How do I know what size my soakaway needs to be?


You’ll find soakaway crates in an assortment of sizes here at Cotterill Civils – but do you know which one to choose?

Knowing which size soakaway you need can be easier said than done but, relax, our team is always on hand to guide and advise you. The most common size for domestic use is 1m³. Specified by most local authorities, this size can effectively drain a roof area of approximately 50 square metres under normal conditions.

To figure out which size you need, the first step is to calculate the area of the roof that will discharge into the soakaway. And to do that, you will need to:

  1. Measure the length of the roof

Using a tape measure, start from one end and work your way towards the other along the eaves and make a note of the measurement.

  1. Measure the width of the gable end

With your tape measure again, find out the width of the gable end (or hip end) and divide it by 2 before jotting the figure down on paper.

  1. Multiply the two together

To get the roof area, times the length of the roof by the width of the gable end after halving it.

That’s the easy bit done.

You then need to consider the pitch of your roof to work out the effective roof area.

Roof pitch

Factor

20.0°

1.182

22.5°

1.207

25.0°

1.233

27.5°

1.260

30.0°

1.288

32.5°

1.319

35.0°

1.350

37.5°

1.384

40.0°

1.419
42.5°

1.459

45.0°

1.500

47.5°

1.547

Using the table above, multiply the initial figure (from step 3) by the factor for your roof pitch. If you’re unsure about the exact pitch of your roof, most tend to be somewhere between 35 and 40°. Choose one that is more likely to be too steep as opposed to too shallow.

Once you know the effective roof area in square metres, you should then be able to work out which size soakaway crates you need.

For full details on soakaway sizes, please feel free to get in touch with our team.

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