How Do I Know What Size My Soakaway Should Be? 

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A soakaway system enables you to collect water and then slowly disperse it into nearby soil without putting additional pressure on any run off and drainage systems, which is particularly important in areas with higher than average rainfall.

Installing a soakaway is fairly straightforward and requires little maintenance – you simply need to dig a uniform hole, ensure that your soakaway crate is wrapped in a geotextile membrane, place it into the ground, and then connect it to your surface drainage system – however it’s crucial that you choose the right size solution depending on your requirements.

In this soakaway size calculator guide we provide guidance on soakaway sizing, along with some additional constraints that you should consider before you get started.

Most Commonly Used Size

For residential applications, the most commonly used size for a soak away is one cubic metre. This is also the size that some local authorities specify within sustainable drainage standards, however as it’s only generally sufficient for a roof size of up to 50 square metres in average UK conditions, it’s advised that you always calculate the correct size you need before making a purchase. It’s also recommended that you seek installation permission from your local authority before you go ahead, and check if there are any building regulations or requirements that you need to follow.

Calculating the Correct Soakaway Size

To calculate the sizing of soakaways that you need, including how many crates, you should carry out a soil percolation test as the results will determine soil conditions – including the clay content – and inform your soakaway design. If this isn’t an option, there are other factors you can look at for a much less precise idea, such as the measurements of the property roof, as this will give you a general indication of the amount of roof rainwater that will be channelled down to the system. You should also account for other impermeable areas such as patios or driveways if that’s where the water will also be flowing from. Unfortunately no automatic soakaway size calculator exists yet – but we are working on this!

picture of sizes of soakaway systems

Measuring a Roof

If you have a gable roof, you can calculate it’s surface area by measuring it’s length from end to end, then measuring it’s slope height from the eaves to the ridge – you may also be able to find these measurements on a copy of the building plans for your home or property. You then need to multiply the length by the slope height, and double the number to calculate the total area – in short, the calculation is Roof Length x Roof Slope Height x 2 = Roof Area.

For more complicated roof shapes, it’s recommended that you split the roof into sections and work out the sizes of each section, then add the measurements together for a total area.

Crate Capacity

Once you have the calculated size of your roof area, you can follow the general guide below to find out how many crates you need depending on the combined capacity of the brand and type of soakaways you’re looking to purchase. Please note that this is a very general rule of thumb for residential projects only and at EasyMerchant, we strongly recommend the use of a drainage percolation test for accurate calculations based on the soil type:

  • 50m² requires crates with a water storage volume of 1.0m³ (1000 litres)
  • 60m² requires crates with a water storage volume of 1.2m³ (1200 litres)
  • 80m² requires crates with a water storage volume of 1.6m³ (1600 litres)
  • 100m² requires crates with a water storage volume of 2.0m³ (2000 litres)

Other Considerations

In addition to choosing the right size soakaway, it should also be installed in a location that’s at least 5 metres from the wall of a house or building and 2.5 metres away from the boundary, so check you have enough space before you begin. It’s also important that a soakaway construction isn’t positioned in an area where the water may become contaminated, and it shouldn’t be located near another rainwater soakaway – where this isn’t possible, you should consider a viable alternative to soakaway crate system.

picture of a space for soakaway crate

Complete Kits to Save Time and Money

At EasyMerchant we offer a range of durable soakaway crates, attenuation tanks and drainage crates, including complete soakaway crate kits which include soakaway crates, non-woven geotextile membrane, waterproof tape, and cable ties to complete a full installation. Our expert team can also offer guidance and information to ensure you order items with the right dimensions and volume.

product image hydrocell soakaway crate system

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