Soakaway Size Calculator
Calculate the right size for your extension
Why Use a Soakaway Calculator?
This soakaway calculator work better if the infiltration of the soil is known via a BRE365 soakaway tests. Results from BRE 365 soakaway testing are used to inform infiltration design within a wider Drainage Strategy for planning, supporting LLFA review.
Soakaway Size Calculator provides an easy way to determine the correct soakaway size for your project, especially for small-scale extensions.
The calculation is only valid for an extension of up to 25m².
This calculation is based on Building Regulations and BRE365; however, it should not be used as a checking tool or as a tool to obtain statutory approval.
The following calculation is only valid for an extension of upto 25m2. This calculation is based on building regulations and BRE365.
Soakaway Size Calculator
The area of the surface that will drain into the soakaway.
Percentage of rainfall that runs off the surface (depends on surface type).
The intensity of the rainfall event used for design purposes.
The duration of the rainfall event.
The rate at which water infiltrates into the soil.
The percentage of void space in the backfill material (e.g., gravel) that can store water. Select the type of backfill material: Gravel Filled (0.3) or Crates (0.95).
The shape of the soakaway to be used in the design.
The length of the soakaway trench or pit.
The width of the soakaway trench or pit.
The height (effective depth) of the soakaway trench or pit.
The diameter of the circular soakaway (manhole).
The depth of the circular soakaway.
The length of the crate soakaway unit.
The width of the crate soakaway unit.
The height of the crate soakaway unit.
Soakaway Size Calculator
How to Determine the Right Size for Your Soakaway
Using a soakaway size calculator is essential for accurately designing a system that manages excess water effectively. Calculating the correct soakaway size involves considering factors such as the catchment area, soil infiltration rate, and storm duration. Below is a step-by-step guide with the equations used in the process:
Step 1: Calculate Inflow Volume
Inflow Volume (m³) = Catchment Area (m²) × Runoff Coefficient × Rainfall Intensity (m/hr) × Storm Duration (hr)
Step 2: Calculate Outflow Volume
Outflow Volume (m³) = Soil Infiltration Rate (m/hr) × 50% Soakaway Internal Surface Area (m²) and no base included × Storm Duration (hr)
Step 3: Determine Required Storage Volume
Required Storage Volume (m³) = Inflow Volume – Outflow Volume
By following these calculations, you can ensure your soakaway is appropriately sized to handle water runoff efficiently. Use our soakaway size calculator to simplify this process and ensure compliance with drainage design standards.
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Frequently Asked Questions
What Are the Soakaway Clearance Rules?
The 5m rule ensures soakaways are placed at least 5 metres away from building foundations to prevent water damage. Additionally, soakaways should be located:
- At least 2 metres from a boundary.
- At least 15 metres from a structure if situated in chalk soil.
How Deep Should a Soakaway Drain Be?
Soakaway depths typically range from 1m to 2m, depending on the soil type and required volume. Using soakaway crates can help achieve the necessary depth while ensuring compliance with Building Regulations.
How Far Should a Soakaway Be from a House?
Soakaways must be positioned at least 5 metres away from a house to avoid potential structural issues.
Can Gravel Be Used in a Soakaway?
Yes, gravel is commonly used in soakaways and is often combined with soakaway crates to enhance capacity and efficiency.
Do Soakaways Need a Membrane?
A geotextile membrane is essential for preventing soil from clogging the system. It helps maintain efficiency, especially when using soakaway crates.
What Happens If a Soakaway Is Full?
When a soakaway becomes full, it overflows and stops functioning effectively. Proper design, sizing, and maintenance can prevent this. Use our soakaway size calculator to ensure accuracy in your design.
Can Soakaways Work in Clay Soil?
Clay soil has poor infiltration rates, making it challenging for soakaways to function effectively. Alternatives like attenuation tanks or infiltration trenches may be better suited.
What Are Alternatives to a Soakaway?
If a soakaway isn’t suitable, alternatives include attenuation tanks, infiltration trenches, and surface water sewers.
Use our soakaway size calculator to design the perfect drainage solution for your project and ensure compliance with UK Building Regulations.
Disclaimer
- No Guarantee of Accuracy: The calculations provided by this tool are based on general guidelines and may not reflect the specific requirements of all projects. Local regulations, site conditions, and specific engineering considerations may result in different requirements.
- Not a Substitute for Professional Advice: This calculator is not a substitute for professional engineering or architectural advice. Always consult a qualified engineer or architect for any specific project requirements before proceeding with design, installation, or construction.
- Use at Your Own Risk: The use of this calculator is entirely at your own risk. We accept no liability for any direct or indirect consequences arising from the use of this tool, including but not limited to design errors, project delays, or additional costs.
- Compliance with Regulations: It is the responsibility of the user to ensure that the final design complies with all relevant regulations and local authority requirements.