How the two differ in kind
Both sit on a flat roof and both reduce the impact of rainfall on the drainage below, and they act on different parts of the problem — which is the whole basis for choosing between them.
A GREEN ROOF retains. Rain landing on it soaks into the growing medium, is held there, and returns to the atmosphere through evaporation and plant transpiration. That water never reaches the drainage system at all. Retention is a genuine reduction in the VOLUME discharged, and it is accompanied by the other benefits a vegetated roof brings: insulation, protection of the membrane from ultraviolet and temperature extremes, biodiversity, and a reduction in the urban heat island.
A BLUE ROOF attenuates. Water is held temporarily on the roof above a restricted outlet — in a void, in a crate system, or simply ponded to a controlled depth — and released slowly over hours. The peak flow leaving the building is cut substantially, and the total volume discharged is unchanged. Attenuation is timing rather than volume.
That distinction maps directly onto what a drainage authority asks for, and the two requirements are different. A limit on the peak discharge RATE is met by attenuation. A target for the volume of runoff, or a requirement to manage the first flush, needs retention. Many schemes are set both, which is why the two are frequently combined as a blue-green roof: storage below, growing medium above, with the green layer retaining and the blue layer attenuating what it cannot.
The caveat on green-roof retention is worth knowing before the figure is relied on. Retention depends on the medium being able to dry out between storms, so a roof that is already saturated from yesterday's rain retains very little of today's. Published retention percentages are seasonal averages, and design-event performance is much lower — which is exactly the opposite of how a peak-flow requirement is assessed.
The factors that actually differ
Show
| Green roof | Blue roof | |
|---|---|---|
| What it does to runoff | Retains — a share of the rainfall never reaches the drain. | Attenuates — the same volume leaves, more slowly. |
| Which requirement it meets | Volume reduction and first-flush management. | Peak discharge rate limits. |
| Performance in a design storm | Much lower than the seasonal average, because the medium may already be saturated. | Predictable — the storage volume and the outlet's flow rate are both known quantities. |
| Dead load | Substantial, and it is the saturated weight that governs — the medium, the plants and the water held in them. | The stored water depth, which for a designed storage depth is a significant and calculable load. |
| Other benefits | Insulation, membrane protection from ultraviolet and temperature swings, biodiversity, amenity, urban cooling. | Essentially none beyond the hydraulics. |
| Maintenance | Real and ongoing: irrigation during establishment, weeding, plant replacement, and keeping outlets clear. | Minimal, but the flow restrictor and the outlet must be kept clear — a blocked restrictor is a flooded roof. |
| Drawdown | By evapotranspiration, over days, and weather-dependent. | By a restricted outlet, over a designed period — which is what the drawdown calculation sets. |
| Roof slope | Works on low slopes, with retention falling as the slope rises. | Needs an essentially level roof, since storage depends on holding a depth of water. |
| Waterproofing demands | Root-resistant membrane, and a leak is expensive to find under a planted build-up. | Membrane designed and warranted for prolonged immersion, with the outlets and upstands detailed for the design water level. |
| Combined | Blue-green roofs pair them — storage below, growing medium above — which is increasingly the answer where both requirements apply. | Identical, from the other direction. |
Which one, and when
Choose green roof when…
- The requirement is volume reduction, or runoff quality, rather than only peak flow.
- The other benefits carry weight — insulation, membrane life, biodiversity credits, amenity.
- The structure can carry the saturated load, which is the first thing to check.
- There is a maintenance regime, because a green roof needs one and does not survive without it.
Choose blue roof when…
- The requirement is a peak discharge rate limit, which attenuation meets directly.
- The structure or the budget will not carry a vegetated build-up.
- Reliable, calculable performance in a design storm matters more than seasonal averages.
- Maintenance capacity is limited, and a simple system with a clear outlet is more likely to keep working.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- What is the difference between retention and attenuation?
- Retention removes volume from the system permanently; attenuation delays it. A green roof retains: water held in the growing medium evaporates or transpires back to the atmosphere and never reaches a drain, so the total discharged over the storm is genuinely less. A blue roof attenuates: every drop that landed eventually leaves, but through a restricted outlet over a longer period, so the peak rate is much lower while the volume is the same. The reason this matters is that drainage requirements are written in both currencies. A limit on peak discharge rate — protecting a sewer from surcharging — is an attenuation requirement. A target for total runoff volume, or a requirement to manage the first flush for water quality, is a retention one. A scheme assessed against the wrong one can be built and still fail its condition.
- Why does a green roof retain less in a real storm?
- Because retention depends on the medium having somewhere to put the water, and a saturated medium has none. The growing layer holds a certain quantity of water against gravity; once it is at that capacity, additional rainfall runs through to the drainage layer and off the roof much as it would from a bare roof. Between storms, evapotranspiration dries the medium and restores its capacity — but that takes days, and it takes longer in the cool, damp conditions in which the largest storms often arrive. So a roof that retains a substantial percentage of annual rainfall may retain very little of a design event falling on an already-wet roof. Published retention figures are seasonal or annual averages, and design-event performance has to be assessed separately rather than inferred from them.
- What load does each add?
- Both are significant and both are governed by the water. For a green roof, the design load is the SATURATED weight of the build-up — medium, drainage layer, plants and the water held in all of them — plus any live load from maintenance access, and the saturated figure is much greater than the dry one, so a design based on dry weight is a design based on the wrong number. Extensive systems with a shallow medium are much lighter than intensive ones with deeper soil and larger plants. For a blue roof, the load is the stored water at the design depth across the roof area, which is a substantial distributed load — and it arrives as the storm progresses, so it is checked against the deflection behaviour of the structure as well as its capacity. Both are structural questions settled before the roof type is chosen.
- What happens if the outlet blocks?
- On a blue roof it is the critical failure, because the entire design depends on a small restricted opening staying open — and a small opening is by definition easy to block with leaves, silt, or debris from the roof itself. The water then rises above the design depth, and the load rises with it. That is why blue roofs are designed with overflows sized to discharge the full flow if the restrictor fails, set at a level the structure can carry, and why the outlet and its screen are a maintenance item with a schedule rather than a fit-and-forget component. A green roof has the same requirement in a milder form: its outlets are protected by inspection chambers that must stay accessible through the planting, and a roof whose outlets have disappeared under vegetation is one blockage from ponding.
- Can they be combined?
- Yes, and a blue-green roof is increasingly the answer where a scheme has both a volume target and a peak-rate limit. The arrangement puts an attenuation layer — a void or crate system with a restricted outlet — beneath the green roof build-up, so the vegetation retains what it can and the storage below attenuates the rest. It has a secondary benefit that is genuinely useful: the stored water is available to the planting through capillary rise or a wicking arrangement, which reduces irrigation demand and improves survival in dry spells, which is the main reason extensive green roofs fail. The costs are a deeper build-up, more load, and a more complex system with more to get wrong — so it is specified where the drainage requirement justifies it rather than as a default.
- What maintenance does a green roof actually need?
- More than the marketing suggests and less than a garden, and the first two years are the demanding part. Establishment requires irrigation through dry spells, and a roof planted and then left through its first summer is the classic failure — the plants die, the medium is exposed, and it erodes and weeds. Beyond establishment: weeding, particularly of woody species whose roots threaten the membrane; replacing failed planting; keeping outlets, inspection chambers and vegetation-free margins clear; and periodic inspection of the edges and upstands. Extensive sedum systems are the lowest-maintenance of the types and are not maintenance-free. The commitment matters because the alternative outcome is not a neutral one: a failed green roof is a weed bed with a drainage risk on top of an expensive membrane.
- Does either protect the waterproofing?
- A green roof does, substantially; a blue roof asks more of it. The membrane under a green roof is shaded from ultraviolet light and buffered against the daily and seasonal temperature swings that age a bare roof, both of which are among the main causes of membrane deterioration — vegetated roofs are commonly credited with a longer membrane life for that reason. The membrane must be root-resistant or protected by a root barrier. A blue roof's membrane, by contrast, spends part of its life submerged, so it must be specified and warranted for prolonged immersion, and the detailing at outlets, upstands and penetrations has to be watertight to the design water level rather than to a normally-draining roof's expectations. In both cases, finding a leak under the build-up is the expensive part.
- How is the drawdown period decided?
- By the drainage requirement and by the need for the storage to be empty before the next storm, and the two pull in opposite directions. A slower release gives a lower peak discharge, which is what a rate limit wants; but a system that takes too long to empty is still partly full when the next event arrives, so its available storage — and therefore its performance — is reduced. Design periods are therefore chosen so the roof drains down within a stated time after the storm, commonly set by the local drainage authority, and the outlet restrictor is sized to achieve it for the design storage depth. The calculation is a straightforward one between the stored volume, the head above the outlet and the outlet's discharge characteristic, and it is what the restrictor is selected from rather than chosen by eye.