Considerations for photovoltaic roofing systems

by Ron Utzler
As the push toward renewable energy continues, roofing suppliers have been receiving an increased number of inquiries from photovoltaic (PV) providers regarding installation of these systems on warranted roof systems. Many of the PV suppliers operate on the assumption membrane manufacturers prefer use of ‘non-penetrating’ ballasted PV systems. However, this is not always the case.

When considering a rooftop PV installation, building owners should always get the roofing material supplier involved early, especially if a warranty is in effect. When receiving a call from a PV supplier, the roofing supplier/warrantor should pull the project records and look for the following elements.

Deck type
If the deck is anything other than a structural concrete deck in sound condition, it is important to remember the added weight of a ballasted PV system can cause deck deflection, resulting in increased ponding of water. A structural engineer should determine the dead load capability for any proposed system to avoid this situation. Further, a rack-mounted system’s supports should be located at structural members to avoid deck deflection.

Insulation type
Less dense, more compressible insulations may also contribute to membrane damage and increase ponding under the added weight of a ballasted PV system. The insulation should be polyisocyanurate (polyiso) board stock as base layer, with 15.8-mm (5/8-in.) gypsum cover board. However, this author generally advises against ballasted PV systems for the reasons cited throughout this article.

Membrane type
If the membrane system is composed of coal tar pitch, styrene-butadiene-styrene (SBS) modified bitumen (mod-bit), or other soft materials, ballasted PV systems could damage the membrane, especially in warm climates. Ballasted PV systems tend to involve more sheet metal assembly on the roof, creating a greater chance of membrane damage (see image above). Generally, all membrane types are vulnerable to physical damage under ballasted PV systems. Temporary protection should be used during roof loading and assembly stages.

Drainage
The installation of a PV system should not interfere with the drainage of the roof system (see image above). Positive drainage is one reason why the National Roofing Contractors Association (NRCA) recommends elevating PV modules, conduits, and other rooftop-mounted PV system components above the roof’s surface (see image below).

Design loads
Before the PV system installation, design loads must be considered. The roof deck and structural framing should be evaluated to verify there is enough load capacity to support the system’s weight and any loads associated with wind uplift (see image below).

Warranties
A planned installation of a solar panel system over an existing roof may require the following information be supplied to the roofing warrantor:
1. The owner may be required to have a roof inspection before the beginning of the solar panel installation, as well as perform any upgrades to the existing roof system determined necessary by the inspection. Depending on the age and condition of the existing roof system, this may be the best time to replace the existing roof system.
2. The roof warranty may be suspended until all aspects of the PV system are completed, inspected, and approved for reinstatement of the warranty.
3. Any alteration or penetration of the roof system (e.g. cutting through the roof for electrical connections) may require design drawings be approved by the membrane supplier in advance.
4. Upon completion of the solar installation, the owner may be required to agree to a follow-up inspection and infrared (IR) scan of the altered roofs to be covered by the warranty. Any damage or wet roofing found would have to be corrected before the warranty would be reinstated.
5. By terms of the typical roofing warranty, the owner would be responsible for removal and replacement of any overburden (i.e. solar panel system) necessary for leak investigation and leak repair (see image below). Removal and replacement can be costly to the owner and temporarily reduce power generation. The owner’s expense to move and replace overburden would be eliminated when an elevated PV system is installed.

6. There may be optional criteria on a case-by-case basis, especially where a ballasted PV system may be used. Due to the extensive array and potential effects on the roof’s long-term performance, the warranty supplier may require the owner to participate in a formal preventive maintenance program for the balance of the warranty term.

Recommendations
According to the NRCA, and some roofing suppliers, a low-risk installation of a solar array system is achieved by mounting the panel system on a raised rack and the rack legs flashed as per the membrane manufacturer’s recommendation (see image below). This installation can add value to the owner, including:

• reducing many of the concerns listed in this article;
• eliminating owner expense to move and replace overburden;
• waiving the requirement for a formal preventative maintenance program; and
• allowing future maintenance (and even re-roofing) without removal of the solar array.

If a building owner is considering the installation of a PV system, all relevant parties should be involved from the beginning.

Ron Utzler has been involved in the technical aspects of commercial/industrial roofing systems for 35 years. He is currently technical director at Viridian Systems in Tallmadge, Ohio. Utzler can be reached by e-mail at ronutzler@live.com.