The greening of the roofs
Roofing considerations for cities
According to the earlier-mentioned RoofPoint Guideline, www.roofpoint.org, each section is comprised of components to take into consideration when deciding on a type of roof:
Energy Management: high roof systems; best thermal practices; roof surface thermal contribution; roof air barrier; rooftop energy systems; and rooftop daylighting
- Material Management: recycled content; materials reuse; roofing waste management; and low-voc materials
- Water Management: roof stormwater retention and roof-related water use reduction
- Durability / Life Cycle Management: durable roof insulation system; roof drainage design; roof traffic protection; increased wind uplift resistance; hygrothermal analysis; construction moisture management; roof system durability enhancement; roof maintenance program; and project installation quality management
- Environmental Innovation in Roofing: innovation in design and exemplary performance
With all due respect to Kermit the Frog who does not consider being green easy, look for some municipalities to disagree due to the increasing implementation of conservational roofs over traditional black ones.
To Rick Duncan, Ph.D., P.E., and technical director of the Spray Polyurethane Foam Alliance, green roofs can mean different things to different building managers.
“To some, a ‘green roof’ can be one where a layer of soil is placed over the roof system and plants or grasses are planted in the soil,” said Duncan, adding that he considered the former a “vegetated roof,” rather than a green roof.
“To others, a green roof is one that has minimal environmental impact. There was a voluntary program called RoofPoint developed by the Center for Environmental Innovation in Roofing that was established a few years ago to help facility managers make environmentally sound roofing decisions.
“Unfortunately, it does not appear to have gained much traction with building managers. However, the criterion RoofPoint uses to score replacement roofing systems is excellent. Full disclosure — I participated as one of many technical advisors for its development.”
Another type of roof, founded by Juan Carlos Pineiro Escoriaza, is called the White Roof Project, which involves a black roof covered with solar-reflective white paint. This type of roof, on a 90-degree day, can reflect up to 90 percent sunlight whereas the typical black roof can only reflect 20 percent of that. A black roof could be up to 180 degrees while a white roof might only be 100 degrees, resulting in up to 40 percent cooling costs.
There are many reasons why building managers should consider “green roofs.” According to Duncan, energy savings, recyclability, water management, durability and environmental impact are all important considerations.
“In many cases, there is no additional first cost with some green roof systems,” said Duncan. “If the first cost is even slightly higher, then it may be offset by energy savings or extended service life — all of which can be estimated by a life-cycle cost analysis.”
He added, “Currently, many building owners and designers are moving away from single-attribute assessment of a roofing system (e.g., high-recycled content or low emissivity) and moving towards using ISO-compliant life cycle assessments and environmental product declarations. LCAs and EPDs provide a ‘big picture’ view of environmental impacts, and enable the customer to easily compare different alternatives. SPFA has an industry-level LCA and EPD that can be used for credit under different sustainable-building programs like USGBC’s LEED and GreenGlobes.
“Also, SPF roof systems require polymeric coatings to protect them from UV damage. One can use ‘cool-roof’ coatings to further improve energy efficiency of SPF roof systems in hot climates.”
Duncan said, “Today, we are seeing a big emphasis on solar PV systems used on low-slope roofs to meet zero-energy goals. These seem to be getting more traction than green roofs. SPF roofing systems are ideal for use with PV systems, as SPF self-flashes around all PV supports and various roof penetrations. SPFA will be publishing a guidance document on SPF and PV later this year.”
Asked whether he could list any downsides to using green — not vegetated — roofs, Duncan said not that he knew of; that it was mainly a matter of product/system selection and design.
“Also, PV can have a high first cost compared to other roofing systems,” said Duncan. “These costs are often subsidized by federal, state and municipal programs, or by utilities. With the current cost of PV, payback periods can be 10-20 years in most of the country. Of course economic payback is not always a primary factor as clean energy production and energy independence are also considerations.”
According to Duncan, these PV systems are the very latest in roofing advances.
Shann Finwell, American Institute of Certified Planners and Environmental Planner for Maplewood, Minn. — population 39,765 — has long been involved in sustainable initiatives for a green city. However, her office building is presently without a vegetated green roof, but Finwall explains that Maplewood’s new Fire Station No. 1 was built under the city’s new Green Building Code and includes a solar-reflective roof and increased roof insulation.
“The code requires that all city-owned buildings be built to the green standards and encourages private building owners to participate,” she said.
Finwall states that, after one year in operation, Fire Station No. 1 had a decrease of 38 percent in natural gas use in 2015 compared to the other fire stations in the city. This energy reduction equates to $5,000 in energy savings per year,” Finwall said.
The Ramsey-Washington Metro Watershed District in Little Canada, Minn., had an extensive-style green roof installed on its garage, according to Simba Blood, natural resources technician.
“We considered other types of green roofing, but our intention was fixed on installing a green roof,” Blood said. “This was an early green roof in the state, intended as a demonstration and a test of the technology.
“The roof is designed at a 3:12 pitch (the extreme limit of pitch recommended at the time) with a very shallow growth medium thickness of 2 inches. We installed about 50 percent Minnesota native plants and 50 percent sedums. With these rather extreme conditions, we had some difficulty getting good plant establishment initially.”
He added, “Currently, the plants are doing well; the roof has required less than one hour of maintenance (weeding) every year for several years. The roof is functioning well; it captures at least the first quarter of rainfall that falls on it and helps keep the garage cool and comfortable in the summer. While we have not explicitly discussed what approach we would take with replacement — we expect to have another 15-20 years before this becomes a concern — we are very pleased with the roof’s performance.”
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