Soakaway Size Calculator (UK Drainage)

Soakaway Design Guide (UK BRE 365)

What is a Soakaway?

A soakaway is an underground structure that collects surface water (from roofs, driveways, patios) and allows it to soak gradually into the surrounding soil. It manages rainwater on-site rather than discharging to the sewer or watercourse. When you need one: new buildings/extensions where surface water can't connect to a public sewer. Driveways and paving (especially since SuDS requirements). Where the local water authority requires on-site disposal. Increasingly required under Sustainable Drainage Systems (SuDS) guidance, which favours managing rainwater on-site rather than sending it straight to the combined sewer.

What's the key thing to understand about Percolation Testing?

The soil's ability to absorb water (percolation rate) is critical. The BRE 365 percolation test: dig a test pit (typically 300mm square, 300mm deep, in the proposed soakaway location). Fill with water, let it drain completely (pre-soak). Refill to 300mm depth. Measure time for water to fall from 75% to 25% full (i.e. the middle 50%). Repeat 3 times, take the average. Calculate Vp (seconds per mm): Vp = time (seconds) / (effective depth drop in mm). Soil type guide (rough Vp values): gravel/sand: drains very quickly. Clay: drains very slowly and may not be suitable for a soakaway at all without significant oversizing.

What do I need to know about Sizing the Soakaway?

BRE 365 method (simplified): the soakaway must store the inflow from a design storm minus what soaks away during the storm. Storage volume needed depends on: drained area (A) — bigger roof/paving = more water. Design rainfall — UK uses specific return-period storms (e.g. 1-in-10-year). Soil infiltration rate (from percolation test). The required storage volume is calculated for several storm durations, and the worst case (largest required volume) governs the design. Construction void ratios: plastic crate systems typically offer around 95% void space for water storage, compared to roughly 30% for a traditional rubble-filled soakaway of the same size.

What's the key thing to understand about Construction and Rules?

Key requirements (BRE 365 / Part H): minimum distance from buildings: 5 metres (to avoid affecting foundations). Distance from boundaries: typically 2.5m+. Below the water table: soakaways don't work if the ground is already saturated — check seasonal water table. Depth: typically 1-3m, within permeable soil, above the water table. Inlet: surface water pipe from gutters/drains. Silt trap/catchpit: recommended before the soakaway to prevent silt clogging it over time. Geotextile membrane: wraps completely around the soakaway structure to prevent surrounding soil particles migrating in and clogging the void space over time.

What should I know about Soakaway Location and Setback Distances?

UK Building Regulations and good practice set minimum setback distances for a soakaway: at least 5m from any building (to avoid undermining foundations or causing damp penetration), at least 5m from a boundary (to avoid affecting neighbouring property), and at least 5m from any other soakaway (to avoid overloading the same area of ground). Soakaways must also sit below the invert level of any nearby foundations and avoid being positioned where surface water could pond against a building during heavy storms before it has time to infiltrate. These distances exist because a soakaway deliberately concentrates large volumes of water into a small area of ground, and placing one too close to a structure risks softening the supporting soil or raising local groundwater levels enough to affect the building's foundations over time.

What do I need to know about Soakaways on Clay Soil?

Soakaways rely entirely on the surrounding soil's ability to absorb and disperse water (infiltration rate), and heavy clay soil — common across much of the UK — often has an infiltration rate too low (sometimes under 1×10⁻⁶ m/s) for a conventional soakaway to work effectively within a reasonable size, even using the maximum practical excavation depth. In these cases, alternative solutions are usually needed: a much larger soakaway with geocellular crates to maximise storage volume relative to excavated area, a combined attenuation and gradual-release system discharging to a watercourse or sewer at a controlled rate, or in the most difficult ground, a sealed storage tank with pumped discharge. A percolation test (filling a test pit and timing how fast the water level drops) is the only reliable way to know which approach is actually needed for a specific site — assuming 'average' soil infiltration rates without testing is a common and expensive mistake.

What do I need to know about Soakaway Crate Systems vs Rubble-Filled Pits?

Traditional soakaways were simply pits backfilled with rubble or hardcore, relying on the voids between stones to store water temporarily while it soaks away — this works but has relatively low storage efficiency, since the stone itself takes up much of the volume, typically leaving only around 30% void space. Modern plastic soakaway crates (sometimes called attenuation or infiltration crates) are hollow, stackable modules wrapped in a geotextile membrane that achieve around 95% void space, meaning a crate-based soakaway can be roughly a third the size of an equivalent rubble-filled pit for the same storage capacity. Crates are also lighter to install, easier to size precisely to calculated requirements, and less prone to the voids silting up over time compared with loose stone, though the surrounding geotextile still needs protecting from fine soil migrating in and gradually reducing capacity. For most modern domestic drainage schemes following the current soakaway design guidance, crate systems are now the more common choice, particularly where space is tight, while rubble-filled pits remain a lower-cost option for simple, non-critical applications such as a single rainwater downpipe on a large plot.

What's the BRE Digest 365 Method for Sizing a Soakaway?

BRE Digest 365 is the standard UK reference method for sizing infiltration soakaways for surface water drainage. It works from a measured percolation value Vp (how fast water infiltrates the specific soil, found by the percolation test) combined with the catchment area draining to the soakaway (roof area, paved area, or both) and a design storm return period (commonly the 1-in-10-year storm for a standard domestic soakaway). The core calculation finds the storage volume needed so the soakaway can hold and then gradually discharge the design storm's runoff without overflowing before the soil can infiltrate it away. In simplified terms: required storage volume ≈ (design rainfall depth for the storm duration × catchment area) − (infiltration rate × storage facility's internal surface area × storm duration), solved for the storage volume that keeps the soakaway from overtopping during the worst-case design storm. This is a more rigorous method than a simple rule-of-thumb sizing table and is what building control or a drainage engineer will expect to see for anything beyond a very small, low-risk soakaway.

Worked Example: Sizing a Soakaway for a 50m² Roof Area

Take a roof area of 50m² draining to a single soakaway, in soil with a measured percolation value Vp giving an infiltration rate of roughly 10×10⁻⁶ m/s (10 mm per hour, a moderate, workable value — sandy soils drain faster, clay soils much slower or not at all). For a 1-in-10-year, 10mm design storm (a simplified illustrative figure; a real design uses local rainfall intensity-duration-frequency data): runoff volume ≈ 50m² × 0.010m = 0.5m³. If the chosen soakaway (say, a crate-based system) can infiltrate water through its basal and side surface area while it's filling, the required stored volume is less than the full 0.5m³ runoff — but as a conservative first check, sizing storage for the full calculated runoff volume of roughly 0.5m³, then confirming with the full BRE 365 method or a drainage engineer, is the safe starting point before finalising crate quantities and excavation size. This illustrates why even a modest roof area needs a meaningfully sized soakaway, and why larger or multiple roof areas draining to one soakaway very quickly require a much bigger excavation than intuition suggests.

Do I Need Planning Permission or Building Regulations Approval for a Soakaway?

A domestic soakaway for roof or surface water drainage usually falls under Building Regulations Part H (drainage) rather than needing separate planning permission, though if the property is on a hillside, near a watercourse, or in an area with known groundwater or contamination issues, additional consent from the Environment Agency or local authority may be needed. Critically, a soakaway must not be sited where it could undermine a building's foundations or a neighbouring structure — minimum setback distances from buildings, boundaries, and trees (commonly at least 5m from any building and well clear of foundations) are a Building Regulations requirement, not just good practice, specifically because an improperly sited soakaway can saturate the ground under a nearby foundation and cause subsidence or heave.