provided and infiltration is not feasible, a build-over or connection agreement with the water company is needed before discharge to a public sewer. See the build-over agreement article for the process.

Common Failure Modes

For diagnostic decision tree on existing failed soakaways see the blocked soakaway diagnostic (in queue).

Frequently Asked Questions

Can I dispose of foul drainage through a soakaway?

No — foul water (toilet, sink, shower, bath waste) cannot discharge to a soakaway. Foul drainage must connect to the public foul or combined sewer, or to a private sewage treatment plant or septic tank discharging in accordance with the General Binding Rules under the Environmental Permitting Regulations 2016. A soakaway is for surface water (rainwater) only. Mixing foul into a surface water soakaway is both an environmental offence and a breach of Approved Document H.

What's the minimum size soakaway I can use legally?

There is no statutory minimum size — the size is whatever the percolation test and rainfall calculation say is required to handle the design storm without overflow. In practice, the smallest typical domestic soakaway is around 1m³ of storage, serving a small extension of around 30–50m² of roof on permeable ground. Anything smaller than this rarely satisfies a 10-year return period storm calculation.

Do I need planning permission for a soakaway?

Generally no — soakaways for domestic surface water disposal are covered by permitted development. However, if the soakaway is associated with a new driveway over 5m² of impermeable surface, planning permission is required for the driveway itself unless the soakaway provides full SuDS-compliant attenuation (or the driveway uses permeable paving). Always check with the local planning authority for sites in conservation areas, listed buildings, or where the property has had previous permitted-development restrictions removed.

How long does a properly installed crate soakaway last?

A crate soakaway with full geotextile wrap, correct backfill and inspection access should last 50 years or more. The plastic itself is rated for permanent burial and the silt-load is excluded by the geotextile. The most common cause of premature failure is omitting or undersizing the geotextile, which lets fine soil particles wash into the void and gradually fill it. Cleaning a clogged crate soakaway is impractical — replacement is usually the only option, which is why the geotextile spec matters more than the crate spec.

Can two houses share a soakaway?

In principle yes, but it is rare. Each property has its own surface water disposal under Building Regulations, and shared infrastructure creates ongoing maintenance liability that crosses property boundaries. New developments sometimes use a shared attenuation basin or oversized communal soakaway under a SuDS Adoption Agreement with the water company or a management company. For two existing houses, the cost and complication of a legal agreement usually outweighs the cost of two separate soakaways.

Regulations & Standards