Commercial rooftops come in many colors. Traditional “black” (dark-colored, often asphalt or tar) roofs absorb heat from the sun and contribute to urban heat islands, which in turn contribute to poor air quality, increased heat-related illnesses, increased energy use (and costs) and more. White or “cool” roofs use reflective materials, which typically are light colored, to reflect the sun’s energy, resulting in rooftops that can be 50 to 60 degrees F cooler than a dark roof on a hot summer day.
The plants and growing media on “green” (vegetated) roofs provide shade and evaporative cooling that also can reduce rooftop temperatures by 50 degrees or more; in addition, these roofs can absorb and delay rainfall runoff. Both green and white roofs can reduce the amount of energy needed to cool the interior space of the floor below by 50 percent or more. “Blue” roofs offer yet another environmental benefit, by capturing rooftop rainwater for nonpotable use onsite or elsewhere. Which color roof is best for which building?
No Simple Answer
The simple answer is, there is no simple answer. Most researchers agree that traditional flat black roofs are bad for the environment and should be phased out as quickly as possible. There is also a growing consensus that whether they should be replaced by green, white or blue roofs is a decision to be made for each individual building based on a range of factors, including building location, type, cost and more. A recent study by researchers at Lawrence Berkeley National Laboratory who used a 50-year life cycle cost analysis found that while white roofs are slightly more cost effective than green ones (due to the green roof’s higher installation costs), the annualized cost premium is just 30 cents per square foot per year, “sufficiently small that the choice between a white roof and a green roof should be based on preferences of the building owner.”
In 2011, Regency Centers retrofitted its 31-year-old Roscoe Square shopping center in Chicago with a green roof, which helped the property earn LEED Silver certification. Photo courtesy of Regency Centers
“Owners concerned with global warming,” the report abstract continues, may want to choose white roofs, which are “three times more effective” than green roofs at reflecting sunlight back into space, directly offsetting climate change. However, both green and white roofs can help reduce greenhouse gas emissions that contribute to climate change by reducing the amount of energy used for building cooling. Owners concerned with local environmental benefits may want to choose green roofs, “which offer built-in stormwater management and a ‘natural’ urban landscape esthetic.” Other benefits of green roofs include a longer rooftop life (since plants and planting media offer protection from the elements), increased property value, potential tax incentives, potential food production as well as bird and wildlife habitat creation and increased employee productivity for those using the roof.
Consider Building Type and Location
Developers and building owners also should consider building type and location. A white or blue roof may be more appropriate for a large, single-story industrial building that has few weight-bearing columns and thus may not be able to handle the weight of a green roof’s growing media and plants. A multistory office building, on the other hand, may easily be able to handle that additional weight — and may benefit from transforming a green roof into a valuable tenant amenity by designing it as usable outdoor space.
Likewise, geography and climate can be a factor in this decision. While white roofs reflect sunlight more effectively than green ones, green roofs also can insulate buildings in cold temperatures, reducing the cost of heating in the winter as well as air conditioning in the summer. Researchers from the University of Arizona and the U.S. Environmental Protection Agency, in a recent study published in the Proceedings of the National Academy of Sciences, report that white roofs generally offer less benefits in northern areas like Chicago, Detroit and the Mid-Atlantic than in southern areas like Texas, Arizona, California and Florida, because in northern cities the energy savings from lower air conditioning use in the summer are offset by more heating requirements in the winter. Keeping green rooftops alive during hot, dry (or too humid) summer months can be more difficult in some southern climates than in more temperate northern ones. Thus green roofs may make more sense in cooler, northern cities, while white roofs may be more appealing choices in hotter southern ones. The amount of rainfall an area receives also may factor into the decision to install a blue roof or to include rainwater-collecting and filtering equipment on a green or white roof.
The bottom line is that there is no one “best” color for rooftops. In fact, a “multicolored” roof — one with a light surface, some vegetated areas and a rainwater collection system — may be the greenest rooftop of all.
For more information:
"Blowin' in the Wind: The Development of a Wind Resistance Standard for Green Roof Assembly," Bas Baskaran, Living Architecture Monitor, Summer 2014
“Economic comparison of white, green, and black flat roofs in the United States,” Julian Sproul et al., Energy and Buildings, Vol. 71, March 2014
“Urban adaptation can roll back warming of emerging megapolitan regions,” Matei Georgescu et al., Proceedings of the National Academy of Sciences, Feb. 10, 2014
“Looking Up: How Green Roofs and Cool Roofs Can Reduce Energy Use, Address Climate Change, and Protect Water Resources in Southern California,” Noah Garrison et al., Natural Resources Defense Council, June 2012