Cambridge Roofscapes

In this StoryMap, you can investigate the distribution of cool and green roofs in Cambridge, and why they matter. You can see how the city's roofs are changing over time as the city encourages more sustainable uses of rooftops.

The interactive 3D City Hall Model below shows details of the streets and buildings, including the shape of rooftops.

Buildings cover a considerable area of cities. In Cambridge, about 28% of the city's developable land is covered by buildings. Utilizing this space for climate resilience strategies can advance the city's goals effectively. Some changes on roofs, such as increasing solar reflectance using solar reflective materials, are relatively accessible in the short term. In this StoryMap you can explore the cool roofs, green roofs, and roof shapes of Cambridge, including how they changed between 2018 and 2021.

To inform heat mitigation policies, Cambridge has measured the roof reflectivity of all buildings in 2018 and 2021. This shows how effective the city’s efforts and policies have been so far, and can inform how the city moves forward with further planning that incorporates the benefits of using roofscapes for climate resilience.

The Solar Reflectance Index (SRI) indicates a roof surface's ability to return solar energy to the atmosphere. Roofing material surfaces with a higher SRI will be cooler than surfaces with a lower SRI under the same solar energy exposure, especially on a sunny day. For more information, see the  EPA's  guidance on the heat mitigation impacts of roofs.

To measure the brightness of roofs in 2018 and 2021, the city has used high-resolution orthoimages. These images provide four bands (blue, green, red, and infrared) that together can be used for estimating SRI. The roof reflectivity is scaled between 0 (very dark) and 100 (very bright). An algorithm was used to match the brightness and contrast of 2018 and 2021 images to make them directly comparable. The following figure demonstrates the range of roof reflectivity measured from the orthoimages. Buildings with a reflectivity value of 60 and above were labeled as cool roofs. Buildings with a reflectivity value below 60 are labeled as dark roofs.

Why do we care about roof types?

Roof shapes define how we can utilize roof space for different purposes. For example, a flat (low-slope) roof can easily be coated with high SRI materials. The installation is relatively low-cost and will take a few days. Yet it can significantly impact the energy consumption of the buildings and associated energy bills, and mitigate the urban heat island effect. However, installing reflective materials on pitched roofs is more challenging but under conducive conditions, such roofs may provide a suitable space for solar panels.

In the following interactive map, you can explore the city's roof typology and see the distribution of low-slope and pitched roofs. This dataset is produced using LiDAR data and the city's 3D building model.

Flat (low-slope) and dark roofs may be a relatively accessible target for increasing roof reflectivity. Flat roofs constitute about 61% of all roof areas in the city. If we focus on the flat roofs, slightly more than a third of them (see Table) had reflectivity in the cool roof range in 2021. This means a considerable roof area in the city remains flat and dark, about 40% of all roof areas. In the following map, you can explore where flat and dark roofs were in 2021, along with other roof combinations from the table.

Green roofs, also known as "living roofs", are roofs of buildings that are partially or completely covered with vegetation and a growing medium planted over a waterproofing membrane. In some cases, modular trays and containers can be used for vegetation and soil without the use of a membrane. Green roofs provide various functions and benefits. They enhance the aesthetics and may provide open space for people and habitat for wildlife (e.g., birds and pollinators) in dense urban areas. They are known to mitigate the urban heat island effect as well as insulate the roofs from extreme temperatures by their larger thermal mass. Green roofs intercept precipitation, slowing down stormwater runoff through infiltration and also improving water quality. With all the benefits that they have, installation and maintenance can be costly; but in the long term green roofs can provide considerable benefits.

There are only a few green roofs installed in the city of Cambridge. Less than ten buildings have installed green roofs as of 2021, and these do not have a significant share of the total roof area. By scrolling through the following figure, you can see details of some of these buildings.

One important use of roof spaces is installing photovoltaic (PV) panels, commonly known as solar panels. PV systems provide a local source of renewable energy. They also shield the roofs from direct radiation. The impact of PV systems on the urban heat island effect is still an open question, but overall, the benefits outweigh the reduction in reflectivity.

In Cambridge, about 700 buildings have deployed solar panels on their roofs. This is about 4% of the total number of buildings in Cambridge (18,000). PV often covers parts of roofs and can be combined with cool or green roofs. This might enhance the performance of PVs by mitigating the heat around them. In Cambridge, 0.8% of the total roof area is covered by PV. Of the total area that PV systems cover in Cambridge, 66% are installed on flat and hybrid roofs. This is because flat roofs provide a more accessible space for installing and maintaining PV. In the following interactive map, you can see which buildings have deployed solar panels, by roof type.

The maps and numbers presented above show how the status of roofs (cool roofs, dark roofs, pitched and low-slope roofs, green roofs, and solar panels) in 2021. This work is informing the city's ongoing efforts toward climate resilience.  Visit the Climate Resilience Zoning page. 

On February 27, 2023, the Cambridge City Council adopted new zoning requirements to address the long-term impacts of increased flooding and heat from climate change. They are based on the City’s climate projections over the next 50 years and will be updated occasionally. One of the main factors in Climate Resilience Zoning is the Green Factor standards. This new performance-based standard encourages heat mitigation through site and landscape designs that have a cooling benefit. Sites will need to achieve a “Cool Score” by including features like trees (both preserving mature trees and planting new trees), shrubs and other plantings, green roofs, shade canopies, and cool pavements. You can review the  draft of the Cool Score Guidelines  for information on how to appropriately design landscape features to have a cooling benefit. These new standards will help the city to achieve a higher resilience in facing extreme heat events.