Last Modified: March 3, 2024

What is a Soakaway

DIY Guide for Rockery Construction

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By Ben Parrot - Landscape Gardener & Amateur Geologist


The humble but often misunderstood soakaway is a simple but very effective drainage solution that is well within the grasp of a DIY landscaper looking  to future-proof their extraordinary gardening feats from flooding and subsidence. Fundamentally, it is an underground system designed to manage excess surface water, primarily rainwater, by dispersing it into the surrounding soil.

This beginners guide aims to enlighten homeowners and budding landscapers alike about the role soakaways can play in creating a cost-effective, gravel drainage system. It covers the crucial elements about design and installation as a primer for planning your project.

Home drainage system next to a wall

Key Takeaways

  • Find out what these simple but ingenious systems are all about.
  • Learn about the main components of a typical, rainwater system.
  • Give consideration to the advantages of incorporating a silt filter into your design
  • Get the installation basics in a five-step summary.
Perforated pipe leading into a crate

What is a Soakaway Drain?

A soakaway drain is a type of rainwater drainage system that is designed to redirect rain water through to a septic tank, sewage or alternate drainage systems and safely away from external areas. It takes the water away via a drainage pipe laid in an underground drainage trench to a hole in the ground containing drainage aggregate or a soakaway crate system.

Soakaways provide an effective solution for managing surface water drainage that can be designed to suit different needs and environments.

The Purpose of Soakaways

The purpose of soakaways is to provide a way for surface water run-off to drain away safely to prevent flooding. This helps to prevent pooling that can lead to waterlogged areas, which may cause damage to property and landscaping features.

Soakaways are designed to gradually infiltrate excess water into the surrounding ground, allowing it to replenish the water table. This helps to maintain a balanced groundwater system and prevents water-related issues such as soil erosion and the compromising of building foundations. For that reason, they play an integral role in sustainable drainage systems while reducing strain on local sewer networks.

This type of drainage system is most commonly found in low-lying garden areas for use with French drains, near the downpipes coming from your roof and around driveways.

Managing Surface Water Run-off From Driveways

Managing surface water run-off from driveways is a crucial part of proper construction. Driveways can contribute to significant amounts of rainwater run-off, which, if not managed properly, can lead to flooding and erosion issues.

To effectively manage surface water run-off from driveways, it may be necessary to design and install a suitable rainwater soakaway system.

Drain grid around a parking area

Soakaway Design Basics

Soakaway design is based around some main components; a collection chamber, distribution pipe network and a pea shingle or gravel-filled pit. The size and capacity are determined by the size of the soakaway drainage area and the average anticipated yearly rainfall. The following are the main components of a typical soakaway system:

The Components of a Soakaway

The primary structure of a soakaway is simple, typically involving a crate system or a pit filled with coarse gravel or shingle. The soakaway crate, also known as a soakaway cell, is a modular unit made from highly durable plastic with a high void ratio, designed to hold and slowly release collected rainwater into the surrounding soil.

The crates are usually wrapped in a porous landscaping fabric that acts as a filter to make sure it doesn't block up by preventing silt and debris from entering the system. Specifically, a non woven geotextile membrane is most suitable for a soakaway, as it is best at preventing soil and silt from clogging the spaces within the gravel or crates.

Each soakaway also requires a drainage pipe to channel water from downspouts or paved areas into the underground structure.

Components of a crate soak away system

Fitting a Silt Trap

Fitting a silt trap is a great way to prevent sediment and debris from entering the system and causing blockages. The silt trap should be installed at the point where water enters the system and works by slowing down the flow of water and allowing particles to settle out. By installing one, you can make sure your system is much more effective and maximise its longevity with just the occasional clearing of debris.

Gravel Trenches

Gravel trenches are used to evenly distribute water, allowing it to soak away into the surrounding soil. After you excavate the trench, you backfill it with coarse 20mm gravel or 20mm shingle, which acts as a reservoir for the water to drain into the soil. The size and depth of the trenches depend on the amount of water and the soil's infiltration rate.

Front garden excavtion in progress

Need to work out how much gravel your drainage project needs? Try our free gravel calculator.

The Basics of Installing a Soakaway

When it comes to installing a soakaway, there are a few key steps to follow:

  1. 1
    Call First: It's important to check for any underground dangers, such as gas pipes or electrical cables, by calling the relevant authorities.
  2. 2
    Planning: Check whether permission is required, as this will vary depending on your location and the size of the system.
  3. 3
    Percolation Test: Needed to determine the soil's infiltration rate, as it will influence the size and type needed for effective drainage.
  4. 4
    Excavate: The trench should be dug to the required depth, which is typically determined by local regulations.
  5. 5
    Installation: The components, including the silt trap, pipework and crate system, should then be installed in accordance with the manufacturer's instructions.
  6. 6
    Tips: Be sure to add 150mm of 20mm pea shingle at the bottom of the pit for the crate system to be laid upon. Also, make sure that the drainage pipe exits above the top of the soakaway.
Pea shingle 10mm close up

Checking for Underground Dangers

Before beginning the installation of a soakaway, it is crucial to check for any underground dangers that may pose a risk. Underground dangers can include gas pipes, electrical cables, or other utilities that may be present in the area. It is recommended to contact the relevant utility companies and use a cable avoidance tool before starting any works.

Planning Permission

Planning permission and building regulations are important considerations when installing a soakaway on your property. In most cases, planning permission is not required for domestic properties as long as it is located within the boundary of the property and does not pose a risk to public health or the environment. However, it is advisable to check with the local planning authority to ensure compliance with any specific regulations or restrictions in your area.

If the property is located in a conservation area, a National Park, or an Area of Outstanding Natural Beauty, there may be additional planning restrictions to consider. It is important to consult with the relevant authorities to determine any specific requirements or permits needed in these areas.

Planning drawings for successful project

Infiltration Rate

The infiltration rate refers to the speed at which water can penetrate into the surrounding soil. It is influenced by the soil type (such as the presence of heavy clay), compaction and any obstacles or impervious layers.

It is essential to assess the infiltration rate of the soil in your garden before installing a soakaway. This can be done through a soil percolation test, which measures how quickly water can seep into the soil. The results will be used to determine the depth and size of a soakaway that can handle the task.

Soil Percolation Testing

To conduct the test, dig a hole, usually around 300mm square and deep, fill it with water for 24 hours, then refill and measure how quickly the water percolates away, typically recording the drop every 30 minutes. The percolation rate, calculated by dividing the total time by the water level drop, determines the soil's suitability for a soakaway system, with local regulations guiding the acceptable rates and test procedures. The test is sometimes repeated in multiple locations to find the most suitable soil conditions.

Digging a hole for soil percolation test

Calculating Roof Pitch

Roof pitch calculation is essential to ensure proper drainage from roof areas. The roof pitch refers to the steepness or slope of the roof. It determines how quickly the soakaway must be able to disperse the water. The roof pitch is usually measured as a ratio of rise to run, such as 1:12 or 5:12.

To calculate the roof pitch, you need to measure the vertical rise and the horizontal run. This can be done by placing a level on the roof and measuring the vertical distance and horizontal distance between two points. Once you have these measurements, you can determine the roof pitch and design the soakaway to handle the amount of water that will flow off the roof.


It is recommended to regularly inspect and clean the silt trap or top of your crates of any debris, leaves, or silt to prevent blockages and make sure that its draining correctly. Additional inspections for any signs of standing water or wet spots can help indicate if the soakaway is working effectively.

Water on limestone paving close up

Can a soakaway get blocked?

Yes, soakaways can get blocked over time due to the build-up of debris, silt and leaves. Regular maintenance is essential to prevent blockages, including clearing debris and inspecting for signs of reduced drainage. Seek professional help if a blockage persists.


In summary, investing in a soakaway is an efficient and cost-effective way to manage surface water run-off and prevent flooding. Proper installation is key to performance and should not be undertaken without following the right processes and identifying any potential underground hazards. Regular maintenance is also required to prevent blockages, which is essential to ensure it operates efficiently.

We hope this beginners guide to soak away system is has been of assistance to you and wish you all the best in your DIY endeavours. Happy gardening!

Polar ice chippings for drainage along a lawn


How deep should a soakaway be?

When it comes to determining the depth of a soakaway, there are several factors to consider. The depth of a soakaway will depend on the volume of water it needs to accommodate and the type of soil present. In general, a soakaway should be at least 1.2 meters deep to ensure effective water dispersal.

However, if the soil has good permeability and the area experiences heavy rainfall, a deeper soakaway may be required. It is crucial to consult with a professional or follow local guidelines to determine the appropriate depth for your specific situation. The goal is to ensure that the soakaway can handle the water volume without causing any flooding or waterlogging issues.

How long do soakaway systems last?

The lifespan of a soakaway system can vary depending on several factors, including the quality of the installation, maintenance, and the soil conditions. On average, a well-designed and properly maintained soakaway system can last anywhere from 20 to 30 years.

However, it is important to note that certain factors, such as high groundwater levels, heavy rainfall, or poor soil permeability, can shorten the lifespan of a soakaway system. Regular inspections and maintenance, such as removing any debris or blockages, can help prolong the life of the system. If issues arise, it may be necessary to repair or replace the components of the soakaway to ensure continued effectiveness in managing surface water.

Can you use limestone for soakaway?

Yes, you may use limestone chippings for soakaways, but they are generally best for a different type. Due to their alkaline nature, limestone chippings are most suited to condensate soakaways as they help to neutralise acidic condensate from condensing boilers.

How do rainwater soakaways work for standing water drainage?

Rainwater soakaways operate as an efficient solution for managing standing water drainage through a subterranean infiltration system. They are designed to collect excess rainwater from surfaces such as roofs, driveways, or patios and allow it to percolate slowly back into the surrounding soil, thereby reducing the risk of flooding and preventing the overloading of public sewer systems.

The process is as follows:

  1. 1
    Collection: Rainwater is directed from impermeable surfaces into a designated pipework that channels it towards the soakaway.
  2. 2
    Storage: Upon reaching the soakaway, which is essentially a pit filled with rubble or coarse stones, the water is stored temporarily.
  3. 3
    Infiltration: The water then gradually seeps through the gaps in the material and is absorbed by the ground.
  4. 4
    Dispersal: The soil acts as a natural filter, removing impurities from the water as it disperses into the earth, recharging groundwater levels.

This method of drainage is both environmentally friendly and effective, provided that the soakaway is correctly sized and installed in a suitable area where soil conditions allow for good absorption and drainage.

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About the author 

Ben Parrot is a landscape gardener and amateur geologist. He is the founder of Outdoor Aggregates, with the mission to create the first encyclopaedia for decorative aggregates. From natural stone through to decorative bark, these aggregates are critical to garden and landscape design, universally.

The problem: There isn't any place where all the different types of decorative landscaping materials and construction aggregates can be found, nor are the specifications of each type readily available. So, enriched by his extensive experience, and inspired by the timeless elegance and raw beauty of stone and rock, etched by nature's artistry over millennia, Outdoor Aggregates was born.