Attenuation or Soakaway: Which Build-Up Does Your Site Allow?

Same StormGrid crate, two different wraps. The ground decides which one you are allowed to use, so answer for the site rather than the preference.

1. Has a percolation test been carried out, and did it pass?

Passed — the ground infiltrates Failed, or the ground is clay Not tested yet

Infiltration performance is a site measurement, not a product property.

2. Can the groundwater table rise above the base of the tank?

No — site investigation shows it stays below Yes, or seasonally Unknown

This drives both the membrane specification and anti-flotation design.

3. Is there a discharge point — a sewer, watercourse or surface water connection?

Yes, with an agreed discharge rate No outfall available

4. Is the ground known or suspected to be contaminated?

No Yes or suspected

Contaminated ground normally rules out infiltration regardless of percolation rate.

Answer the four questions

Each answer narrows the build-up. Nothing here replaces a site investigation or a drainage design.

Scope of this tool. It routes you to the likely build-up so you can price the right components. It does not size the tank, set a discharge rate, or constitute a SuDS design. Approval routes differ across the UK — Wales operates a statutory approval body regime while England’s national standards sit as non-statutory guidance alongside lead local flood authority requirements.

Get a free module layout drawing for this build-up