A growing body of research proves that replacing black roofs with a green roof or white roof can yield both economic and environmental benefits.
Many agree that black roofs should be phased out, perhaps with the exception of colder regions where black roofs may be preferable due to their capacity for heat retention.
No consensus has been reached, however, when it comes to determining the best alternative to black roofs. In fact, the debate over whether it is better to build with a green or white roof has become more intriguing given their varying characteristics and benefits.
Both white and green roofs alike are successful in mitigating the adverse effects of dark impervious urban surfaces, which contribute to an urban landscape’s heat island effect, or man-made areas that become hotter than adjacent rural areas.
Of the many kinds of green roofs and white roofs, which one is right for you? Before making your decision, let’s compare green roofs and white roofs and look at their overall economic and environmental differences.
What are Green Roofs and White Roofs?
A green roof, also known as a “living roof,” is partially or completely covered with vegetation and a growing medium. Because these roofs must be planted on a waterproofing membrane, they may also contain layers of root barriers, drainage and irrigation systems.
Green roofs are generally grouped into three categories:
- Extensive green roofs: These roofs require minimal maintenance since they contain native plants that have either low maintenance or self-sustaining characteristics.
- Intensive green roofs: These can somewhat resemble traditional gardens, and therefore may require semi-regular maintenance.
- Semi-intensive green roofs: Designs for these roofs usually prioritize aesthetic appeal over factors like seasonality or mimicry of local biodiversity. This requires regular irrigation, fertilization and maintenance.
The benefits of white roofs are determined partly according to their solar reflectance index, or SRI. A roof’s SRI indicates its capacity for reflecting solar heat. SRI measures reflectance (reflection of the sun’s rays) and thermal emittance (the roof’s ability to radiate absorbed heat).
SRIs are scored on a scale from 0 to 100: 0 being most likely to absorb and radiate heat, 100 being the most reflective or least likely to heat up.
Now that we have defined the two roofs, let’s explore their functionality. There are three factors you should consider prior to making and investing in this decision: economic consequences, environmental effects, and aesthetics and function.
Should I Choose a Green Roof or White Roof?
The Berkeley Lab Report analyzed 22 commercial flat roof projects in the United States in which two or more roof types were considered.
The research consisted of a 50-year life cycle cost analysis (LCCA), assuming a 20-year service life for white and black roofs and a 40-year service life for green roofs.
Researchers found that in comparison to black roofs, white roofs provide a 50-year net savings (NS) of $25/m2 ($2.40/ft2), while green roofs have a negative NS of $71/m2($6.60/ft2).
It was discovered that over the course of 50 years even the cheapest green roofs cost roughly $7 per square foot more than black roofs, while white roofs save $2 per square foot. Essentially, white roofs cost $9 per square foot less than green roofs, which amounts to $0.30 less per square foot each year.
The annualized cost premium is just $3.20/m2-year ($0.30/ft2-year). The annual difference in savings is so small that the choice between a green roof or white roof should be based on preferences of the building owner rather than economic analysis alone.
Keep in mind that during the warmer spring and summer months, green roof owners can count on plant evaporation to help cool their buildings. By the time winter rolls around, green roofs can reverse course to insulate and retain heat.
“Both white and green roofs do a good job at cooling the building and cooling the air in the city, but white roofs are three times more effective at countering climate change than green roofs,” said Rosenfeld.
While it does not affect the bottom lines of building owners directly, in some states and municipalities green roofs may also be eligible for tax credits, deductions or incentives.
As stated by Benjamin Mandel of the Berkeley lab report:
“What makes the monetary comparisons difficult …is that green roofs offer many environmental benefits that are difficult to objectively measure… while the economic results are interesting, it also highlights the need to include factors such health and environment in a more comprehensive analysis. ‘We’ve recognized the limitations of an analysis that’s only economic,’ Mandel said. ‘We would want to include these other factors in any future study.’”
Ultimately, a green roof can generate a return on investment (ROI) — just not for the short term investor.
Even over an extended period, the installation premiums for green roofs do not offset their long term benefits in contrast to white roofs.
As mentioned previously, white roofs reflect roughly three times as much sunlight as green roofs. They may be preferable if the building owner is particularly concerned with global warming.
That said, green roofs do offer more benefits to their immediate environment. Green roofs may not reflect sunlight as well as other roofs, but they increase and maintain local biodiversity by improving — and not disturbing — their surroundings. Other green benefits include capturing C02, supporting new and pre-existing ecologies and so on.
Although white roofs are effective at reflecting sunlight, reflected sunlight can also impact nearby buildings, potentially increasing their respective air conditioning loads.
When it comes to heat island effect, both roof types are successful at reducing urban heat differentials. Green roofs do this by way of their plants’ photosynthetic properties, while white roofs do this by allowing very little of the sunlight to be absorbed by their reflective surfaces.
Green roofs also reduce the impact of each new building on municipal storm drainage systems and the surrounding watershed. They mitigate flooding, erosion and artificial heating of water, which helps preserve fisheries and other forms of aquatic life.
The soil in green roof systems acts like a sponge, absorbing excess rainwater. Research has shown that extensive green roof systems can reduce runoff by up to 90 percent annually (varies with climate, soil and pitch of roof).
Aesthetics and Function
Green roofs allow architects to create aesthetically pleasing, eco-friendly roofs that can contribute many points toward LEED certification.
These 10 beautiful green roofs demonstrate what is possible. One look at Enric Ruiz-Geli’s Villa Bio, or McGlashan Architecture’s Mill Valley Residence, and it becomes clear that it is nearly impossible to match that type of beauty with a white roof.
Beyond aesthetics, green roofs’ greater availability of functional usable space is an additional bonus that gives them an edge in this category.
With enough creativity and design acumen, you can still build and design a white roof that’s usable for recreational purposes. But will it have the same effect as a garden-type setting? Probably not.
A green roof in the heart of Denver. Region 8 Green Office Building includes one of the first green roofs of its kind in the state of Colorado. The roof provides air and water quality benefits and reduces building heating and cooling needs.
Green Roof or White Roof? Apples or Oranges?
In my opinion, when it comes to the environmental benefits of a green roof or white roof, we are comparing apples and oranges.
If you’re concerned with metrics alone, white roofs seem to perform better based on their ability to deflect sunlight back into the atmosphere. However, green roofs arguably have more indirect — and hence more immeasurable — effects on the environment that are just as positive.
I believe that good marketing plays a large role in the emergence of white roofs on the eco-friendly roofing scene. I am not negating the positive effects that white roofs have on the environment, but I personally prefer the aesthetics and impact of green roofs.
Aside from the fact that my architectural background draws me to a beautiful roof that resembles a garden, the well roundedness of a green roof’s environmental benefits appeal most to me.
Both roof types yield economics benefits over the long run (although the white roof gets a firm upper hand in this category); both roofs can are good for the environment; and both can be aesthetically pleasing.
Ultimately, in my honest opinion, the green roof trumps the white roof — which is why if I were a building owner, I would choose to go green.