How Do I Know What Size My Soakaway Should Be?

OThe most common size of soakaway for residential use is 1 Cubic Metre (1m³), as this is what most Local Authorities seem to specify. As a broad brush a Soakaway Size of 1m³ is sufficient to drain a roof area of approximately 50 square metres under normal conditions.

To work out what size soakaway is required for your property, use the method below to give you an accurate guide. We recommend that you should always seek approval from your local planning authority prior to commencing any works. To work out what size of soakaway is required, you firstly need to calculate the area of the roof that will discharge into the soakaway. This is calculated as follows;

Calculating the Roof Area

Measure the length of the roof from one end to the other along the eaves. Write this figure down on a notepad.

Then measure the width of the gable end, or the hip end, and divide this number by two. Write down on the notepad the figure that you get after dividing by 2.

Now multiply together the two figures on your pad. Here’s an example;

Roof measurements for soakaway

The first figure you write down will be 10. The second figure will be 3 (6 divided by 2). Multiplying these two together gives 30 (10 x 3).

Now for the clever bit. To allow for the slope of the roof you need to multiply your roof area figure by the factor shown in the table below, depending upon the pitch of your roof.

Roof pitch for soakaway

In our example above, let’s assume we have a roof pitch of 45°. Simply multiply our initial figure of 30 by 1.5 to give an effective roof area of 45m(30 x 1.5). 

Note: If you are unsure of the exact pitch of your roof, choose a pitch that is more likely to be too steep than too shallow – most domestic roofs are pitched somewhere between 35° and 45°.

Sizing the Soakaway

Once you have calculated your effective roof area, you can use the table below to work out how many crates you may need.

Please note the numbers below are for guidance on residential projects - for full details on soakaway sizes please contact us today.

Effective Roof Area (Square Metres) Size of Soakaway Required   (Cubic Metres) Number of Rainsmart Ellipse Double Crates Required Number of Rainsmart Ellipse Shallow Crates Required Number of Polystorm Crates Required
25 m² 0.5m³ (500 litres) 2 4 3
30 m² 0.6m³ (600 litres) 3 5 3
40 m² 0.8m³ (800 litres) 3 7 4
50 m² 1.0m³ (1000 litres) 4 8 5
60 m² 1.2m³ (1200 litres) 5 10 6
80 m² 1.6m³ (1600 litres) 7 13 8
100 m² 2.0m³ (2000 litres) 8 16 10

Our Rainsmart Ellipse Double Cubic Metre Soakaway is a great option for your residential property. It comes flat-packed to save on delivery and only takes 10 - 12 minutes to put together without the need for any heavy machinery. 

Items you will need