Soakaway Sizing and Design: BRE Digest 365, Percolation Testing and Part H

Quick Answer: A soakaway must be sized to discharge the design storm event into the ground within 24 hours. Use BRE Digest 365 percolation tests to measure soil infiltration rate (f, in m/s), then size the soakaway storage volume from V = A_d × i × D − A_b × f × 50 × D (BRE Digest 365 simplified). Minimum 5m from any building (Approved Document H3) and 2.5m from a boundary. For a typical 50m² roof on moderately draining soil (sandy clay, f = 5×10⁻⁶ m/s), expect a soakaway storage volume of around 1.0–1.5m³ — typically four 1m³ crates or a 1.5m × 1.5m × 1.5m granular pit.

Summary

A soakaway is an underground void that receives surface water — typically roof drainage from a domestic property — and allows it to infiltrate slowly into the surrounding subsoil. Where mains surface water sewers exist, Building Regulations Part H1/H3 specifies a discharge hierarchy: 1) infiltration to ground (soakaway); 2) discharge to watercourse; 3) discharge to surface water sewer; 4) combined sewer (last resort). Connection to a public surface water sewer requires Section 106 approval from the water authority and is now actively discouraged where ground infiltration is feasible. As a result, almost every new domestic surface water disposal — and most retrofits where capacity is added or roof areas extended — uses a soakaway as the primary route.

The single most common mistake on domestic projects is sizing the soakaway by guess or rule of thumb rather than to BRE Digest 365. A "small" 1m³ soakaway is acceptable for a porch or single garage roof but inadequate for a typical 75m² rear extension. Undersized soakaways back up, surcharge upstream gullies, and lead to localised flooding of patios, lawns and (worst case) the property's wall foundations. Conversely, oversized soakaways cost the customer money for no benefit.

Part H also requires soakaways to be sited away from buildings and boundaries to prevent water-saturated ground from undermining foundations or affecting neighbouring property. The 5m minimum distance from buildings is a simple rule that ignores soil type — on clay subsoils prone to seasonal shrinkage and swelling (Mercia Mudstone, Boulder Clay, London Clay), a soakaway closer than 7–10m can drive seasonal foundation movement. Always assess the geology before siting.

Key Facts

• Approved Document H3 (2015) — sets surface water disposal hierarchy and minimum 5m distance from any building; soakaways serving a single dwelling can be designed without a structural engineer where ground conditions are simple
• BRE Digest 365 (2016) — the standard percolation test method for domestic soakaway design, also adopted in BS EN 752 and CIRIA C753 (SuDS Manual)
• BS EN 752:2017 — Drain and sewer systems outside buildings — general references soakaway design
• CIRIA C753 (2015) — The SuDS Manual; more sophisticated design approach for development projects, supersedes C523
• Section 106 Water Industry Act 1991 — connecting surface water to public sewer requires sewerage undertaker approval
• Design return period — typically a 1 in 10-year event for domestic soakaways (CIRIA recommends 1 in 30 for larger developments)
• Design storm duration — typically 15-minute storm for small catchments; longer durations needed for larger areas. The critical duration is the one that produces the largest required storage volume — try multiple
• Soakaway lifespan — 25–40 years typical; can fail by silting (sediment-loaded surface runoff) or biofouling (overgrown root systems penetrating crates)
• Granular pit (Victorian) soakaway — pit filled with hardcore (25–75mm angular crushed concrete or stone) wrapped in geotextile. Storage void ratio ≈ 30% — needs 3× larger pit than crates for same storage
• Modular crate soakaway — proprietary plastic crates (Wavin AquaCell, Polypipe Polystorm, Hydro Civil) with ≥90% storage void ratio. Wrap in geotextile and surround with 200mm angular granular bed
• Soak hole / rubble-filled trench — informal alternative for small loads (porch, garage); not adoption-grade
• Percolation test — three repeat tests in three trial pits across the proposed soakaway footprint, depths matching the bottom of the proposed soakaway
• Time to 75% empty (t_p75) — the BRE Digest 365 metric: soakaway must drain from 100% to 25% full within 24 hours
• Infiltration rate f (m/s) — soil property derived from the percolation test. Sands 10⁻⁴ to 10⁻⁵; sandy clays 10⁻⁵ to 10⁻⁶; silty clays 10⁻⁶ to 10⁻⁷; pure clay 10⁻⁷ to 10⁻⁹ (effectively impermeable — soakaway unviable)
• Watertable — soakaway base must be ≥1m above the highest seasonal water table (BS EN 752, Approved Document H3)

Quick Reference Table

(All figures assume 1:10-year design storm, 15-minute duration, 35mm/h intensity for SE England — adjust for postcode rainfall via FEH or BRE rainfall maps.)

Detailed Guidance

Step 1 — Percolation testing per BRE Digest 365

The test must be done before designing the soakaway, not after. Three repeat tests in three separate trial pits, ideally during the wettest seasonal period (winter/early spring) to ensure the result represents the worst-case soil condition.

Trial pit construction:

1. Excavate three pits at the proposed soakaway location, each 0.3m × 0.3m × 1.0m–3.0m deep — depth must match the proposed soakaway base level.
2. Stand a graduated rule (or vertical scale) inside the pit.
3. Fill the pit with clean water to a known level (typically 0.5–1.0m above the base).
4. Measure the time for the water level to fall from 75% full to 25% full (the t_p75-25 time).
5. Repeat — refill and re-test — at least three times per pit, take the longest of the three.
6. Repeat in two more pits.

Calculate the infiltration rate for each pit:

f = (V_p75 - V_p25) / (a_p50 × t_p75-25)

where:

• V_p75, V_p25 = water volumes at 75% and 25% (m³)
• a_p50 = internal surface area of pit at 50% depth (m²)
• t_p75-25 = time taken (s) to drain from 75% to 25%

Use the lowest (worst) f value from the three pits for design.

Step 2 — Design storm rainfall depth

For a domestic soakaway, the typical design event is the 1 in 10-year, 15-minute storm. The total depth (mm) varies by location:

Convert mm to m by dividing by 1000. For SE England, the design storm is 0.014 m of rainfall in 0.25 hours (15 min) = 900 seconds.

For more accuracy use FEH (Flood Estimation Handbook) rainfall depth-duration-frequency data, or the BRE rainfall map in Digest 365.

Step 3 — Soakaway storage volume

The BRE Digest 365 simplified design equation is:

S = (A_d × i × D) - (a_s50 × f × D)

Where:

• S = required soakaway storage volume (m³)
• A_d = drainage area (catchment) in m²
• i = rainfall intensity (m/s) — for 15-min 14mm storm, i = 0.014 / 900 = 1.56×10⁻⁵ m/s
• D = storm duration in seconds (e.g. 900 for 15-min storm)
• a_s50 = soakaway side area at 50% depth (m²) — depends on geometry
• f = infiltration rate (m/s)

The first term is total rainfall volume on the catchment over the storm; the second is the amount infiltrated during the storm. The difference is the required storage.

Try multiple durations (15-min, 30-min, 60-min, 4-hour, 24-hour) and the critical duration is the one giving the largest S. Soakaway storage is sized for that worst-case.

Worked example — typical 75m² rear extension, sandy clay

• A_d = 75 m² (catchment from new extension roof)
• f = 5×10⁻⁶ m/s (from percolation test)
• Try 15-min storm: i = 14mm in 900s = 1.56×10⁻⁵ m/s
• Assume soakaway is 1.5m × 1.5m × 1.5m granular pit
  • a_s50 = 4 × (1.5 × 0.75) = 4.5 m² (side area at half-depth)

S = (75 × 1.56×10⁻⁵ × 900) - (4.5 × 5×10⁻⁶ × 900) = 1.053 - 0.020 = 1.03 m³ storage required

A 1.5m³ granular pit (3.375m³ volume × 30% void ratio = 1.01m³ storage) is on the limit — try 1.8m × 1.5m × 1.5m pit (giving 1.21m³ storage), or two 1m³ crates (90% void ratio = 1.8m³ storage) which comfortably exceeds the design requirement.

Check 24h drain-down: at the start of drain-down the side area is the full a_s = 4 × 1.5 × 1.5 = 9 m². Initial outflow rate = 9 × 5×10⁻⁶ = 4.5×10⁻⁵ m³/s. To drain 1.03 m³ at this rate = 22,888s = 6.4 hours — within 24h target. Pass.

Step 4 — Geometry and construction

Granular pit (Victorian) construction:

1. Excavate to the design depth, typically 1.5–2.5m deep with sides battered or shored per CDM.
2. Wrap the void with non-woven geotextile (Terram 1000 grade or similar) — overlap edges by 300mm.
3. Fill with clean, washed, angular 40–75mm crushed concrete or hard stone (NOT pea gravel — too small a void ratio and silts up).
4. Inflow pipe (100mm or 150mm) terminates inside the geotextile, with a perforated section at the top of the granular fill.
5. Inspection chamber upstream of the soakaway is required for Part H compliance.
6. Top the soakaway with geotextile, then 300mm of cover soil to grass level (or to base of footing detail).

Modular crate construction:

1. Excavate to the design depth — usually 1.2m deeper than the pipe invert level to allow crate height plus 150mm bedding.
2. Lay 150mm of 20mm angular crushed stone as bedding; level and compacted.
3. Wrap crates fully in non-woven geotextile (Terram 1000 or equivalent) — sealing all overlaps.
4. Stack crates per manufacturer instructions; connect to inflow pipe via proprietary inspection cap.
5. Backfill sides with 200mm of 20mm angular crushed stone before granular surround backfill.
6. Cover with geotextile, then minimum 500mm of cover for non-trafficked area (1m for vehicle loading — and use trafficked-grade crates).

Step 5 — Siting (Part H3 compliance)

• 5m minimum from any building (front, side, rear)
• 2.5m minimum from a property boundary
• 5m minimum from any tree with mature spread (and ideally outside the rooting zone of mature trees per NHBC 4.2)
• 1m minimum above seasonal high water table
• No soakaway on clay slopes >1 in 10 without engineering assessment — risk of slope instability from saturated ground

On clay subsoils with seasonal shrink-swell behaviour, increase the 5m distance to 7–10m where possible — saturated clay near foundations can drive seasonal heave/subsidence.

When a soakaway is not viable

• Clay subsoils where f < 1×10⁻⁶ m/s — soakaway becomes uneconomically large and unreliable
• High water table where seasonal water reaches within 1m of soakaway base
• Heavily contaminated ground (urban brownfield, former industrial)
• Listed building where excavation is restricted

Alternative discharge routes:

• Permeable paving with integrated infiltration (SuDS approach per CIRIA C753) — sub-base acts as soakaway
• Lined attenuation tank with throttled outflow to surface water sewer
• Connection to watercourse under Land Drainage Consent (Environment Agency / Lead Local Flood Authority)
• Last resort: Section 106 connection to surface water sewer with sewerage undertaker approval
• Rainwater harvesting tank with controlled overflow to soakaway — reduces effective catchment

Frequently Asked Questions

Do I need a percolation test for a small porch soakaway?

Strictly per BRE Digest 365 — yes, even for small soakaways. In practice for porch and small garage roofs (<20m² catchment), a single test is acceptable, and many local Building Control surveyors accept a rule-of-thumb assumption (e.g. 1m³ soakaway for porches on suspected clay subsoil). Always do the test on bigger jobs — the cost is small relative to the risk of an undersized soakaway needing rebuilding.

Can I discharge a foul drain to a soakaway?

No. Soakaways accept surface water only. Foul drainage must connect to a foul sewer, septic tank or package treatment plant per the Environment Agency General Binding Rules. A combined sewer surcharge from foul into a soakaway is a serious public health risk.

What's the minimum depth a soakaway needs to be?

The soakaway top should be at least 500mm below ground level (for clear cover and frost protection). The minimum total depth is usually 1.2–1.5m — small shallow soakaways (under 1m deep) struggle to access sufficiently permeable subsoil under the topsoil and rooting zone. A soakaway in topsoil only is largely ineffective in a saturated rain event.

How long do soakaways last before needing replacement?

A well-designed and constructed soakaway with appropriate geotextile and clean angular fill should last 25–40 years. Failure modes are:

• Silting — fine sediment carried by surface runoff blocks the void space. Use a silt trap or filter chamber at the inlet to extend life.
• Biofouling / root ingress — tree roots penetrate the geotextile and clog the void. Avoid siting under or close to mature trees with aggressive root systems (willow, poplar, ash).
• Soil consolidation — clay subsoils slowly compact around the soakaway and infiltration rate reduces.

Modular crate soakaways with inspection caps allow periodic CCTV inspection and jetting maintenance — extending life beyond the granular-pit norm.

Does my insurer/mortgage care about soakaway siting?

If the soakaway is closer than 5m to a building, or if the property has experienced subsidence and the soakaway is suspected as a contributing factor — yes. Saturated ground around foundations is a known subsidence trigger. Conversely, well-sited soakaways are routinely accepted by insurers and lenders without comment. Document the design and percolation test in case of a future query.

Regulations & Standards

• Approved Document H (2015) — Drainage and waste disposal; H1 foul drainage, H3 rainwater drainage and surface water disposal

• BRE Digest 365 (2016 revision) — Soakaway design

• BS EN 752:2017 — Drain and sewer systems outside buildings — general principles

• BS 8000-14:2011 — Workmanship on building sites; Code of practice for below-ground drainage

• BS EN 1610:2015 — Construction and testing of drains and sewers

• CIRIA C753 (2015) — The SuDS Manual (sustainable drainage systems design)

• CIRIA C609 — Sustainable drainage systems; hydraulic, structural and water quality advice

• Water Industry Act 1991 — Section 106 surface water connection to public sewer

• Flood Estimation Handbook (FEH) — Centre for Ecology and Hydrology rainfall depth-duration-frequency

• Environment Agency General Binding Rules — small sewage discharge / surface water discharge requirements

• Approved Document H drainage and waste disposal — Part H text

• BRE Digest 365 product page — soakaway design method (paid publication)

• CIRIA C753 SuDS Manual — sustainable drainage systems guidance

• Environment Agency General Binding Rules — sewage and surface water rules

• Centre for Ecology and Hydrology — UK rainfall data — FEH design rainfall

• soakaways — overview article (complementary, less detailed on sizing)

• underground drainage — pipe sizing and gradient for inflow to soakaway

• manhole construction — inspection chamber upstream of soakaway

• rainwater harvesting — alternative to soakaway with controlled overflow

• permeable paving — integrated SuDS sub-base infiltration

• drainage landscaping — wider site drainage and French drain options