How Big does a Soakaway Need to be?

The size of a soakaway depends on several factors, including the surface area that drains into it, the soil permeability, and the amount of rainfall in your area. Here’s a step-by-step guide to help you calculate how big your soakaway needs to be:

1. Calculate the Drainage Area

The first step is determining the total area that will drain into the soakaway. This could include:

Roof Area: If your soakaway is for a roof, measure the length and width of the roof area that will be directing water to the soakaway.
Driveways or Patios: If your soakaway is also taking runoff from a driveway, patio, or other impermeable surfaces, include those areas in your calculations.

For example, for a roof with a surface area of 50 square meters, and a driveway with an area of 30 square meters, your total drainage area would be 80 square meters.

2. Determine Rainfall Intensity

To calculate the volume of water the soakaway needs to handle, you must account for local rainfall levels. Rainfall is typically measured in liters per square meter. In the UK, for instance, you can use an estimate of around 0.05 meters (50mm) of rain for a heavy storm.

The calculation to determine the total volume of water (in cubic meters) is:

$(m)\text{Rainfall Volume} = \text{Drainage Area (m²)} \times \text{Rainfall (m)}$

For example:

Drainage Area = 80 m²
Rainfall = 0.05 m (or 50mm)

$\, \text{m²} \times 0.05 \, \text{m} = 4 \, \text{cubic meters}$

This means that for a heavy storm, the soakaway will need to accommodate up to 4 cubic meters of water.

3. Consider Soil Permeability

The type of soil where the soakaway will be installed greatly affects its efficiency. Percolation tests are commonly used to determine how quickly water will drain through the soil.

Percolation Test:

Dig a Test Hole: Dig a hole approximately 300mm wide by 300mm deep in the location where the soakaway will go.
Fill the Hole with Water: Fill the hole with water and let it drain completely.
Refill the Hole: Refill the hole again and this time, time how long it takes for the water level to drop by 25mm (1 inch).
Calculate Percolation Rate: If it takes more than 150 seconds for the water to drop by 25mm, you may have poor draining soil (e.g., clay), and the soakaway will need to be larger.

The faster the water drains, the smaller the soakaway can be. If your soil drains poorly, you will need a larger soakaway to handle the same volume of water.

4. Soakaway Crate Volume

Most modern soakaways are built using soakaway crates, which are modular plastic units that create voids for storing water while allowing gradual seepage into the surrounding soil.

Standard Soakaway Crates: Typically have a storage capacity of around 190 to 220 liters (0.19 to 0.22 cubic meters) per crate.
You can calculate how many crates you need by dividing the total volume of water by the capacity of each crate.

For instance, if you need to store 4 cubic meters of water and each crate holds 0.20 cubic meters, you will need:

$\, \text{cubic meters} \div 0.20 \, \text{cubic meters per crate} = 20 \, \text{crates}$

5. Guideline for Soakaway Size Based on House Size (General Rule of Thumb)

Small house roof (e.g., 50 – 100 m²): A soakaway of around 1 cubic meter is often sufficient, or about 5-6 crates.
Medium-sized house roof (100 – 150 m²): Soakaway size of 1.5 to 2 cubic meters, or around 8-10 crates.
Large house roof (150 – 200 m²): Soakaway size of 2.5 to 3 cubic meters, or about 12-15 crates.

6. Building Regulations (UK Example)

In the UK, the Building Regulations recommend the following:

At least 5 meters from any building or structure.
The soakaway should be large enough to contain 1 cubic meter of water for every 30 square meters of roof area draining into it.
Percolation Test must be passed to ensure the soil can handle the water.

Conclusion

The size of your soakaway depends on your drainage area, local rainfall levels, and soil type. Typically, for a small household, a soakaway of 1 to 2 cubic meters is sufficient, while larger projects may require 3 to 4 cubic meters or more. A series of soakaway crates is the most efficient way to create a soakaway, with the number of crates varying depending on the volume of water the soakaway must manage.

By performing a percolation test, you can also fine-tune your soakaway size to ensure it drains efficiently in your specific soil type.