PV Racks for Sloped, Asphalt-Shingled Roofs: Page 3 of 5


Inside this Article

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.

If installing under the 2014 or 2017 National Electrical Code, you may need to mount equipment for rapid shutdown (RSD) at the array. If not using RSD-compliant MLPEs, a separate device is needed at the array. This can be rail-mounted. Even without RSD requirements, most installations transition at the roof from outdoor-rated wire to less-expensive THWN-2. Here again, the rail provides an easy place to mount junction boxes, saving the need to cut or drill more holes in the roof to flash or secure a junction box.

Rooftop PV systems require some maintenance and most rail-based systems make this straightforward. Since the modules are secured with top-down clamps, removing a module from the array—while leaving the others in place—is uncomplicated. With most modern systems, the ground path is maintained through the rail, so no additional bonding jumpers are required when a module is removed for service or replacement.


A disadvantage to rail-based systems is the expense of the rails and their transportation—long, heavy pieces of metal can be expensive to ship. Most modules are also encased in an aluminum frame, similar to a rail, so it’s material redundancy to add an external rail where one already exists.

Rail-free racks were first popularized by Zep Solar. This system has modules with specially grooved frames that were connected with a splice bar, which fit into those grooves. The system’s flashed leveling feet also attached to the grooves. These systems required PV modules with proprietary grooved frames, plus specialty tools and parts, which added cost but saved installation time.

Other manufacturers have since created a variety of “rail-free” systems (although, in most, a small section of rail or channel structure supports the modules and allows for some adjustability). This lets the attachments follow the shingle line and the roof surface, which often dips and rises due to wood framing inconsistencies below.

Most of these newer rail-free systems do not require a special module frame, so they can be used to support almost any PV module. Most rail-free racks use a top-down clip or clamp similar to those used in rail-based racks. The difference is that this clamp is part of a bracket assembly that also elevates the module off the roof, and connects to the roof frame with a lag screw and integrated flashing. Typically, only one attachment kit is needed per module at the upper and lower sides of each row. If more than one row is installed, then there will be a shared bracket assembly between module rows. With many systems, a coupling then connects adjacent modules at their corners to create a rigid support structure.

This greatly reduces the number of attachments needed to support an array—especially if modules are installed in a landscape layout, since only one connection is needed per 66 inches or so for a typical 60-cell module. If modules are in a portrait layout, then one connection may be needed every 40 inches or so. With fewer attachments and no rails, the entire rack system can sometimes ship in a single box—saving several hundred dollars in freight costs, and reducing time spent on the roof.

The downside to fewer attachments is that all of the dead and live loads are transferred to fewer points in the roof supports. In snow country, this could result in hundreds of additional pounds per connection, which could necessitate adding connections or even structurally reinforcing the roof. With support in fewer places on the long side of the module, the frame’s ability to resist wind and snow loads is decreased, which means these rack systems are not deployable in all areas, with all modules.

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