Southwest Environmental Limited

BRE 365 Soakaway Testing

We cab undertake soakaway tests for foul or surface (rain) water infiltration systems. BRE 365 is the method used for specifying disposal of rain water from roof and other impermeable surfaces.

Typically the process will involve:

1. Call SWEL and agree a price.
2. Send over your project drawings.
3. We come to site dig holes and do BRE 365 Tests.
4. About 3 days later you will get your report.

The report will show sizes of soakaway required, according to the BRE 365 calculation. If you are on sandy / gravelly soil they may be quite small, if you are on clay soils, they may be huge.

If your results are very bad, and the soak-away seams unfeasibly expensive then we can make an argument on your behalf that water should go to a river of stream, if there is one nearby.

In extreme cases the planning office may allow for connection to surface water sewers, and very very rarely foul water sewers. But you will need attenuation volume on site.

We are quick, and practically minded, and can help find a way forward for you. Digging holes and filling them with water can make quite a lot of mess, we try to keep this to minimum, we can arrange to repair surfaces / replace turf at cost but do not ordinarily do this.

Please call for a chat. No obligation.

Is Sandy Soil Good for BRE 365 Soakaway?

Nice clean washed sand, especially coarse sand, would make for a wonderful soakaway. The water will flow between the grains and away, leaving you with a set of good results, and hopefully a relatively small soakaway.

But most sand is mixed with something else (clays or silts) and has a range of grain sizes meaning that the spaces between grains are filled, and soakaway rates are lower.

So for example sands with perhaps 20% clay in them, which may still look like sand when dry, will not give very good results, and as we move towards sands with 40% clay in them the results will become similar to that of "pure" clay. 

So in summary sandy soils can make for a good soakaway, but not always.

Is BRE 365 for Roofs or Driveway

The BRE 365 soakaway method can be used to account for run off from any impermeable surface. Roof water and water run off from driveways are two options. You simply add up all of the surface with 0% permeability and put the total area in the model.

Some software can split these down a little further for example if you were to use permeable paving then a 30% impermeability figure can be input which will reduce the require volume of attenuation, and soak-away size overall.

Will BRE 365 tell me the size of Soakaway?

Yes it will provided your proposal meets the relevant minimum criteria. For example the top of the soak-away "should" be 1m below ground level. So technically we should not carry out calculation if soakaway top is less than 1 meter deep. But if the calculations are carried out then a soak-away size will be given at the end, and sometimes a selection of sizes, so that you our your client will have some flexibility in where to put the soakaway.

What are Access Requirement for BRE 365 Soakaway?

For typical sites we would use a 2.5ton digger, and an IBC (Intermediate Bulk Container). Sometimes we will need a breaker for concrete. This additional to normal soil investigation kit such, as spades, buckets and CAT scanner.

The holes can be dug by hand, in any position, but there still remains the task of getting 1000 - 15000 liters of water to the hole, and putting it in quickly. The world is you oyster here. We have used, tractor and tanker, land rover and bowser, dragged an IBC next to hole and fill with hose, drag several rain water butts up a hill and fill with buckets from nearby river. . . . . 

So in short you can do a BRE 365 soakaway tests just about anywhere, but setup can take 10 times longer in some spots.

The below section was written by a machine.

Technical Guidance: BRE 365 Soakaway Testing and Design Criteria

BRE 365 is the industry-standard methodology for designing soakaways and infiltration systems to manage surface water runoff from roofs and impermeable surfaces. As a core component of any site-specific flood risk assessment (FRA), determining the ground's ability to absorb water is essential for regulatory compliance. By following the BRE Digest 365 procedure, developers can ensure that drainage systems are sized correctly to prevent localized flooding and meet the requirements of local planning authorities.

The Testing Process and Site Requirements

Conducting a valid soakaway test requires precise on-site measurements and specialized equipment. The process involves excavating trial pits to the anticipated depth of the proposed soakaway and filling them with water to observe the discharge rate. Several key factors influence the outcome of these tests:

• Infiltration Rate Calculation: The core objective is to calculate the soil's permeability. Sandy and gravelly soils often yield high infiltration rates, resulting in smaller, more cost-effective soakaways, while clay-heavy soils may require significantly larger attenuation volumes.
• Test Conditions: Standard BRE 365 procedure requires the pit to be filled three times on consecutive days, or as close as possible, to simulate saturated ground conditions. This ensures the design is robust even during prolonged rainfall events.
• Access and Logistics: For most sites, a 2.5-ton excavator and a substantial water supply (often via bowser or IBC) are required to complete the testing accurately. We provide comprehensive support to manage these logistics efficiently on-site.

Integration with SuDS Strategy and Planning

Infiltration is the preferred method for managing surface water according to the Sustainable Drainage Systems (SuDS) hierarchy. If BRE 365 results indicate that the ground is unsuitable for soakaways, a robust technical argument must be presented to justify alternative discharge routes, such as nearby watercourses or sewers. Integrating these results into a broader SuDS strategy or drainage strategy is a critical step in securing planning consent.

Expert Consultancy for Drainage Design

With over 15 years of industry experience and 900+ projects delivered across the UK, SWEL provides the technical authority and professional accreditation (IES/FGS) required to navigate complex drainage challenges. Whether you are testing for a minor residential extension or a major commercial development, our team offers the professional oversight needed to secure your project's future. Our reports are designed to be clear, actionable, and ready for submission to building control and environmental regulators.