blogTL;DR: A soakaway is an underground drainage system that collects surface water from your roof, driveway, or garden and lets it slowly percolate back into the ground. It reduces pressure on public sewers, prevents surface flooding, and is the first-choice drainage solution required by UK Building Regulations. This guide covers how soakaways work, the types available, when they fail, and what to do about it.
If you’ve noticed water pooling on your lawn after rain, or your downpipes emptying onto a waterlogged drive, you may already have a soakaway problem without knowing it. A soakaway is one of the most common and important drainage components on any UK property, yet most homeowners have never seen one. It sits entirely underground, does its job silently, and only becomes visible when something goes wrong.
Understanding what is a soakaway, how it works, and when it starts to fail can save you from costly flooding, subsidence, and structural damage. Cornwall’s high rainfall, clay-heavy ground, and granite bedrock make soakaway performance especially critical here. This guide covers everything you need to know, from the basic principle to the regulations that govern installation, and what professional diagnosis looks like when things go wrong.
What Is a Soakaway?
A soakaway is an underground pit or chamber that collects rainwater and surface runoff from a property and slowly releases it back into the surrounding soil through a process called percolation. It is not connected to the foul sewer. It handles clean surface water only.
The most straightforward version is a gravel-filled pit. Water enters through a pipe connected to your roof drainage system, fills the void space between the stones, and then filters out through the surrounding soil at a controlled rate. Modern systems replace the gravel with engineered plastic crates, which offer far greater storage capacity in a smaller space.
Soakaways are installed beneath gardens, driveways, and open land within a property’s boundary. They work without pumps, power, or moving parts. The driving force is gravity and the natural porosity of the ground. When the soil can absorb water faster than rainfall delivers it, the system works well. When it can’t, you start to see the signs.
How Does a Soakaway Work?
A soakaway works by temporarily storing rainwater underground and releasing it gradually into the soil, reducing the peak flow that would otherwise overwhelm surface drains or public sewers during heavy rain.
The process follows a simple sequence. Rainwater falls on roofs, drives, and patios and runs into gullies and downpipes. Those pipes carry the water to an inlet, typically via a silt trap that catches leaves and debris before they reach the soakaway chamber. The water then fills the chamber, whether that’s a void between gravel pieces or the internal space inside plastic crates. From there, it seeps outward through the soakaway walls and into the surrounding soil.
The rate at which it disperses depends on the soil type. Sandy or gravelly ground absorbs water quickly and suits a soakaway well. Clay soil holds water and drains slowly, which creates problems because the soakaway can fill faster than the ground can absorb. In parts of Cornwall, where china clay subsoils, shillet rock, and high water tables are common, soakaway performance requires careful assessment before installation.
The system recharges naturally between rain events. A well-designed soakaway should drain from full to half-volume within 24 hours, giving it capacity to handle the next downpour.
What Types of Soakaway Are There?
There are three main types used in the UK: traditional rubble-filled pits, modern plastic crate systems, and borehole soakaways for specialist applications.
Traditional Rubble and Gravel Soakaways
The original design is a pit dug into the ground and backfilled with broken stone, rubble, or coarse gravel. The voids between the aggregate pieces store water while the stone supports the sides of the excavation. Rubble soakaways are straightforward to create, but they are prone to building up silt over time, which shortens their working life. It should be expected that a domestic rubble-filled soakaway may need to be renewed about every ten years.
Plastic Crate Systems
Modern, more efficient soakaway design replaces the aggregate backfill with sturdy interlocking plastic crates. These are also known as sustainable drainage systems (SuDS) and stormwater attenuation crates. The crates connect to your drainage pipework and sit wrapped in a geotextile membrane that filters out fine silt while allowing water to pass freely.
The most obvious advantage of using crates is sheer capacity, as they offer up to 95% void capacity compared to gravel infill, which provides as little as 30% capacity. That means the same excavation can store nearly three times more water. Properly installed crate systems with geotextile protection can maintain performance for 50 years or more.
Borehole Soakaways
Much rarer and requiring greater regulatory approval, a borehole soakaway can discharge surface water deeper underground. These are specialist installations used where topsoil permeability is poor but a suitable stratum exists at depth. They’re uncommon in domestic settings.
For most Cornwall properties, modern crate systems are the preferred solution where ground conditions allow.
Does a Soakaway Need to Meet Building Regulations?
Yes. Any new soakaway in England must comply with Approved Document H of the Building Regulations, which governs drainage and waste disposal for all building work.
Approved Document H3 sets out a clear hierarchy of acceptable surface water destinations: first, infiltration to the ground via a soakaway or similar device; second, discharge to a watercourse; third, discharge to a surface water sewer; and as a last resort, discharge to a combined sewer with the sewerage undertaker’s consent. Infiltration is first on the list because it most closely mirrors the natural water cycle and reduces pressure on public infrastructure.
Compliance note: This section covers requirements under Approved Document H and associated standards. The client should verify all regulatory details with Building Control or a qualified drainage engineer before installation. This content is flagged for accuracy review before publishing.
Key location rules apply to all soakaways:
- A soakaway must be positioned at least 5 metres from any building or road, and at least 2.5 metres from a boundary. It must not be sited in unstable land or in ground where the water table reaches the base of the soakaway at any time of year.
- Soakaways are designed for surface water only and must not be used to discharge foul water or sewage under any circumstances. Surface water and foul drainage must always be kept separate.
- A soakaway does not usually need planning permission, but Building Regulations apply. Installing one as part of a new build or extension will normally require building control approval.
Before any soakaway is installed, the ground must be tested. A BRE Digest 365 soakaway test measures the soil’s natural infiltration rate. Local planning authorities and lead local flood authorities often require this test during the planning application stage to ensure the ground can adequately absorb surface water runoff. The design of the soakaway, including its depth, volume, and position, is calculated from these test results.
For soakaways covering more than 25 square metres of drained surface, the design must reference BRE Digest 365 directly, the Building Research Establishment’s standard guidance document for soakaway design and percolation testing.
What Are the Signs of a Failing Soakaway?
A soakaway that is failing will usually give visible warning before it collapses entirely. The earlier you spot the signs, the simpler the fix tends to be.
Persistent water pooling is a classic indicator. It means the soakaway is overwhelmed, blocked, or waterlogged. If your garden remains boggy for 24 to 48 hours after rain, drainage investigation is needed.
Other common warning signs include:
- Water pooling around gullies and pipes, the ground near the soakaway staying soaked and waterlogged, and drains or pipes connected to the soakaway overflowing.
- Overflowing downpipes or gutters that were previously draining correctly
- Dips or soft patches in the ground above the soakaway, which indicate compaction or collapse
- Persistent backing up at the base of downpipes after rain, or internal drains running slowly during wet weather
Soakaway failure has two main causes. The first is siltation: fine particles carried by stormwater build up in the gravel or around the crate membrane over time, eventually blocking the passage for outward percolation. The second is soil saturation, where the ground around the soakaway becomes permanently waterlogged and simply cannot absorb any more water. In clay-rich soils, soakaways can fail relatively quickly because clay has very poor porosity and cannot drain at an adequate rate.
In Cornwall, where clay subsoils and high water tables are widespread across many areas, soakaway performance needs to be assessed carefully. A system that worked well for years can deteriorate as surrounding soil structure changes or as rainfall events become more intense.
Why Does Soil Type Matter So Much?
The soil around a soakaway determines whether the whole system works at all. A soakaway cannot function if the surrounding ground cannot absorb water, regardless of how well-designed the chamber itself is.
Free-draining soils such as sand and gravel allow water to disperse easily, helping the soakaway last longer. Clay-heavy soils drain slowly, which can cause water to sit in the system and lead to early failure.
This is a significant issue across Cornwall. Much of the county sits on varying geology: china clay spoil in mid-Cornwall, shillet and slate in the north and west, granite bedrock across Bodmin Moor and West Penwith. These ground conditions affect drainage in different ways, but clay-based subsoils in particular create conditions where soakaways underperform or fail early.
High water tables, common in low-lying coastal areas and valley bottoms across the county, are another complicating factor. A soakaway must not be used in ground where the water table reaches the bottom of the soakaway at any time of year. Where the water table is close to the surface for months at a time, as it often is in Cornish winters, standard soakaway design may not be suitable without careful engineering.
This is why a proper percolation test, carried out at the right depth and in the right location, is not just a regulatory formality. It’s the only way to know whether the ground can support a soakaway before any excavation begins.
How Clear Stream Handles Soakaway Problems in Cornwall and Devon
Clear Stream carries out soakaway diagnosis and assessment across Cornwall and Devon, identifying whether a failing system can be remediated or needs replacement, and what soil conditions mean for any new installation.
The starting point is always investigation. A CCTV Drain Survey of the pipework feeding the soakaway will confirm whether the problem is in the connecting drains rather than the soakaway chamber itself. Root intrusion, pipe collapse, or a blocked inlet can all mimic the symptoms of soakaway failure while leaving the chamber itself intact. Getting an accurate diagnosis first avoids unnecessary excavation.
Where a blocked drain is feeding the soakaway incorrectly, High-Pressure Jetting can clear the line and restore flow. Where the soakaway chamber itself is silted or collapsed, replacement with a modern crate system is usually the most cost-effective long-term solution. In cases where the ground conditions make a standard soakaway impractical, alternative drainage routes can be assessed and planned.
All diagnosis work from Clear Stream comes with a fixed-price quote before any work begins. There are no call-out fees and no hidden costs. For urgent drainage problems, a Clear Stream engineer can be on site anywhere in Cornwall and Devon within one to two hours.
Where soakaway failure is affecting septic tank drainage fields on rural properties, the two systems need to be assessed together. A surface water soakaway and a septic drainage field are separate systems with different regulatory requirements, but ground saturation from one can directly affect the performance of the other.
For properties with cracked or displaced drain pipework contributing to the problem, drain pipe relining offers a no-dig repair option that restores the pipe without excavation.
Conclusion
A soakaway is a simple system with a demanding job. It handles every drop of rainwater that falls on your roof and hard surfaces, year after year, without any visible machinery. When it works, you never notice it. When it fails, the consequences arrive fast: flooding, waterlogged ground, structural movement, and in some cases, drain backups inside the property.
The key points to take away:
- Soakaways handle surface water only, never foul sewage
- UK Building Regulations require infiltration to be the first drainage choice for new builds and extensions
- Soil type determines whether a soakaway will work at all, which is why percolation testing is essential before installation
- Warning signs include persistent pooling, overflowing downpipes, and waterlogged ground
If you’re seeing any of these symptoms, or you’re planning work that involves new drainage, get the right diagnosis first.
Call Clear Stream on 01872 222555 or visit clearstreamdrainage.co.uk to book a site assessment. No call-out fee, fixed-price quote, and an engineer on site within one to two hours anywhere in Cornwall and Devon.
Frequently Asked Questions
What is the difference between a soakaway and a drainage field?
A soakaway is designed to manage clean surface water, typically from roofs, driveways, and gutters, by allowing it to percolate into the surrounding soil. A drainage field is a network of perforated pipes that disperses treated effluent from a septic tank or sewage treatment plant. They serve completely different purposes and have separate regulatory requirements. Connecting a surface water soakaway to foul drainage is not permitted under Building Regulations.
Can I install a soakaway in clay soil?
Clay soil drains very slowly, which makes it poorly suited to soakaway installation in most cases. A percolation test will confirm whether the soil can absorb water at a sufficient rate. In clay-heavy ground, the soakaway chamber can fill faster than the surrounding soil can empty it, causing the system to fail quickly. Alternative drainage solutions may need to be considered where clay soils make infiltration impractical.
How close to my house can a soakaway be?
Under Approved Document H of the Building Regulations, a soakaway must be positioned at least 5 metres from any building or road, and at least 2.5 metres from a boundary. It must also be sited in ground where the water table does not reach the base of the chamber at any time of year. These distances protect foundations and neighbouring land from waterlogging and structural movement.
How do I know if my soakaway has failed?
The most common signs are persistent puddles that remain for more than 24 to 48 hours after rain, overflowing gutters or downpipes, waterlogged ground around gullies, and internal drains that run slowly during wet weather. A CCTV drain survey of the pipework connecting to the soakaway is the most reliable way to diagnose whether the problem is a blocked pipe, a collapsed chamber, or saturated surrounding soil.
Do I need planning permission to install a soakaway?
In most domestic situations, a soakaway for surface water does not require planning permission. However, it must comply with UK Building Regulations, specifically Approved Document H, which sets out requirements for drainage design, location, and minimum distances from buildings and boundaries. If the installation forms part of a new build or extension, Building Control approval is normally required. Additional restrictions may apply in protected areas or where ground conditions raise environmental concerns. Always check with your local authority before starting work.
How long does a soakaway last?
This depends on the type of system and the soil conditions around it. Traditional rubble-filled soakaways can last around 10 years before siltation reduces their performance significantly. Modern plastic crate systems, installed correctly with a geotextile membrane and silt traps, can perform for 50 years or more. Ground conditions matter considerably: clay soil and high water tables will shorten the life of any system, which is why correct site assessment before installation is essential.
Can a soakaway be repaired, or does it need replacing?
It depends on the cause of failure. If a blocked inlet pipe or collapsed drain is the problem, jetting or pipe relining may restore the system without touching the chamber. If the chamber itself is silted up or collapsed, or if the surrounding soil has become permanently saturated, replacement is usually the better option. A CCTV survey and site assessment will confirm which applies before any work is priced.
