Roofing material versus service life
Generally, a module’s warranted life is approximately 25 years, but the products themselves often exceed three decades of use. The service life of the roof should exceed the service life of the solar system. Otherwise, it needs to be replaced—a move that necessitates the de-commission, removal, and re-assembly of the solar system. This presents serious financial and logistic impacts beyond the cost of a traditional membrane roof replacement.
The mean service duration of conventional roofing materials can range. Survey results published by Carl Cash of Simpson, Gumpertz & Heger (SGH) in 2005 suggest the following life expectancies:
- built-up roof (BUR): 19.8 years;
- atactic polypropylene (APP): 16.1 years;
- styrene-butadiene-styrene (SBS): 17.7 years; and
- thermoplastic polyolefin (TPO): 13.6 years. (Some specific TPO products are warranted for 15, 20, or 25 years, however.)
The service life of other commercial roof types typically falls between 12 and 20 years At that point, they have to be replaced, even though the solar array atop their assembly is still performing.
This means the reroofing work would involve:
- removing the modules;
- removing the mounting and racking system;
- de-commissioning the system during the reroof;
- traditional re-roof work;
- re-installing the PV system; and
- re-commissioning the system.
When combined with the fact PV components will likely be damaged and require replacement, and the loss of power production during the work, this becomes a very expensive process.
It is vital to understand the relative costs of the roof asset (i.e. the mounting platform) and the solar asset (i.e. the revenue generator), as they have a profound effect on each other. For example, with a new TPO roof of significant size (i.e. 2322 m2 [25,000 sf] or larger), covered with PV, the initial cost is approximately $30/m2 ($3/sf). Calculating the value of the solar array at about 12 Watts per square foot at a typical cost of $2.50/Watt places the cost of the array at $300/m2 ($30/sf). The initial cost of solar is approximately 10 times that of a conventional roof. The aggregated cost of this total system is $330/m2 ($33/sf).
The cost of reroofing is not only the cost of the re-roof, but also the expense of dismantling and reassembling the solar array, in addition to the loss of power production while this process takes place. Estimating the re-roof cost at $20/m2 ($2/sf) and the cost of removing and reinstalling the solar array at 130/m2 ($13/sf), the total is $150/m2 ($15/sf) for the complete re-roof. This means the aggregated cost of this total system over time is now $480/m2 ($48/sf).
While these estimates are based on the previously mentioned Cash survey, along with professional experience and current market costs, the final numbers will vary based on state, union or non-union labor, and the project’s size and parameters.
Standing seam metal roofs
This calculation can be contrasted with another example—a 33-year-old Galvalume standing seam metal roof (Figure 5). Recent field studies gathered empirical data from 14 roofs aged over 30 years in five different U.S. climate zones affirm such roofs can be in the 60- to 70-year range, far outliving the life of a solar PV system.
There are also other benefits to specifying a standing seam metal assembly for a roof that needs to be solar-ready. For example, instead of expensive ballasted or penetrating racking systems required for membrane (or any non-metal) roofs, clamps can be used to mount the system.
Mounting methods to standing seam are actually lower in cost—15 percent less in some cases—than with conventional roof types. In many instances, enough money is saved in mounting costs to pay for the higher-grade roof, and still have net savings.
The cost of a mid-sized, low-slope commercial standing seam roof is about $40/m2 ($4/sf) and, as mentioned, it should outlive the solar array, avoiding all the costs of roof replacement. Consequently, the aggregated cost of ownership of the roof over the life of the solar array is approximately $310/m2 ($31/sf) versus the aggregated cost of ownership over the same time span of $480/m2 ($48/sf) on a conventional commercial membrane roof.
