How to Build Soakaway

A soakaway is a subsurface pit that receives surface water runoff, typically from a roof via downpipes, and disperses it slowly into the surrounding soil. Under Building Regulations Approved Document H (requirement H3), a soakaway or other infiltration system is the first choice for rainwater: only where that is not reasonably practicable should it go to a watercourse, and only after that to a sewer.¹

Getting it right is not complicated, but there are two steps that most DIYers skip: the percolation test to check whether your soil can actually absorb water, and the sizing calculation to make the pit large enough. Both are covered below.

When Is a Soakaway Appropriate?

Soakaways are suitable where:

• The soil percolation rate is adequate (see test below)
• The water table does not reach the bottom of the soakaway at any time of year¹. That is the national test. Individual councils often ask for a margin below it: Wiltshire, for example, wants 1 m of unsaturated ground beneath the base²
• The soakaway can be positioned at least 5 m from any building or road, which is the Part H3 minimum¹. There is no national distance to a boundary. Councils set their own, and Copeland for instance asks for 2.5 m³
• It is not near any drainage field, drainage mound or other soakaway, so the ground’s overall soakage capacity is not exceeded³

They are not suitable in clay-heavy soils where the percolation test gives a Vp above 100 (more than 100 seconds for the water to fall 1 mm), in areas with high groundwater tables, on brownfield land with potential contamination, or on steeply sloping ground where lateral migration could undermine structures.

Percolation Test

Before designing your soakaway, you must establish the soil’s infiltration rate. For a soakaway serving a larger area Part H3 sends the design to BS EN 752 or BRE Digest 365¹. For a domestic soakaway, Approved Document H lets you use the percolation test from its drainage-field guidance (Part H2).¹ Council building control guidance, such as Copeland’s, sets out the steps for rainwater soakaways:

1. Dig a hole 300 mm square at the proposed soakaway position, to a depth 300 mm below the invert level of the pipe that will feed it.³
2. Fill the bottom 300 mm of the hole with water and let it seep away overnight.³
3. Next day, refill it to at least 300 mm and time, in seconds, how long the water takes to fall from 75% full to 25% full (a drop of 150 mm).³
4. Divide that time by 150 to get Vp, the average time in seconds for the water to fall 1 mm.³
5. Carry out the test at least three times, using at least two trial holes, and take the average.³

A Vp of between 12 and 100 is the range Approved Document H sets for a drainage field, and building control commonly applies the same window to soakaways.¹ Below 12 the ground drains too fast for the soil to filter the water; above 100 it drains too slowly and the soakaway will back up.

Clay Soils

Much of London and the Home Counties sits on London Clay. A Vp above 100 is common. If your percolation test fails, you will need an alternative drainage route, a French drain to a watercourse, an attenuation tank, or an adoptable surface water connection.

How to Size a Soakaway

A soakaway must store the difference between the rain that flows in during a storm and the water that soaks out through its sides over the same time. The inflow is the rainfall depth multiplied by the area drained.¹ Approved Document H says soakaways should be designed for a storm with a return period of once in ten years, trying storms of different lengths to find the one that needs the most storage.¹ The outflow depends on the area of the pit sides when half full and on the soil infiltration rate, which the guidance relates to your percolation value as f = 10⁻³ ÷ (2 × Vp).³ For larger soakaways the full method is set out in BRE Digest 365, Soakaway design, last revised in 2016.⁴

For a simpler practical guide, use the table below based on roof area:

For a soakaway serving 25 m² or less, Approved Document H allows a design rainfall of 10 mm in 5 minutes, which means storing about 0.25 m³ (25 m² × 10 mm).¹ The larger figures are indicative only and assume slow-draining soil: always calculate from your own percolation test.

Rubble Soakaway vs Polypropylene Crate System

Traditional Rubble Soakaway

A rubble soakaway is a pit filled with clean angular aggregate (20–40 mm crushed stone or hardcore), with the incoming pipe feeding into the top and water dispersing from the voids in the stone. It is the older method and is still an accepted form of soakaway for small areas.³ It takes more digging, though: a rubble pit has to be much larger than a crate system to hold the same volume of water, because clean stone leaves only about 20–30% of the pit as open space, against the 95% or so that crate makers declare.³ A domestic rubble soakaway may also need renewing about every ten years.³

Best for: small areas (up to 30 m²), properties with easily accessible open ground, lower budgets.

Polypropylene Crate System

Plastic soakaway crates (also called modular infiltration units) are interlocking polypropylene boxes with near-total void volume. They are wrapped in geotextile membrane to prevent soil ingress and sit at the bottom of a relatively compact excavation. A common domestic crate measures 1,000 × 500 × 400 mm and, with a void ratio of around 96%, stores about 190 litres.⁵

Best for: larger drainage areas, constrained plots (shallower footprint), gardens where surface disruption must be minimised.

Cost Comparison (2026)

Crate prices are for supply only: the Naylor AquaVoid Home 20T crate (1,000 × 500 × 400 mm), for example, was listed at £37.50 including VAT in September 2026.⁵ Contractor totals are estimates; get at least three quotes.

Tools and Materials

Tools

• Spade and mattock (or hire a mini-digger for larger excavations)
• Spirit level and tape measure
• Hacksaw or pipe cutter
• Rubber mallet
• Wheelbarrow

Materials

• Soakaway crates or clean angular aggregate (20–40 mm)
• Geotextile membrane (non-woven, 90–120 g/m²)
• 110 mm UPVC drainage pipe and fittings
• Inspection chamber and cover (recommended)
• Sharp sand (for bedding pipe)
• Cable ties or geotextile tape

Step-by-Step: Building a Crate Soakaway

Step 1: Confirm Utilities

Before digging, check for buried cables and pipes with a free search on LinesearchbeforeUdig (LSBUD), which maps many utility networks, and use a cable locator on site if in doubt.⁶ Check your title deeds for any drainage easements.

Step 2: Mark Out and Excavate

Mark the soakaway footprint at least 5 m from the building. Dig to a depth that allows for: the crate stack height, plus 150 mm of granular material beneath, plus at least 200 mm of cover above. Allow 300 mm extra around the perimeter for membrane wrap. The base of the excavation must be below the invert level of your incoming drain.

Step 3: Prepare the Base

Compact the base firmly. Lay a 150 mm bed of clean angular aggregate (20–40 mm). Level with a spirit level, the crates must sit flat.

Step 4: Wrap in Geotextile

Line the entire excavation, base and sides, with geotextile membrane. Leave enough membrane hanging over the top edges to fold over and cover the top of the crate stack once assembled. This prevents soil migration into the voids over time.

Step 5: Assemble the Crates

Stack and clip the crates together following the manufacturer’s instructions. Most systems use push-fit connectors. Ensure the inlet pipe knockout is positioned to accept your 110 mm UPVC drain.

Step 6: Connect the Inlet Pipe

Run your 110 mm UPVC drain from the downpipe to the crate inlet. The pipe should fall at no less than 1:100 (10 mm per metre), the Approved Document H minimum for a 100 mm rainwater drain; 1:80 (about 12 mm per metre) gives a little more margin.¹ Bed the pipe in sharp sand. Fit an inspection chamber upstream of the soakaway, especially on a longer run, to allow rodding access.

Step 7: Install an Inspection Chamber (Strongly Recommended)

An inspection chamber at the junction between your downpipe and the soakaway run allows you to check water is flowing and to rod the pipe if a blockage occurs. A basic 100 mm chamber and cover costs £30–£60 at any builders’ merchant.

Step 8: Fold Over Membrane and Backfill

Once all pipe connections are made, fold the membrane over the top of the crates and secure with tape or cable ties. Backfill with the excavated soil in 150 mm layers, compacting each layer as you go. Do not use a heavy plate compactor directly over the crates, hand-tamp or use a lightweight wacker plate on the layers above 500 mm of cover.

Step 9: Reinstate Surface

Restore topsoil and turf, or lay whatever surface is appropriate. Avoid paving over the soakaway, the ground above should remain permeable to allow air and some moisture movement.

Building Regulations and Planning

Building Regulations Part H3 requires that new drainage to a soakaway serving an extension or new dwelling is built to comply with the standard. For extensions and new builds, you will typically need to notify your local authority building control (LABC) or a registered building control approver. Soakaways serving only roof drainage from small extensions (under the permitted development thresholds) are often covered under building notice rather than full plans.

BS EN 752 sets out the framework for drain and sewer systems outside buildings.⁷ Approved Document H refers designers of soakaways serving larger areas to it, or to BRE Digest 365.¹ Copeland Council’s building control guidance notes that BRE Digest 365 is the most commonly used.³

Planning permission is not usually required for a soakaway itself, it is underground infrastructure. However, if you are also paving a front garden, a new hard surface of more than 5 m² needs planning permission unless it is permeable or the rainwater runs to a permeable area such as a lawn or border.⁸

Adoption

A domestic soakaway is a private drainage asset and is not normally adopted by the water company. Maintenance is your responsibility.

Common Mistakes

• Skipping the percolation test. Building a soakaway in clay soil that fails the test results in a pit that fills with water and never empties, backing up into downpipes and causing damp.
• Under-sizing. Using a rule-of-thumb crate count without calculating from actual roof area and Vp leads to soakaways that overflow in moderate rain.
• Too close to the house. The 5 m distance in Approved Document H is a minimum, not a target.¹ Water migrating toward shallow foundations causes subsidence over time.
• No inspection access. A soakaway with no upstream inspection chamber is impossible to diagnose or rod when something goes wrong.
• Poor pipe fall. A fall flatter than 1:100 lets debris settle and block the inlet pipe.

When to Get a Professional

DIY is feasible for small domestic soakaways (single downpipe, roof area under 50 m²) where the percolation test passes and access is clear. Consider using a contractor when:

• Your soil type is uncertain or the percolation test is borderline
• The excavation is deep enough for its sides to collapse: HSE warns that no ground can be relied upon to stand unsupported in all circumstances⁹
• The soakaway needs to receive multiple drainage sources or a large impermeable area
• Building control requires drawings or a drainage design report
• Ground conditions are variable (made ground, filled areas, sloping sites)

Quotes from drainage contractors for a complete soakaway installation on a standard residential roof, including supply, dig and reinstatement, commonly run from about £500 for a small rubble pit to £1,500 for a larger crate system: treat these as estimates and get several quotes.

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.