PV Racks for Sloped, Asphalt-Shingled Roofs: Page 4 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.

Usually, wire management is easier with modules installed in a portrait layout. In this orientation, there is always enough wire length to securely clip the leads to the module frame while avoiding drooping wires. For the same size array, there are also usually fewer rows of modules in a portrait layout than in a landscape layout. This decreases the number of jumpers needed between rows as modules are wired into source or output circuits. Most rail-free systems, then, are at a disadvantage, since they encourage modules to be installed in landscape orientation. The addition of MLPEs, which have even more wiring to deal with, makes this more complicated. With rail-free systems, MLPEs need to be connected to the module frame with an accessory bracket, which adds time and cost, but can be done on the ground before the modules are lifted to the roof. Even with the unique challenges of rail-free racks, the material cost and  time savings can outweigh the potential disadvantages. This is especially true for lower-sloped roofs and in locations where heavy snow loads are not a significant factor.

Technical Considerations

When deciding what type of rack system to use and from which manufacturer, there are a number of considerations that can help ensure a smooth project from start to finish.

Compatibility. The rack system needs to fit the specific site characteristics and the PV system’s electrical design. If you’re installing many systems, a rack preference should be versatile enough to work with many roof types, pitches, and module manufacturers. It is not uncommon to design a system for a specific module and then have that model no longer available by the time the permit is approved and parts are ordered. The more universal the system, the easier it will be to accommodate changes without a major system redesign. With rail-free systems, it is critical to verify that the modules are compatible, as some PV module frames’ clamping zones may be too small for many rail-free systems.

Design support. Many rack manufacturers offer online tools to assist with the system design and verify module compatibility. The best tools allow you to design a virtual system.

With a few site-specific inputs like location, module model, roof height and pitch, and attachment spacing, the design tool should provide an engineering report that can be used to verify that the structure can support the live loads specific to that area, based on local code requirements. This report should provide uplift and shear forces on the attachments, which can be used to help select the appropriate lag for the project.

More sophisticated online tools provide drawings that define array area, rail lengths, and attachment locations, and provide custom views of the module layout and array height above the roof. These can help expedite engineering and permit review. If the drawings are accurate, they can also be referenced during installation. Some tools also provide a complete “balance of materials” report for the entire rack installation.

Attachment methods. Most rack manufacturers have an attachment method and compatible flashing designed for asphalt shingles—but not all flashings are created equal. Small, thin flashings can bend easily when lag-screwed into the framing, creating a concave shape that encourages water to run toward the penetration. The best flashings are wide, not too flexible, and elevate the mounting hardware above the roof with a raised base so that water flows around the penetration.

If coordinating the PV installation with installing a new roof, consider preinstalling posts and suitable flashing before the roofing material is laid down. Post-and-base installations are compatible with several different shingle types, and most roofers are familiar with flashing these, since they are similar to working around a vent pipe. The posts also allow for later reroofing while leaving all the rack attachments in place, which maintains the integrity of the roof framing and the lag-screwed posts.

Materials. Rack systems should be suitable for the environment in which they are installed, which means most are made from anodized aluminum with stainless steel hardware. However, as manufacturers try to be cost-competitive, more coated steel products are entering the market. Few, if any, of these steel racks have been in the field for the length of their warranty, so only time will tell how well this material will perform over the long term.

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