Resources
Green roof statistics cover very different kinds of evidence. A measured rooftop temperature is not the same as a modeled citywide benefit. Both can be useful when their locations, assumptions and reporting years stay attached.
A 2026 study in npj Urban Sustainability estimated that urban roof greening could reduce daytime land surface temperatures by 0.57–1.58°C across its global scenarios. That is modeled potential, not a measured temperature reduction already achieved by cities.
This reference brings that recent analysis together with a field study and a US government stormwater case study. It keeps site observations separate from scenarios so readers can use the figures without turning them into guarantees.
Key Green Roof Statistics at a Glance
- Bai and colleagues' 2026 study modeled 0.57–1.58°C daytime urban land-surface cooling across roof-greening scenarios.
- Bai and colleagues' 2026 study modeled 0.14–0.39°C nighttime urban land-surface cooling.
- Bai and colleagues' 2026 study modeled a 0.39–1.10°C reduction in daily temperature range.
- Baryla and colleagues' 2019 paper reported a maximum 31.3°C roof-surface difference at the Warsaw site during April–September 2015.
- EPA's 2018 Kansas City report modeled annual runoff of 3.3 inches for green roofs versus 32.3 inches for conventional roofs in its 2020 scenario.
- EPA's 2018 Kansas City report modeled up to 29 inches of annual stormwater retention in that 2020 scenario.
- EPA's 2018 Kansas City report modeled annual electricity savings of 601,502 kWh across its total 2020 green-roof scenario.
- EPA's 2018 Kansas City report modeled annual gas savings of 2,930 therms across its total 2020 green-roof scenario.
How much urban cooling could green roofs provide?
The 2026 Bai and colleagues study estimated 0.57–1.58°C in daytime land-surface cooling across scenarios with different extents of urban roof greening. Its nighttime range was 0.14–0.39°C.
The study compared scenarios covering 20%, 40% and 60% of each roof with vegetation. Its city models used average summer land-surface temperature and vegetation information from 2016–2020. Its temperature measure is land surface temperature. That is distinct from the temperature inside a building, the air beside a roof, or a person's heat exposure.
The modeled daily temperature range narrowed by 0.39–1.10°C. These findings describe the study's scenarios. They should not become a promise that adding vegetation to an individual building will produce a specified citywide change.
How large was the measured rooftop temperature difference in Warsaw?
Researchers reported a maximum 31.3°C surface-temperature difference during the 2015 summer observations at the University of Warsaw. The paper was published in 2019.
This was a maximum comparison at a particular site, not a seasonal mean or a national estimate. The study compared conventional and vegetated roof surfaces using thermal imaging.
The measurement answers a narrower question than the global model. It describes how two roof surfaces differed during the field observations. The global scenarios describe possible changes across urban areas. Combining the maximum of one with the spatial scale of the other would create a claim neither source supports.
How much stormwater did the Kansas City model retain?
EPA's 2018 Kansas City case study estimated up to 29 inches of retained stormwater annually. Its modeled annual runoff comparison was 32.3 inches from conventional roofs and 3.3 inches from green roofs.
This was a scenario looking toward 2020, not a later verification that the forecast occurred. EPA treated the runoff results as an order-of-magnitude estimate sensitive to the growing medium and its condition.
Annual retention and performance during a single storm answer different questions. A roof can affect annual runoff without retaining every storm. A design review needs the local rainfall pattern and the intended drainage function, rather than a percentage borrowed from another climate.
How much energy did the Kansas City scenario save?
The EPA scenario modeled annual savings of 601,502 kWh of electricity and 2,930 therms of gas across its total modeled green-roof system of 734,826 square feet. EPA's inputs describe existing office buildings, four-inch growing media and no irrigation. Those are scenario totals, not per-building savings.
A total-area result should remain attached to its modeled installation area and assumptions. Dividing it across an unrelated portfolio would imply a level of transferability the study does not establish.
Current building decisions need current costs and an asset-specific comparison. The study is useful as an example of how water, energy and environmental effects can be evaluated together. Its historical scenario is not a current construction price or a guaranteed operating return.
Turning research into a roof decision
The questions for an owner extend beyond the performance headline. The roof assembly needs to remain accessible and maintainable after the vegetation is installed. The proposal needs a clear responsibility for the waterproofing, drainage, vegetation and any work needed later.
CRA's guide to vegetative and solar roofs explains why the underlying membrane and the equipment above it should be reviewed together. A roof condition evaluation can organize findings before an owner commits to an added rooftop system.
For a reader using these statistics in a report, the most useful habit is to keep the measure beside its scope. “Surface temperature,” “modeled annual runoff” and “scenario-wide energy saving” carry more information than the number alone.
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