blogTL;DR: There are three main types of soakaway used in UK residential drainage: traditional rubble-filled pits, modern plastic crate systems, and French drain trenches. The right choice depends on your soil type, garden size, and the volume of water you need to manage. A percolation test confirms whether your ground can support any soakaway at all. This guide covers each type, where it works, and when to call a drainage specialist.
Not all soakaways are equal. Choose the wrong type of soakaway for your ground conditions, and you’ll end up with a waterlogged garden, a failed system, and repair costs that far exceed what a properly designed installation would have cost in the first place.
Soakaways manage surface water by collecting rainwater runoff from roofs, driveways, and patios, then slowly releasing it into the surrounding ground. But “the surrounding ground” is where things get complicated. Cornwall’s heavy clay subsoils, coastal water tables, and granite bedrock all affect how water moves underground. What works on a sandy Devon hillside might be completely unsuitable for a Falmouth garden sitting on compacted clay.
This guide explains the main types of soakaway available in the UK, what distinguishes each one, how to tell which is right for your property, and what the regulations require before you install anything.
What Is a Soakaway and How Does It Work?
A soakaway is an underground drainage structure that collects surface water runoff and disperses it back into the ground at a controlled rate, rather than sending it to the main sewer network. It works by storing water during peak rainfall and releasing it gradually once the surrounding soil can absorb it.
The principle has stayed the same for decades. Dig a pit, create void space for water to sit in, wrap it in a permeable membrane to filter silt, and let the water slowly percolate outward. What has changed is the materials and engineering behind that void space.
There are three common formats: rubble-filled pits, modular plastic crate systems, and trench-based French drains. Each manages water differently and suits different site conditions.
One thing all soakaways share: they handle surface water only. Foul drainage from toilets or septic tanks must never enter a soakaway. That’s not just good practice. It’s a legal requirement under the Water Resources Act 1991.
Compliance note: The distinction between surface water soakaways and foul drainage fields is governed by building regulations and environmental law. Client should verify current requirements with Building Control before installation. See GOV.UK: Approved Document H for the current guidance.
What Are the Main Types of Soakaway?
The three main types of soakaway used in UK residential drainage are rubble-filled pits, plastic crate systems, and French drain trenches. Each has different void space, longevity, installation requirements, and suitability for different ground conditions.
Rubble-Filled Soakaways
The traditional method. A pit is excavated and filled with broken stone, coarse gravel, or hardcore. The voids between the rubble create storage space for incoming water.
Rubble soakaways are simple pits filled with rubble and gravel. They are straightforward to create, but are prone to silting up, which shortens their working life.
Rubble soakaways typically offer only 30-35% void space, meaning a larger pit is needed to achieve the same storage capacity as a modern crate system. When heavy loads are placed above a rubble soakaway, the rubble can compress, which reduces the void space and diminishes drainage performance.
Rubble soakaways are rarely the right choice for new installations. They’re harder to size accurately, quicker to fail, and more disruptive to replace. They’re still found under many older Cornwall properties, and they’re one of the most common reasons a drainage assessment gets called in.
Plastic Crate Soakaways (Geocellular Systems)
The modern standard for residential soakaway installation. Modular polypropylene crates are stacked and arranged in a pit, then wrapped in a geotextile membrane.
Plastic crates provide void ratios of 93-97%, making them far more compact than rubble systems for the same storage volume. Water enters through gutters, channel drains, or gullies, travels by pipe to the crate structure, and percolates outward through the geotextile membrane into the surrounding ground.
The geotextile membrane filters silt before it enters the crate structure, extending service life considerably compared to traditional rubble methods.
Crates come in different load ratings. Green crates are suitable for non-traffic areas; blue crates are rated for traffic areas such as driveways. Load rating matters, especially if the soakaway sits beneath a driveway or hard-standing.
This system is the default recommendation for most residential properties where ground conditions support it. Compact, predictable, and long-lasting.
French Drain Trenches
A French drain is a perforated pipe laid in a gravel-filled trench, typically running along the length of a garden or driveway edge. It intercepts waterlogged ground and channels water either to a soakaway, a watercourse, or a surface water drain.
French drain trenches with perforated pipe in gravel work particularly well in narrow gardens where a traditional square pit would be impractical.
French drains are often used alongside crate soakaways rather than instead of them. The trench captures waterlogged ground and feeds into a crate system that handles the disposal. In Cornwall gardens with drainage problems spread across a large area, this combination is often more effective than a single pit soakaway.
Does Your Ground Actually Support a Soakaway?
Before installing any soakaway, a percolation test is required to confirm the ground can absorb water at a sufficient rate. Skipping this step is one of the most common reasons soakaways fail.
A percolation test, conducted to BRE Digest 365 methodology, measures how quickly water drains through your soil at the depth the soakaway will sit. For soakaways serving areas greater than 25 square metres, Building Regulations direct reference to BRE Digest 365 as the standard design method. The test involves digging a pit, saturating the surrounding soil, then timing how long it takes for the water level to drop 150mm. The result, expressed as a value called Vp, determines whether the soil can support a soakaway at all.
If the Vp value exceeds 100 seconds per millimetre, a soakaway will not work, and alternative discharge options are needed.
This is where Cornwall’s geology creates real problems for property owners. Permeable soils such as sands, gravels, and some chalks support soakaway drainage well. Impermeable soils including clays, silts, and certain rock types prevent effective infiltration. Soakaways in impermeable soils are unlikely to function because they simply fill with water.
Large parts of inland Cornwall sit on heavy clay subsoils. Coastal areas often have high water tables. Properties on granite bedrock may hit rock at very shallow depths. A percolation test removes the guesswork. It also provides the documentation Building Control will need if you’re connecting a soakaway to new drainage works from an extension or new build.
Compliance note: Percolation testing requirements and design standards should be verified with Building Control. Client to confirm current BRE 365 requirements apply to the relevant project scope before publishing.
What Are the Rules Around Where and How You Install a Soakaway?
Soakaway installation in the UK is governed by Building Regulations Approved Document H. Most surface water soakaways for single dwellings do not require planning permission, but building regulations still apply, and specific siting rules must be met.
The key requirements under Approved Document H:
- A soakaway must be positioned at least 5 metres from any building and at least 2.5 metres from any boundary.
- Surface water only may enter a soakaway. Foul drainage must not enter a soakaway. This is a legal requirement under the Water Resources Act 1991.
- The base of the soakaway should remain above the seasonal high water table.
- Where a soakaway is connected to new drainage works from an extension or new build, building control notification is required even if planning permission is not needed. Where a soakaway serves more than one property, is in a flood zone, or is within a conservation area or listed building setting, the local planning authority should be consulted first.
The 5-metre setback from buildings is particularly relevant for Cornwall properties where gardens can be tight and boundary distances limited. A site survey before installation prevents costly repositioning later.
Compliance note: Planning and building regulations requirements should be verified with the relevant local authority or Building Control body before any installation. This section should be reviewed before publishing.
When Is a Soakaway Not the Right Solution?
A soakaway is not always the correct answer. Certain site conditions make infiltration drainage impractical or ineffective regardless of which soakaway type is chosen.
Clay soils have a critical weakness: low permeability means water has nowhere to go. Clay particles are tightly packed, and water cannot infiltrate through them at a sufficient rate. A successful soakaway requires soil with good permeability, typically sand or gravel. In clay, water simply sits in the pit with nowhere to drain.
Other situations where a standard soakaway is unsuitable:
- High seasonal water tables that would bring groundwater up to or near the soakaway base
- Contaminated runoff from commercial yards or roads that must not enter the ground
- Sites where the 5-metre setback cannot be achieved without directing water toward a building
- Catchment areas too large for the site to accommodate an adequately sized system
Where infiltration drainage is not possible, attenuation tanks are purpose-built underground storage systems that hold large volumes of surface water temporarily and release it slowly into the drainage network at a controlled rate. They don’t depend on infiltration at all, making them effective in clay soil conditions. Connecting to an existing surface water drain or a permitted discharge to a watercourse are also options, depending on proximity and Environment Agency consent.
A CCTV Drain Survey is often the fastest way to understand what drainage infrastructure exists on a site before deciding which solution to pursue. It identifies existing pipes, confirms their condition, and rules out connection options that might not be visible above ground.
How Clear Stream Handles Soakaway Assessment and Garden Drainage
Clear Stream engineers diagnose the root cause of surface water problems before recommending any installation. That means assessing soil conditions, existing drainage routes, site constraints, and the volume of water the system needs to manage.
For Cornwall and Devon properties, that diagnosis always accounts for local ground conditions: the clay-heavy subsoils across much of mid and west Cornwall, granite bedrock that limits excavation depth, coastal water tables that shift seasonally, and rural sites where connecting to a public sewer is simply not an option.
The process typically works like this:
- Site assessment: Engineers survey the property, identify where water is accumulating, and establish what’s causing it.
- Percolation testing: Where a soakaway is the proposed solution, a BRE 365 percolation test confirms the ground is suitable before any installation work begins.
- Design and specification: The correct soakaway type, size, and position is determined based on the catchment area, soil absorption rate, and site constraints.
- Installation: Clear Stream carries out the work with fixed-price quotes agreed in advance. No surprise invoices on completion.
- Long-term performance: All repair and installation work carried out by Clear Stream comes with a 5-Year Guarantee.
If a soakaway cannot be installed due to ground conditions, Clear Stream’s garden drainage services cover alternative surface water management solutions, including French drain installation, channel drainage, and attenuation. There are no call-out fees, and engineers reach properties anywhere across Cornwall and Devon, typically within 1-2 hours.
If the issue involves an existing drainage system, a CCTV Drain Survey provides a complete picture of what’s underground before any ground is broken.
Conclusion
The right type of soakaway depends on three things: your soil, your site, and the volume of water you need to manage. Modern plastic crate systems outperform traditional rubble fills on almost every measure, but neither will work if the ground can’t absorb water fast enough in the first place. A percolation test is not optional. It’s the essential first step.
Cornwall’s geology means a significant number of properties are not suitable for standard infiltration drainage. That’s not a problem without a solution; it just means the solution is different, and that’s exactly what a proper site assessment is for.
If you’re dealing with a waterlogged garden, a failed soakaway, or surface water you can’t control, Clear Stream Drainage Solutions will assess your site, carry out the testing, and recommend the right drainage strategy. Fixed-price quotes, no call-out fees, and all repair work backed by a 5-Year Guarantee.
Call 01872 222555 to arrange a site assessment, or visit clearstreamdrainage.co.uk/garden-drainage-services to find out more about garden drainage across Cornwall and Devon.
Frequently Asked Questions
The three main types used in UK residential drainage are rubble-filled soakaways, plastic crate (geocellular) soakaways, and French drain trenches. Rubble-filled pits are the traditional method, using broken stone and hardcore to create void space. Plastic crate systems are the modern standard, offering higher void ratios and longer service life. French drain trenches intercept waterlogged ground across a larger area and typically feed into a crate soakaway or discharge to a surface water drain.
A percolation test, following BRE Digest 365 methodology, measures how quickly water drains through your soil at the depth the soakaway will sit. The result determines whether the soil absorbs water fast enough to support a working soakaway. Clay-heavy soils common in parts of Cornwall often fail this test, in which case alternative drainage solutions are needed. A qualified drainage engineer can carry out the test and interpret the results.
Under Building Regulations Approved Document H, a soakaway must be positioned at least 5 metres from any building and at least 2.5 metres from any property boundary. It should also be sited so that its base remains above the seasonal high water table. These rules apply regardless of soakaway type.
In most cases, surface water soakaways for single dwellings do not require planning permission and fall under permitted development. However, building regulations still apply, and building control notification is required where the soakaway is connected to new drainage works, such as an extension. If your property is in a conservation area, flood zone, or is a listed building, you should check with your local planning authority before proceeding.
No. Soakaways are designed for surface water only, such as rainwater from roofs, driveways, and patios. Foul drainage from septic tanks or sewage treatment plants must discharge through a properly designed drainage field, not a soakaway. Using a soakaway for foul water is a legal offence under the Water Resources Act 1991.
The most common reason is ground conditions. Heavy clay subsoils across much of Cornwall prevent water from percolating away quickly enough, causing the soakaway to fill and overflow. High seasonal water tables, particularly on coastal and low-lying sites, can also reduce effective storage capacity. Older rubble-filled soakaways often fail as silt gradually fills the voids over time, reducing their ability to hold and drain water.
Where ground conditions make infiltration drainage impractical, the main alternatives are attenuation tanks, which store water and release it slowly into the sewer network; French drain systems connecting to a surface water drain or watercourse; and in some cases, a pumped discharge arrangement. The right solution depends on what drainage infrastructure exists on or near the site. A CCTV Drain Survey can identify existing connections and inform which options are available.
