blogTL;DR: A French drain moves water sideways through a gravel-filled trench and perforated pipe. A soakaway stores water underground and lets it slowly seep into the surrounding soil. They solve different problems. French drains collect and redirect; soakaways absorb and disperse. Get the diagnosis wrong and you’ll end up with the same waterlogged garden you started with.
Wet winters are Cornwall’s default setting. But a garden that sits underwater for days after heavy rain, or paving that floods every time a weather system rolls in off the Atlantic, is not something you have to accept. The fix depends on understanding the difference between a french drain vs soakaway, because these two systems work in completely different ways. Choosing the right one comes down to your soil, your site, and where the water is actually coming from. This post explains how each system works, when to use which, how they interact with UK building regulations, and why Cornwall’s ground conditions complicate the decision.
What Is a French Drain?
A French drain is a gravel-filled trench containing a perforated pipe that collects water and redirects it away from problem areas. The pipe sits at the bottom of the trench, wrapped in geotextile membrane to stop silt and soil from clogging the holes. Water seeps in through the gravel, enters the pipe, and flows by gravity to a safe discharge point elsewhere on the site.
Think of it as a transport system. Its job is movement, not storage.
French drains are typically laid at a depth of 300 to 600mm, depending on where the water is entering the ground. They work well in several situations:
- Water building up against a house wall or along the base of a retaining wall
- Saturated soil on a sloped garden where groundwater tracks downhill towards the property
- Lawn areas that stay boggy for days after rain because the topsoil has nowhere to drain
- Paved areas or driveways where surface water pools along one edge
The system needs a gradient to function. Without a consistent slope along the trench, water won’t flow. It also needs somewhere safe to discharge: a drainage ditch, a watercourse, a surface water sewer, or a soakaway at a lower point in the garden. A French drain without a proper outfall just moves the problem.
What Is a Soakaway?
A soakaway is an underground system that stores surface water and releases it slowly into the surrounding soil. It does not move water sideways; it holds water temporarily while the ground absorbs it from below and around the structure.
Modern soakaways are usually built from modular plastic crates (sometimes called storm cells or infiltration crates), which are wrapped in geotextile fabric and buried in an excavation. Older installations used rubble or clean stone, though crate systems are now the standard for domestic installs because they offer far greater void space relative to their size.
Soakaways suit situations where rainwater needs somewhere to go vertically, not laterally. Common applications include:
- Collecting roof water from a downpipe that cannot connect to a surface water sewer
- Managing runoff from a new patio, driveway, or extension under Building Regulations
- Dispersing water from a French drain that has collected it from across the site
One important distinction: a soakaway handles disposal, not collection. It waits for water to arrive, then releases it gradually. If the issue is water travelling towards your property across a wide area, a soakaway on its own won’t intercept it. You need a collection system first.
What Does UK Law Say About Soakaways?
Under Approved Document H of the Building Regulations, rainwater from roofs and paved areas should be managed in a specific order of priority. A soakaway or other infiltration system comes first. A watercourse comes second. A sewer is the last resort. This hierarchy applies to new builds, extensions, and significant drainage alterations, and Building Control will inspect the work before you can backfill.
For soakaways to comply, there are clear placement rules. A soakaway must sit at least 5 metres from any building and at least 2.5 metres from any boundary. The base of the structure must remain above the winter water table at all times. Foul water must never enter a soakaway; it can only accept surface water or rainwater. That separation is a legal requirement under the Water Resources Act 1991.
Sizing a compliant soakaway requires a percolation test carried out to the BRE Digest 365 methodology. The test involves excavating a trial pit, filling it with water, and measuring how quickly the water level drops. The result gives a soil infiltration rate that determines how large the soakaway needs to be for the catchment area it will serve. For areas exceeding 25 square metres, a full BRE 365 calculation is required.
Note for clients: This section references legal and regulatory requirements. Please verify compliance details with your local planning or building control authority before publishing.
Does Cornish Ground Make This Harder?
In most of Cornwall, yes. The county’s geology creates conditions that complicate soakaway design more than most UK regions.
Large parts of Cornwall sit on heavy clay subsoils. Clay particles are tightly packed and absorb water very slowly. In some clay ground, infiltration rates are so low that a standard soakaway will never empty fast enough to accept the next rainfall event. Water sits in the crate system until it backs up and resurfaces, defeating the purpose entirely.
Granite bedrock adds another layer of difficulty. Igneous rocks like granite are effectively impermeable. Where granite lies close to the surface, there is simply no pathway for water to disperse downward. A soakaway dug into granite-dominated ground will not work.
Cornwall’s high winter water table compounds both problems. During November through March, the water table across much of West and Mid Cornwall rises significantly. Soakaways positioned too close to the seasonal water table lose their storage capacity precisely when demand is highest.
This is why the BRE 365 percolation test matters so much in Cornwall. What looks like a suitable site in a dry August can fail a winter test completely. A drainage assessment that includes proper soil analysis is not a formality here; it is the only reliable way to confirm whether a soakaway is even viable before money is spent installing one.
French drains are generally less affected by these ground conditions, because they move water laterally rather than relying on the ground to absorb it. However, they still need a viable outfall. If that outfall is a soakaway and the soakaway won’t work in the local soil, the whole system needs redesigning around an alternative discharge route.
Can You Use Both Together?
Yes, and for many properties in Cornwall it is the most effective approach. A French drain and a soakaway are not alternatives to one another; they serve different stages of the same drainage chain.
The French drain acts as the collection network. It intercepts water as it moves through the soil or accumulates along a problem edge, and channels it through a perforated pipe to a lower point on the site. The soakaway sits at that lower point, receives the collected water, and disperses it into the ground gradually.
This combined setup works well in gardens where multiple problems overlap: groundwater tracking downhill towards the house, surface water pooling from a paved area, and a low corner where water can safely discharge. Each system handles the part of the problem it is designed for.
The risk with a combined system in Cornish clay or granite ground is overloading the soakaway. If the French drain collects water faster than the soakaway can release it, the crates fill and the problem shifts rather than resolves. That is why ground assessment before installation matters. A system that works on well-draining sandy soil in East Cornwall may not work at all in the heavy clay belt through the centre and west of the county.
How Does Clear Stream Diagnose and Design Garden Drainage Solutions?
Clear Stream engineers assess garden drainage problems at the root cause. That means looking at where water is actually entering the site, what the ground conditions are, and whether an existing system (a silted-up old soakaway, a collapsed land drain, a blocked gully) is contributing to the problem before any new work is designed.
For properties across Cornwall and Devon where clay subsoils and granite bedrock are a factor, this assessment stage is not skipped. Installing a soakaway that won’t drain, or a French drain that has nowhere safe to discharge, would leave you with the same problem and a bill for work that didn’t fix it.
Clear Stream’s garden drainage services cover the full range of solutions, including French drain installation, soakaway design and installation, and combined systems for properties where both are needed. If an existing system needs inspection before a new one is designed, a CCTV drain survey can identify what is already underground without unnecessary excavation.
All work is quoted at a fixed price before any excavation begins. There is no call-out fee for assessment visits across Cornwall and Devon, and all drainage repairs carry Clear Stream’s 5-Year Guarantee. Engineers are on site within 1 to 2 hours of your call anywhere in the coverage area.
Which System Do You Actually Need?
The right system depends on three questions: where the water is coming from, where it needs to go, and what the ground underneath will allow.
Use a French drain when:
- Water is travelling across or through the soil towards your property and needs intercepting before it reaches a vulnerable area
- You have a sloped garden where groundwater consistently tracks downhill towards the house
- A long stretch of lawn or border stays saturated and needs water moving sideways to a safe outfall
- Surface water from an adjacent road, driveway, or neighbouring property is entering your site
Use a soakaway when:
- Rainwater from a roof, patio, or extension needs a compliant discharge point under Building Regulations
- Water pools in a specific low spot with no slope to direct it elsewhere
- The soil percolation test confirms the ground can absorb water at a sufficient rate (particularly relevant in Cornwall where this is not guaranteed)
Use both when:
- A wide area collects water that needs gathering first (French drain) before it can be dispersed underground (soakaway)
- An extension or new paved area generates roof and surface runoff simultaneously, with no nearby watercourse or sewer connection available
If you are unsure, a site assessment is always the correct first step. Assuming a soakaway will work without testing the ground is one of the most common (and expensive) mistakes in domestic drainage.
Conclusion
French drains and soakaways are not competing products. They solve different parts of the same problem. A French drain moves water; a soakaway absorbs it. Many sites in Cornwall need both, and many sites in Cornwall have ground conditions that demand a proper assessment before either is installed.
Key takeaways:
- A French drain without a safe discharge point solves nothing
- A soakaway without a percolation test is a gamble, and clay or granite ground in Cornwall means that gamble often loses
- Building Regulations require soakaways to be the first-choice discharge route for rainwater, but only where ground conditions allow
- A combined system is often the best answer for complex gardens
For advice on garden drainage in Cornwall or Devon, or to arrange a site assessment, call Clear Stream Drainage Solutions on 01872 222555 or visit clearstreamdrainage.co.uk. Engineers cover the full county with no call-out fee and fixed-price quotes before any work begins.
Frequently Asked Questions
What is the main difference between a french drain and a soakaway?
A French drain collects water and redirects it through a pipe to a discharge point elsewhere. A soakaway stores collected water underground and releases it slowly into the surrounding soil. They perform different jobs: one moves water, the other disperses it. Many drainage systems use both in sequence, with the French drain collecting and the soakaway receiving.
Do I need planning permission or building regulations approval for a soakaway?
New soakaways serving roof areas or paved surfaces on extensions typically require Building Regulations approval under Approved Document H. The regulations set a drainage hierarchy where a soakaway is the preferred first option, but compliance requires evidence from a BRE 365 percolation test. Simple domestic soakaways for existing properties may not need formal approval, but it is worth checking with your local Building Control office before installation.
Will a soakaway work in clay soil?
Often not, or at least not without significant design modifications. Clay soils drain very slowly, and standard soakaways can fill faster than they empty during heavy rainfall, causing water to surface and defeat the purpose of the system. A BRE 365 percolation test will confirm whether your clay ground can support a soakaway. In many parts of Cornwall, the clay subsoil means alternative discharge routes need to be considered.
How far from my house does a soakaway need to be?
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. It also must not be installed in land prone to instability, and the base of the soakaway must remain above the winter water table. These are minimum distances; site conditions may require greater separation.
Can a French drain discharge into a soakaway?
Yes. This is one of the most common setups for domestic gardens. The French drain collects and channels water through the site, and the soakaway at the end of the run absorbs it into the ground. For this to work, the soakaway must be correctly sized to handle the volume of water the French drain can deliver, which again depends on a percolation test for the specific site.
How do I know if my existing soakaway has stopped working?
Signs of a failing soakaway include surface water returning to areas it previously drained, waterlogging that takes several days longer than usual to resolve after rain, and soft or boggy patches of ground over or near the soakaway location. Soakaways can silt up over time as fine particles pass through the geotextile membrane and accumulate in the crate structure. A drainage inspection can confirm whether the issue is the soakaway itself or a problem upstream.
Does Cornwall’s geology affect which system is right for me?
Yes, significantly. Heavy clay subsoils are widespread across Cornwall, and granite bedrock lies close to the surface in large parts of the county. Both conditions limit the effectiveness of soakaways because water has limited capacity to absorb downward. This makes proper site assessment and percolation testing especially important before any garden drainage system is installed in Cornwall. French drains are less affected by these conditions, as they rely on gravity-fed lateral movement rather than ground absorption.
