Page 14 - North American Clean Energy July/August 2020 Issue

solar energy
The Unknown Perils of Ballasted Solar Systems
by Kevin Kervick
Most commercial solar rack systems are designed around ballast as the preferred method for securing a commercial rooftop solar array. While ballasted systems offer many benefits, solar installers and designers should carefully review all the variables before specifying the securement method. This includes knowing what type of commercial roof is installed, as well as how that system was installed.
and contraction and will expand/contract with a downhill bias. As a result, it is not uncommon that ballasted systems “walk” downhill over time.
FM also outlines specific instructions based on whether the roof was fully adhered or mechanically attached. Knowing which method was used to install the roof is critical for the installer and specifier to understand.
“2.1.1.6 Install ballasted, rigid roof-mounted PV panels over fully adhered roof covers.
There is no consensus wind design method for installing ballasted PV arrays over mechanically fastened, single-ply roof covers. When a ballasted PV array is proposed over an existing mechanically fastened single-ply roof cover that is relatively new, do one of the following:
a) Locate the array with a minimum setback distance equal to twice the roof height and fasten each module around each outside edge of each array.
b) Mechanically fasten PV panels throughout the entire array, such as by fastening each module around each outside edge of each array, and then intermittently fasten the remaining PV modules in the interior of the array
at every second module in their long dimension and every third module in their short dimension.”
“Fully adhered” refers to a roof system where the waterproofing is adhered to the insulation or coverboard using adhesive
or asphalt, which support a 100 percent ballasted PV system. Mechanically attached roof covers (i.e. membrane systems secured with screws and plates) are subject to ‘billowing’ or fluttering during windy
Figure 1.
Most companies and installers will tell you that they prefer ballast over a mechanical anchor to preserve the roof warranty. However, many industry veterans do not realize that ballasted systems can easily void roof warranties, do not eliminate rooftop rack movement, and can create more problems than they solve.
Factory Mutual and Solar
Factory Mutual Approvals (FM Approvals) is closely involved with commercial construction techniques, methods, and testing to mitigate risk. “FM Approval” is often considered the gold standard for testing and approving building products and assemblies.
For rooftop solar, FM outlines (in its Loss Prevention Data Sheet 1-15) the installation best practices and requirements for installing a PV array.
FM on Ballast
According to the best practices referenced above, FM places clear limits on the acceptable slope for a ballasted PV array on a commercial facility. (Figure 1).
“2.1.1.5 Install ballasted rigid PV roof-mounted solar panels on roofs with a maximum roof slope of 1/2 in. per foot (2.4°). A higher slope is not recommended for ballasted PV panels as it will decrease frictional resistance to wind forces and increase sliding forces from gravity loads, weakening wind resistance.”
While commercial roofs are frequently described as ‘flat,’ most are designed with some small degree of slope to ensure that the roof does not allow ponding water. Too often, ballasted solar systems are installed on commercial roofs with steeper slopes than FM outlines. This is dangerous and should be avoided - all PV systems are subject to thermal expansion
Figure 2.
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JULY•AUGUST 2020 ///
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