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. For most UK properties without access to a public surface water sewer, a correctly designed soakaway is the approved drainage method under Building Regulations Approved Document H (Part H3) and the supporting standard BS EN 752.¹

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
• There is no risk of contamination from nearby septic tanks or cesspits (minimum separation 5 m)

They are not suitable in clay-heavy soils where water drains slower than 1 minute per 10 mm (the BS test threshold), 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, most local authorities and building inspectors accept the field method set out in Approved Document H:

1. Dig a trial pit 300 mm square by 300 mm deep at the proposed soakaway location and at the proposed soakaway base depth.¹
2. Fill the pit with water and allow it to drain completely, repeat twice to saturate the soil.
3. On the third fill, measure the time in seconds for the water level to drop 10 mm.
4. Calculate Vp (the average of three tests): Vp = time in seconds ÷ 10.

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

Soakaway volume is calculated from your roof (or paved area) drainage area and the percolation test result using the formula from BRE Digest 365:⁴

A = (Imp × D × Vp) ÷ (5 × 1000)

Where:

• A = required soakaway storage volume (m³)
• Imp = impermeable area draining to soakaway (m²)
• D = storm duration factor (use 10 for standard residential; hours)
• Vp = your percolation value

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

Values assume Vp of 50 and moderate UK rainfall intensity. 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, still an acceptable infiltration device under Part H¹, and cheaper in materials, but takes more digging: a rubble pit has to be larger than a crate system to hold the same volume of water, because clean stone gives around 30% voids against the 90-95% that crate manufacturers declare.

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 1,000 × 500 × 500 mm crate typically holds around 190–250 litres of void volume.

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

Cost Comparison (2026)

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, contact Dial Before You Dig (dial 0800 096 8399 or use the LSBUD map) and check for buried cables and pipes. 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 1:80 minimum (about 12 mm drop per metre).⁵ Bed the pipe in sharp sand. Fit an inspection chamber upstream of the soakaway if the run is longer than 10 m, this allows 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.⁶ For soakaways, this standard governs design life (typically 60 years), design storm return period, and documentation.

Planning permission is not usually required for a soakaway itself, it is underground infrastructure. However, if you are also installing a new driveway or patio covering more than 5 m² in non-permeable material that connects to the soakaway, that surfacing work may need permitted development compliance.⁷

Adoption

Soakaways are private drainage assets and are never 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 minimum from foundations is a legal minimum, not a target. 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 shallow fall (less than 1:80) allows debris to settle and block the inlet pipe, typically within five years.

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 depth exceeds 1.2 m (health and safety requirement to consider shoring)
• 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)

A drainage contractor typically charges £300–£800 for a complete soakaway installation including supply, dig, and reinstatement for a standard residential roof drainage job.

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.