PV Racks for Sloped, Asphalt-Shingled Roofs

Intermediate

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A rack system has the simple job of supporting the modules in a PV array.
Early rack solutions often included poor grounding techniques and inadequately protected roof penetrations.
Rail-based racks provide an easy way to level and align PV modules, as well as attaching module-level inverters and optimizers.
Rails run vertically for PV modules mounted in landscape orientation.
Top-down racking, like this system by RBI Solar, involves clamps that grip the module frames from above and supporting rails beneath. There are a variety of extruded rail configurations and clamping attachment systems.
Rail-free rack systems like this Quick Rack by Quick Mount PV use top clamps that integrate with mounting-foot locations.
Like others of its kind, this Roof Tech system allows subtle adjustment of horizontal and vertical alignment.
Material and shipping costs may be reduced by choosing rail-free systems. However, some systems may require more roof attachment points and more critical alignment.
Clamps at the module corners of this rail-less system by Quick Mount PV secure the module frames to the roof attachments. The gray rail is actually a trim strip, providing an aesthetic finish for the bottom of the array.
Clamps at the module corners of this rail-less system by Quick Mount PV secure the module frames to the roof attachments. The gray rail is actually a trim strip, providing an aesthetic finish for the bottom of the array.
Quick Mount PV’s online design tool.
A solid structural attachment directly to the roof rafters is ideal. However, attachment to blocking or even sheathing is possible with the proper product choices and engineering.
A typical flashing for a rack’s foot.
Post feet can be installed prior to roofing material, allowing reroofing without compromising the structure.
Various options for wire management can be integrated with racks, including clips, zip ties, and tucking wires into rail cavities. No matter what method is used, care must be taken to protect the wire’s insulation.

A rack system has the simple job of supporting the modules in a PV array. While that’s straightforward, options available may make choosing a manufacturer or method more complex. As installed costs for PV systems continue to decrease, racks become a larger portion of the cost, making it important to select racks that will install quickly and easily, be durable and reliable—and will not result in unexpected costs before, during, or after the installation.

In the early days of PV systems, rack system requirements were much simpler—there were no relative UL standards, no permits that needed to be pulled, and rarely were inspectors questioning materials and installation methods—PV systems weren’t covered in the National Electrical Code (NEC). Structures to support an array were custom-built from whatever materials were available. Most roof-mounted systems were tilted to capture every possible watt-hour from a handful of expensive PV modules.

Many of these early installations have lasted decades without problems, but that is not always the case. One of the issues with early systems is that the racks’ connection to the roof rarely included flashing to create a waterproof seal and maintain the roof warranty. Wind and snow loads can result in hundreds of pounds of force on this connection, and thermal cycling adds stress from expansion and contraction. Commonly, sealant was applied under and around a piece of angle aluminum, L-foot, wood block, or whatever anchor was used to hold the system in place. This method is not acceptable, as it will void the roofing warranty. Plus, it does not meet International Building Code (IBC) requirements or the National Roofing Contractors Association best-practice recommendations for sealing roof penetrations.

Another issue common in early rack systems was grounding and bonding. Since the rails and hardware to secure the modules did not necessarily create an effective ground path, a separate wire was needed to bond the modules together. A lay-in lug was typically used to bond the modules to the bare wire, but the installation methods for these lugs were not well understood by all installers, so it was common to see improperly bonded modules, or incorrectly rated parts and hardware used to make this connection.

As array voltage increased from 12 to 600 V, and as PV systems increasingly came under the scrutiny of the authority having jurisdiction (usually, the local electrical inspector) and the National Fire Protection Association, custom racks fell out of favor. With higher voltages, greater importance was placed on ensuring that the modules and rack were properly bonded, so that ground-fault protection systems in the inverter would function properly and shock and fire hazards would be reduced.

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