By Harry J. Lubitz, CSI, CDT
In trying to achieve specific outcomes within the realm of sustainability, architects and designers are increasingly learning to pair technologies for improved efficiency or performance. One case in point can be found with the combination of solar photovoltaic (PV) power systems mounted to a standing-seam metal roof—a result that brings together renewable energy with longevity.
The cost of solar generated electric power is continuing to become a more attractive alternative to electric power generated by traditional methods because of reduced cost for components and new developments in cost-saving mounting techniques. Coupled with choosing the right platform for those solar panels, the long-term costs can make a solar power system a very attractive proposition.
Importance of the platform
Since the expected life of a solar power system is 30 to 35 years, it is well beyond the lifespan of most conventional roofs. Generally, the service life of a built-up roof (BUR) or membrane roof is anywhere from 15 to 20 years with proper maintenance. (This estimate comes from the “2005 Roofing Industry Durability and Cost Survey,” written by the late Carl G. Cash, PE, of engineering firm, Simpson Gumpertz & Heger [SGH].)Consequently, there is a mismatch between the longevity of the base below the solar array and that of the solar array itself. This means there will be significant cost incurred during the lifespan of that solar power system due to the teardown and disassembly of the system during a reroof cycle.
This can be especially daunting considering the cost of this activity. One must remove all the solar panels, mounting hardware, racking, conduit, condenser boxes, and wiring. Then, these components must be taken off the roof and placed in a safe storage place. At this point, the old roof surface is torn off and likely sent to a landfill. Then, an entirely new roof surface is installed. Finally, the PV power system must be retrieved from its safe storage, brought back up on to the roof, and entirely reinstalled.
Added to the expense of this activity are some hidden costs. First, there needs to be replacement of any inevitably damaged solar components spoiled during the process. Second, there is the loss of power generation while the solar system was decommissioned during the reroofing and PV disassembly/re-assembly processes.
Much of these costs can be avoided by simply considering the best roof and solar platform in the initial design phases of the project. Unlike other roofing materials, a standing-seam metal assembly is impervious to ultraviolet (UV) degradation. Consequently, a well-maintained metal roof can last 50 to 60 years and even longer—in most cases, it should outlast the solar power system itself. (For more, see the 2014 report from the Metal Construction Association [MCA], “Service Life Assessment of Low-Slope Unpainted 55% Al-Zn Alloy Coated Steel Standing Seam Metal Roof Systems.)
While more costly in the initial installation, the long-term savings outweigh the upfront expense of a metal roof. Many traditional non-metal roof products have an initial cost advantage of $1 to $2 per square. This is miniscule when considering the estimate of approximately $14 (CDN) per square for decommissioning and re-commissioning of the PV system. This cost, added to that of the tear-off and reroof of the roof surface estimated at $4 per square, effectively makes the metal roof platform far more cost-efficient over the entire life cycle of the solar power system.
A more sustainable base
In addition to being highly reflective, metal roofs are 100 per cent recyclable at the end of their lifespan. Whether made from steel or aluminum, these decks generally comprise 85 to 90 per cent recycled material, which contributes to those projects seeking related points under the Leadership in Energy and Environmental Design (LEED) program.
Another overlooked side benefit is the rooftop temperature stability of solar panels installed over a metal roof. Raised solar panels cast shade on the metal roof, reducing rooftop temperature and cooling costs. A potential 30 per cent (or greater) drop in rooftop temperature can be realized, offering significant savings during the warmer season. This shading effect could promote fungal growth on roof surfaces in humid climates, but metal roofs contain a zinc component that kills fungus. As long as organic matter is not allowed to collect on a metal roof, it should remain fungus-free.
When crystalline solar panels are attached to a typical standing-seam metal roof, the result is a convective plenum, drawing air up under the bottom and back out through the top of the solar array. This ‘stack-effect’ ventilation below the solar panels adds more cooling effects to the metal roof; it also keeps the solar panels cooler to affect greater power generation efficiency.
No penetrations means no leaks
Solar PV systems mounted on standing-seam metal roofs have another important benefit—a non-penetrating installation. When installing solar panels on non-metal roofs, the designer has two attachment methods from which to choose:
- weighting the panels down using concrete ballast on flat roofs; or
- fastening them down on sloped roofs.
Ballasted systems require a significant amount of racking hardware and ballast, adding tremendous cost for both the mounting system and the extra building support structure to accommodate the additional weight. Non-ballasted or through-fastened systems require penetration of the mounting systems through the roof surface into the structure—adding holes in the roof and numerous potential places for leaks.
With a standing-seam metal roof, one can install an entire crystalline solar panel system without any penetrations, and at a huge savings in hardware and labour costs. A solar panel roof can have over tens of thousands of attachment points. A non-penetrating attachment system utilizing a standing-seam clamp will simply and effectively attach the crystalline solar panels without voiding the metal roof warranty and causing a place for a leak.
These standing-seam clamps should be load-to-failure-tested by an independent testing laboratory and can be ‘engineered’ for the specific wind uplift requirements of the project location. One example is a ETL-listed to UL 1703, Standard for Flat-plate Photovoltaic Modules and Panels; it allows crystalline solar panels to be directly attached to standing-seam metal roof panels, eliminating traditional rack and rail methods, and electrically bonding the panels together, eliminating copper lug wires.