How to Build a Soakaway: A Step-by-Step Guide

What Is a Soakaway and How Does It Work?

A soakaway is a subsurface drainage structure that collects surface water runoff and allows it to percolate slowly into the surrounding soil. Water enters via a drainage pipe, fills the void space inside the soakaway, and then filters outward through the walls into the earth.

The key is balance. The soakaway must store incoming water fast enough during a storm, then release it into the ground slowly enough that the surrounding soil is not permanently saturated. Most modern domestic soakaways use modular plastic crates rather than gravel or rubble. The crates provide roughly 95% void space, compared with 30-40% for a rubble-filled pit, so a much smaller excavation achieves the same storage volume.

Soakaways handle surface water only: rainwater from roofs, gutters, and paved areas. They are not suitable for foul water (sewage) or wastewater from sinks and appliances. Connecting foul drainage to a soakaway is illegal without specific consent, and the consequences for soil contamination can be serious.

Do You Need Planning Permission or Building Regulations Approval?

In most cases you do not need planning permission to install a surface water soakaway, but Building Regulations almost always apply. These are different things, and confusing them is one of the most common mistakes homeowners make.

Approved Document H governs drainage and waste disposal in England. It sets out the rules for soakaway location, design, and sizing. If you are installing a soakaway as part of a new build, extension, or significant drainage alteration, you will typically need Building Control approval, and the work will be subject to inspection before backfilling.

Even if your extension falls under Permitted Development, Building Regulations still require you to manage the additional surface water it creates. Failing to notify Building Control is a common oversight that can cause problems when you come to sell the property.

How Do You Test Whether a Soakaway Will Work on Your Land?

A percolation test tells you how quickly your soil absorbs water. It is not optional. Skipping this step and installing a soakaway that the ground cannot drain is one of the most expensive mistakes in domestic drainage work.

The standard method in the UK is the BRE Digest 365 percolation test:

1. Excavate a trial pit, at minimum 300mm x 300mm in cross-section, to the proposed soakaway depth.
2. Pre-soak the pit by filling it with water and allowing it to drain. This saturates the surrounding soil to simulate real-world conditions rather than testing dry ground.
3. Fill the pit again and measure the time for the water level to drop from 75% full to 25% full.
4. Repeat the test at least three times, and carry out tests at a minimum of three separate locations across the proposed soakaway area.

The result, the infiltration rate, tells you how fast water moves through your soil. That figure drives the sizing calculation.

What If the Test Fails?

If your soil absorbs water very slowly, or not at all, a conventional soakaway may not be viable. This is common in Cornwall, where clay subsoils and shillet layers are widespread. In those cases, your options include:

• A larger or deeper soakaway designed around the actual infiltration rate, accepting a slower drain-down
• A drainage trench or French drain that spreads the load across a wider area
• Connection to a surface water sewer or watercourse (requires authority consent)
• A garden drainage system that manages water above ground before it reaches the soil

If the test shows the ground simply cannot absorb water at an acceptable rate, installing a soakaway anyway will result in waterlogging, ground movement, and eventual system failure. Professional drainage advice at this stage costs far less than remedial work later.

How Do You Size a Soakaway Correctly?

Soakaway size depends on three factors: the area draining into it, the rainfall intensity your system needs to handle, and the infiltration rate of your soil. All three must be calculated together.

BRE Digest 365 is the definitive design standard for soakaways in the UK. It provides the methodology for calculating the required storage volume based on your measured infiltration rate and a design storm event.

The required storage volume is also referenced in BS 8582:2013, which states that rainwater soakaways should be designed in accordance with BRE Digest 365 or CIRIA Report 156.

For soakaways serving areas up to 25m², Approved Document H allows a simplified approach: a design rainfall of 10mm in five minutes is used as the worst-case figure. For anything larger, the full BRE Digest 365 calculation is required.

Key inputs to the calculation:

• Catchment area: The roof or paved area, in square metres, that drains into the soakaway
• Design rainfall: In Cornwall, high seasonal rainfall means your design storm figure will be higher than the UK average
• Infiltration rate: Your measured percolation test result
• Climate change allowance: Current guidance requires the soakaway to handle a 1-in-100-year storm event plus a 40% allowance for climate change

The soakaway must also be capable of draining from full to half-volume within 24 hours. This ensures sufficient capacity is available before the next significant rainfall event.

Step-by-Step: How to Build a Soakaway

Before you excavate, confirm two things: your percolation test shows the ground is suitable, and you know the required storage volume from your BRE Digest 365 sizing calculation.

Step 1: Choose the Right Location

Position matters as much as size. The minimum required distances are:

• At least 5 metres from any building (to protect foundations from the increased moisture)
• At least 2.5 metres from any property boundary
• Below or at the same level as the building being drained (so water flows toward the soakaway naturally)
• Away from large trees, whose roots can damage the system over time
• Clear of any underground services: gas mains, water mains, electrical cables, drainage pipes, and street lighting cables

Check the location thoroughly before you break ground. Your local utility providers can confirm the position of buried services.

Step 2: Excavate the Pit

Dig to the dimensions required by your storage volume calculation. For clay soils, consider going wider rather than deeper: the sides of the pit drain more effectively than the base in low-infiltration ground, and depth also increases the risk of hitting the water table.

If you are using mechanical equipment, take care not to smear the pit walls. Smearing compresses the soil and reduces its permeability. If smearing occurs, roughen the walls with hand tools before installation.

Step 3: Lay the Geotextile Membrane

Line the pit with a non-woven geotextile fabric before any crates go in. The membrane prevents fine soil particles from migrating into the soakaway and clogging the system over time. It is one of the most important details in a long-lasting installation.

The membrane should extend up and over the sides of the pit, with enough excess to fold back over the top once the crates are in place.

Step 4: Install the Soakaway Crates

Lower the plastic crates into the lined pit and connect them according to the manufacturer’s instructions. Standard crates typically measure around 1,000mm x 500mm x 400mm, each providing approximately 0.2m³ of storage. Clip or tie adjacent crates together to prevent movement as fill material is added around them.

Wrap the excess geotextile membrane back over the top of the crates before any backfill goes in. This creates a fully enclosed system that keeps fine soil out on all sides.

Step 5: Connect the Inlet Pipe

Run a perforated or solid drainage pipe from the downpipe, gutter outlet, or surface drain into the soakaway. The pipe should be laid with a continuous fall toward the soakaway: no dips or level sections where sediment can accumulate.

Use a non-return valve on the inlet where possible. This prevents water backing out of the soakaway into the drainage pipe if the system fills during heavy rainfall.

Step 6: Backfill and Reinstate

Backfill around and over the crate assembly with clean material, typically sharp sand or clean angular gravel in the immediate surround. Compact the backfill in layers to prevent settlement.

Finish the surface to match the surrounding ground. Soakaways installed in lawns or garden areas can be turfed over. Those under paved areas need a load-bearing specification: confirm the crates you have chosen are rated for the expected surface load before paving over them.

How to Tell If Your Soakaway Is Failing

A failing soakaway shows above-ground signs before any underground inspection is needed. Knowing what to look for saves time and reduces the risk of structural damage to your property.

Common signs of soakaway failure:

• Persistent pooling water around the soakaway area after rain, or in the garden generally, that does not disperse within a few hours
• Slow drainage from gutters, downpipes, or gullies connected to the soakaway, even during moderate rainfall
• Ground subsidence or dips in the lawn or garden above the soakaway, caused by soil compaction or wash-out beneath the surface
• Gurgling sounds from drainage pipes, indicating water is backing up rather than flowing freely
• Waterlogged ground that returns quickly after dry spells, suggesting the soakaway is no longer releasing water into the surrounding soil

The three most common causes of soakaway failure are silt and sediment build-up (usually a sign that the geotextile membrane was poor or absent), compacted surrounding soil that can no longer absorb water, and a collapsed inlet pipe that has disconnected from the system.

If any of these signs are present, a CCTV Drain Survey will confirm whether the fault is with the inlet pipework or the soakaway itself, before any excavation is required.

How Clear Stream Handles Soakaway Assessment and Garden Drainage

Clear Stream engineers carry out site assessments and garden drainage surveys across Cornwall and Devon. The starting point is always diagnosis, not assumption.

When you contact us about a soakaway problem or a new installation, we arrange a site visit and assess:

• Where water is collecting and why
• Current ground conditions and likely soil type
• Whether an existing soakaway has failed or is undersized
• Where surface water can realistically be discharged

For sites where ground conditions are unclear, we carry out percolation testing to confirm whether a soakaway is viable before any design work or excavation begins. If the soil cannot support a soakaway, we recommend the right alternative rather than installing a system that will fail.

All work is quoted at a fixed price before we start. No call-out fees, no mid-job surprises. Our engineers cover all of Cornwall and Devon, with a standard 1-2 hour response window. Every drainage repair we carry out is backed by our 5-Year Guarantee.

For properties with septic tanks, soakaway drain fields are a critical part of the system. We assess those separately, as they operate under different regulations and have specific siting requirements. If you suspect your septic soakaway is failing, call us rather than attempting to investigate it yourself.

For wider surface water management, see our garden drainage services, which cover everything from French drains and channel drainage to full site surveys and remedial works.

Conclusion

Building a soakaway is not complicated, but it has to be done in the right sequence. The percolation test comes first. The sizing calculation comes next. Only then does excavation make sense.

Key takeaways:

• Soakaways are governed by Approved Document H and almost always require Building Control notification
• BRE Digest 365 is the recognised sizing standard for soakaways serving areas larger than 25m²
• Clay soils and high water tables, common in Cornwall, can make a standard soakaway non-viable without a modified design
• A properly installed crate soakaway with geotextile membrane should last significantly longer than a rubble-filled pit

If you are unsure whether your ground is suitable, or you have an existing system that is not performing, we can carry out the site assessment for you.

Call our team on 01872 222555 or visit clearstreamdrainage.co.uk for a fixed-price quote with no call-out fee.

Frequently Asked Questions

How deep should a soakaway be?

Depth depends on your required storage volume, which is calculated from the catchment area and the infiltration rate of your soil. Most domestic soakaways sit between 1 metre and 1.5 metres deep. In areas with clay subsoil, going wider rather than deeper is often more effective, as the sidewalls of the pit tend to drain better than the base in slow-infiltration ground.

Can I build a soakaway myself?

You can, but Building Regulations apply in most cases, which means Building Control notification is required and the work is subject to inspection before backfilling. Carrying out the percolation test correctly and sizing the soakaway to BRE Digest 365 standards requires some technical care. For small installations, a competent DIYer with the right guidance can manage the physical build. For anything serving a roof area larger than 25m², or any site with complex ground conditions, professional involvement is strongly recommended.

How far does a soakaway need to be from my house?

Approved Document H requires a soakaway to be positioned at least 5 metres from any building and at least 2.5 metres from any property boundary. These distances protect foundations from the increased ground moisture a soakaway creates and ensure water does not cross onto neighbouring land.

Will a soakaway work in clay soil?

Standard soakaways are not recommended in heavy clay because clay absorbs water very slowly. In some cases, a soakaway can still be installed in clay-dominant ground if it is significantly oversized to compensate for the low infiltration rate. Percolation testing is essential before committing to any design. If testing confirms the infiltration rate is too low, alternative drainage options such as a French drain, attenuation system, or connection to a surface water sewer should be considered.

How long does a soakaway last?

A well-designed plastic crate soakaway with a properly installed geotextile membrane should last 30 years or more if installed correctly. A traditional rubble-filled soakaway has a much shorter lifespan: Approved Document H notes that domestic rubble soakaways may need to be renewed approximately every ten years, as soil particles gradually fill the void space between the stones.

Do I need a soakaway if I already have a septic tank?

A septic tank and a soakaway serve different functions. The soakaway drain field connected to a septic tank handles treated effluent, not surface water. Surface water from roofs and paved areas should never enter a septic tank system. If your property lacks a surface water sewer connection, you need a separate surface water soakaway in addition to any septic drainage field. The two systems must be kept entirely separate.

What should I do if my soakaway smells?

A surface water soakaway should not produce noticeable odours under normal operation. A persistent smell usually indicates that foul water or surface water contaminated with organic material has entered the system, or that the soakaway is failing and water is stagnating rather than draining. If you notice an odour from a soakaway or the ground above it, contact a drainage specialist for an inspection. Do not attempt to investigate the pit yourself without proper safety equipment.