A soakaway is an effective way to disperse surface water when connecting to a surface water system, such as a gutter downpipe, is impractical or impossible. The soakaway works on the principle of a “reverse well”, collecting and draining water rather than holding it.
Even when a connection to a downpipe exists, a soakaway may still be a suitable solution, especially if the existing drainage is already overloaded. Often referred to as part of a Sustainable Urban Drainage System (SUDS), soakaways can help improve drainage where traditional methods fall short.
Factors Affecting Soakaway Success
For a soakaway to be effective, there are two critical considerations:
- Size of the area feeding the soakaway
- The rate at which the water drains into the ground
The depth of the water table also plays a role. If the area is prone to waterlogging, a soakaway may not work unless the issue is caused by an impermeable layer (like clay). In such cases, excavation down to porous material could make the solution viable, although this may be more disruptive.
Key Requirements for Installing a Soakaway
When planning your soakaway, certain criteria must be met:
- It must be lower than the area it is draining.
- It must sit above the existing water table.
- It should be at least 5 metres away from the nearest building.
- It must not be located where it could affect a building’s foundation.
- It must not interfere with existing drainage systems.
- There must be no risk of contamination from pollutants.
Before installation, it’s wise to assess the ground’s suitability for a soakaway. Digging a trial pit can help you determine whether the soil is porous enough. Hard rock or heavy soils may render the soakaway ineffective, while sandy soils or porous rock types will ensure success.
What Size Should the Soakaway Be?
The usual formula for calculating the size of the soakaway required is this:
Vol = A x (rainfall rate/3000)
Or, for example and in simpler terms, with the assumed UK rainfall rate:
Area to be drained x (50/3000)
To save you from having to do this calculation yourself, see the table below. To be safe, it’s best to round up when using pre-fabricated soakaway systems.
| Size of Area to be drained | Size of Soakaway Required (m³) |
| 25m² | 0.4m³ |
| 30m² | 0.5m³ |
| 40m² | 0.7m³ |
| 50m² | 0.8m³ |
| 60m² | 1.0m³ |
| 80m² | 1.3m³ |
| 100m² | 1.7m³ |
Contact Us for a Site Visit
At Edwards Surfacing, we’re experienced road surfacing contractors and drainage specialists. If you’re unsure whether you need a soakaway or if you should integrate with your existing drainage, our road surfacing company can help. We offer site visits to assess your property and recommend the best solution for your needs. Contact us today to discuss your drainage project with our team and explore options like tarmacing for your driveway or car park areas.