Green Roof & Blue Roof Storage Calculator
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Green Roof & Blue Roof Storage Calculator
Estimate the rainfall a green roof retains in its substrate, the attenuation a blue roof provides, and whether the build-up meets the ~5 mm interception target. Based on The SuDS Manual (CIRIA C753, Chapter 12 & Section 24.8).
Green roof retention & interception
The substrate stores rainfall, later lost to evapotranspiration. Retention available for the next storm = depth × (water-holding capacity − antecedent moisture).
Blue roof attenuation storage
A blue roof stores water above or within the build-up and releases it slowly through a flow control. Storage = area × storage depth × void ratio.
Reference & notes
Typical figures and how green and blue roofs sit in a SuDS strategy.
| Type | Substrate depth | Indicative retention* |
|---|---|---|
| Extensive (sedum / low planting) | ~80–150 mm | ~20–40 mm |
| Intensive (deeper planting / accessible) | >150–300+ mm | more, & heavier |
*Retention available depends on how dry the substrate is before the storm (evapotranspiration between events). It is not a one-off store – it recovers as the roof dries.
| Storage medium | Void ratio |
|---|---|
| Open water (restricted outlet) | ~1.0 |
| Attenuation cells / void formers | ~0.90–0.95 |
| Lightweight aggregate | ~0.4 |
| Gravel | ~0.3 |
Interception. Delivering about 5 mm of rainfall retention mimics the greenfield response for small, frequent events (C753 Section 24.8). Green roofs are well placed to meet this at source.
Roles. A green roof mainly retains (loses water to ET, reducing volume and providing interception); a blue roof mainly detains (attenuates the peak via a flow control). The two are often combined ("blue-green roof"). The flow control sizing is an orifice/throttle calculation; structural loading (saturated weight) must be checked by an engineer.
This blue roof attenuation calculator estimates the rainfall a green roof retains in its substrate, the attenuation a blue roof provides, and whether the build-up meets the roughly 5 mm interception target. It is based on The SuDS Manual (CIRIA C753, Chapter 12 and Section 24.8). Managing rainfall at roof level is source control under the national SuDS standards.
Green roof retention and interception
A green roof's substrate stores rainfall, which is later lost to evapotranspiration. The retention available for the next storm is the depth times the water-holding capacity, less the moisture already there. Enter the roof area, substrate depth and moisture state and the calculator returns the rainfall retained and whether you meet the interception target.
Blue roof attenuation storage
A blue roof stores water above or within the build-up and releases it slowly through a flow control. The calculator estimates the attenuation storage the roof provides for a design event, so you can see how much it takes off the rest of the drainage system.
How roofs fit the SuDS hierarchy
Green and blue roofs sit at the top of the SuDS hierarchy because they manage rain where it falls, reducing both the volume and the rate that reach ground-level storage. That can shrink the attenuation tank or pond the site needs downstream.
Further reading: green SuDS: nature-based drainage · the 2025 national standards for SuDS · gov.uk SuDS technical standards.
Frequently asked questions
A green roof retains rainfall in a planted substrate; a blue roof temporarily stores water and releases it slowly through a flow control. Some roofs combine both.
Roughly the first 5 mm of rainfall managed on site so that small, frequent storms produce no runoff. The calculator checks the build-up against it.
It reduces it, not always removes it. The calculator shows how much volume and rate the roof takes off, which informs the downstream storage.
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