Which Sustainable Roof Is Best For My Building? 'It Depends'
Here is something everyone can agree on: The market for sustainable roofs is big and getting bigger.
Driven by factors ranging from building code requirements to concerns about climate change, nearly $1.5B was spent globally in 2019 on the submarket of sustainable roofs known as green or vegetative roofs. Spending on this segment alone is projected to reach about $4.2B by 2027.
Where the consensus ends, however, is with the questions of what actually constitutes a sustainable roof for a low-slope commercial building, and which type is the best match for a particular building’s needs.
“What is a sustainable roof?” asked Jason Wilen, an architect and senior associate with architectural and structural engineering firm Klein & Hoffman. “It depends on who you ask.”
Wilen, who advises building owners and developers on selecting the best roofs, said that “sustainable” encompasses a range of roof types or attributes: white reflective roofs, when used in the right situation; green or vegetative roofs; rooftop solar; and blue roofs designed to retain some moisture during storms. It can also include fairly conventional roofs fabricated from sustainable materials or roof designs that incorporate multiple sustainable attributes.
Each has its pros and cons depending on the materials used and the owner’s expectations. If building owners want to reduce their energy consumption or gain other benefits of sustainable roofing while also getting a dependable top for their building that will last for decades, they need to do a little homework upfront.
“People get very excited about the sustainable part and get very bored by the roof part,” Wilen said. “But to have a successful roof, you really think about both sides of the equation.”
To Allison McSherry, an architect and associate with Klein & Hoffman, a successful sustainable roof does more than keep the weather out: It achieves a balance between potentially conflicting goals.
“It is a matter of finding balance between what the project goals are in terms of sustainability but also taking maintenance into consideration,” McSherry said. “The roof has to meet energy and building codes using proven and tested systems that withstand wind and other stresses — and should last many years with minimal maintenance.”
If a building owner is considering a roof with high thermal resistance or R-value, McSherry recommended modeling the potential new roof’s energy performance. That will help owners choose the best option and bring to light any unintended consequences.
“It really is about finding a balance between the performance of the roof and the overall building’s performance goals,” McSherry said.
That is why it is important to understand the options available and the potential trade-offs involved, she added.
A white, reflective roof is the type that might come to mind for most people when sustainability is mentioned. Wilen said a reflective roof can transfer less heat into spaces below the roof line, thus reducing a building’s dependence on air conditioning. However, this is less true when roofs are well-insulated.
Reflective roofs are also mandated by some local jurisdictions because reflective roof surfaces are thought to combat the urban heat island effect by reflecting solar energy back into the atmosphere.
But Wilen said there are also things to watch out for. Like any white surface, reflective roofs typically soil easily, reducing their purported benefit. In more northerly areas, they may also contribute to condensation in the building in some situations.
Vegetative roofs, too, have advantages and drawbacks. Wilen said vegetation provides added protection for the roof surface and reduces the building’s urban heat island impact — in most situations more effectively than reflective roofs. It can also provide an attractive tenant amenity and a habitat for local birds and pollinating insects.
Drawbacks include the expense and added weight, especially when being considered for an existing building. Vegetation can also complicate repairs when a roof leak needs to be addressed.
A sustainable roof can also work against the owner’s goals if it was not selected carefully.
“You might find that a sustainable roof gets you extra R-value, but if you have a very high internal load-dominated building use, you actually may be trapping heat and causing your energy usage to go up,” McSherry said.
A building owner must also think carefully about the products used when constructing a sustainable roof, Wilen added. Some environmentally friendly products have shorter service lives or a shorter or unknown track record compared to traditional products. This raises a question: If a sustainable roof needs to be repaired or replaced more frequently than a more traditional roof, is it really the greener option?
The bottom line is that a sustainable roof can benefit the building owner and tenants. But it is not a one-size-fits-all solution that can simply be installed without much thought and then ignored for 20 years.
“The best roof design occurs when knowledgeable roof system designers are free to select a roof system that best serves an owner’s needs while also utilizing proven materials and installation methods,” Wilen said. “In addition, those methods must be evaluated during construction to assist an experienced contractor in achieving the best possible installation.”
For technical guidance, the Klein & Hoffman team recommended consulting documents such as ANSI D7851-17, the “Standard Guide for Design of Sustainable, Low-Slope Roofing Systems,” and NSF/ANSI 347, the “Sustainability Assessment for Single Ply Roofing Membranes.”
ASTM also has published guidance for recycling roof coverings, membrane and shingle materials, ASTM D8013-16(2021). In addition, the International Energy Conservation Code sets minimum standards for energy-efficiency of roofs and other building components that are the basis for local energy codes in many places.
This article was produced in collaboration between Klein & Hoffman and Studio B. Bisnow news staff was not involved in the production of this content.
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