If you're planning to install a soakaway, it's crucial to determine the correct size for your specific needs. For residential properties, the most common soakaway size is 1 cubic metre (1m³), as this is typically specified by Local Authorities. As a general guideline, a soakaway crate can drain approximately 50 square metres of roof area per 1m³ of crate, under standard conditions.
However, if your property is larger than average or you want to check you have sufficient capacity for your your roof area, our step-by-step soakaway sizing guide will come in useful. This should be only used as a guide; always seek approval from your local planning authority before starting any work.
Your soakaway may also need to be larger if you live in an area with higher than average rainfall or duration; in which case, this guide may not be helpful. If you have the statistics for your area, we can help create an exact design – feel free to contact us.
The Drainfast Step-by-Step Guide to Calculating Your Soakaway Size
1. Measure the Roof Area
To calculate the required soakaway size, you must first determine the area of the roof that will discharge rainwater into the soakaway. Follow these steps:
- Measure the roof length along the eaves (the bottom edge of the roof where rainwater collects).
- Measure the width of the gable or hip end and divide this measurement by two.
- Multiply A by B to calculate the basic roof area.
Example Calculation:
- Roof length = 10m
- Gable/hip width = 6m
- Half of the gable width = 3m (6 ÷ 2)
- Basic roof area = 10m × 3m = 30m²
2. Adjust for Roof Pitch
The pitch (slope) of your roof affects the total catchment area for rainwater. Use the table below to find the appropriate adjustment factor based on your roof pitch:
Roof pitch | 22° | 22.5° | 25° | 27.5° | 30° | 32.5° | 35° | 37.5° | 40° | 42.5° | 45° | 47.5° |
Adjustment factor | 1.182 | 1.207 | 1.233 | 1.26 | 1.288 | 1.319 | 1.35 | 1.384 | 1.419 | 1.459 | 1.5 | 1.547 |
To adjust for roof pitch, multiply your basic roof area by the appropriate factor.
For example, if the roof pitch is 45°, the factor is 1.5. Using our earlier calculation, the effective roof area = 30m² × 1.5 = 45m²
Tip: If you're unsure of your roof pitch, err on the side of caution by selecting a slightly steeper pitch, as most domestic roofs range between 35° and 45°.
3. Use our Soakaway Sizing Table
Now that you have your effective roof area, use the table below to determine the required soakaway size and the number of soakaway crates needed. These calculations are based on using Hydrocell Heavy Duty Storm Water Crates.
| Effective Roof Area (m²) | Soakaway Size (m³ / litres) | No. of Crates |
| 25 | 0.5 / 500 | 3 |
| 30 | 0.6 / 600 | 3 |
| 40 | 0.8 / 800 | 4 |
| 50 | 1.0 / 1000 | 5 |
| 60 | 1.2 / 1200 | 6 |
| 80 | 1.6 / 1600 | 8 |
| 100 | 2.0 / 2000 | 10 |
Need Assistance?
These calculations provide a useful guideline for average residential properties in regions with average rainfall volumes and duration, but specific project requirements may vary. For expert advice and a tailored solution, get in touch with Drainfast today!
Written by
Bob Stone
Technical Sales
Heading up our Technical Estimating Department, Bob is our in-house quantity surveyor.
