PV Rack Strategies

Intermediate

Inside this Article

Advance planning, accurate measuring, a good eye, and an extra pair of hands all contribute to a successful PV array installation.
Complex roof angles, obstructions, and shading all affect options for array position. (Note the screen under the array to keep out varmints.)
Laying out the array and rail position with a chalkline and tape measure.
Finding rafter position by tapping with a hammer. This is an acquired skill that professional installers quickly master.
Drilling a pilot hole to verify rafter position. If the drill punches through after going in less than an inch, you didn’t hit a rafter. Size your pilot hole to match the necessary hole size for your lag bolts (1/8 in. is common).
Leveling mounting rails with a string line.
On tile roofs, locating mounting-foot positions becomes trickier. Tile roofs are among the most challenging roof types to work with. Here, the tiles are pried up to allow a tile to be removed in the region where the rafter is believed to be running.
Once the tile is removed and you’ve confirmed your best bet for rafter location, it’s time to probe with a drill to verify.
After a rafter is verified, you can mount the roof attachment hardware (in this case, a standoff) with a lag screw. Even with flashed attachments, roof sealant is used underneath the standoff.
Finish the weatherproofing of the standoff by sliding flashing over it. Some installers take the extra step of trimming the tiles to fit back in tightly around the standoff, which is quick and easy with a cutting wheel, shown here.
Sighting along the edge is often enough to align PV modules accurately.
While two rails might each be straight, they may not lie on the same plane. This can result in an unsightly twist, commonly called “potato chipping.” This is easily avoided by visually checking rail alignment during the leveling process.
It’s often easiest to wire and ground PV modules as you go. Accessing the backs of modules after a full array is installed may be difficult.
This rack allows array wires to be tucked into channels within the mounting rail. This provides protection to the wires and a neat installation.
Some rack manufacturers provide end caps for rails and module clips that attach to the rail underneath the module, so rail ends are flush with the array.

THE PROS can make mounting modules to rooftops look easy, but a lot goes into making that array look like a million bucks on your home’s roof. Here, we look at what makes a rooftop installation go smoothly and quickly, ensuring years of trouble-free energy production—with no leaks.

Getting the Details

Roof type is a major determinant in the installation hardware required. Composition shingle roofs are most prevalent, but certain regions, such as California and the Southwest, have lots of other roof types, like tile, that are more challenging to work with. Make sure you consider the roof type when choosing your attachment methods.

Most systems on the market offer flashing for penetration points—which is always advisable for any region. You’ll sleep better at night knowing you installed flashings over all of those bolt holes in the roof.

Rafters typically run up-slope, but not always! Installation tends to be easiest if your rails run perpendicular to the rafters. Crawl into the attic to check, and pay close attention to how rafters are arranged—their size and spacing. In particular, look for any discrepancies in the spacing pattern. Lighter-duty roof construction (typically 2 by 4s at 24 inches on center or greater) will be more susceptible to point-loading issues, particularly in regions with snow-loading requirements, where there will be substantial limits on the spacing of mounting points (increasing their quantity).

Some jurisdictions require rafter attachments to be staggered. For example, for a pair of rails in a single row of an array, attachment points would have to alternate rafters to more evenly distribute the loading on the building. This also increases the quantity of attachment points due to the unavoidable duplication of attachments at the ends of rows.

In snowy regions, it may be wise to install snow fences or guards. Arrays that are installed over doors or walkways are prime candidates to create dangerous snowfall conditions below the array. Leaving room for a snow fence or guard may also affect your layout.

Wiring runs and electrical component placement should be considered when you’re designing your initial layout. Take note of where you will mount any J-boxes and how you will route conduit from the roof. Some rack systems have channels built into the rails for easy wire management, facilitating an easier installation.

Grounding can be simplified with products like Wiley Electronics’ WEEB or the rack manufacturer’s own version of these grounding washers. These can save time and cost over using copper wire and lugs for grounding on each module. However, these products are not accepted by all jurisdictions. Check into these details before committing to a grounding strategy, since some jurisdictions will only approve grounding solutions that are listed as approved in the manufacturer’s documentation.

Array height is variable with most rack systems. As a general rule, systems that are closer to the roof can be more aesthetically appealing, but more distance under the array assures better airflow and higher efficiency, since the cells are cooler. Having the module glass 6 to 8 inches above the roof surface is typical and offers a good balance of aesthetics and system performance.

Maintenance and code requirements must be followed carefully. To ensure firefighter safety, more jurisdictions are restricting installations close to roof edges and ridges, or requiring access pathways on larger arrays and installations. In some cases, access will be required or desired for other reasons, such as roof equipment or skylights. While these details can be frustrating to accommodate, they are much more easily accounted for early on in the process, before installation.

There is a wide offering of rack systems on the market, with various advantages to each. This article considers rail-mounting systems only. The industry standard, they are easy to install and level, and are the most versatile in accommodating the majority of modules on the market.

Roof Layout

Grease pens or chalk are great for marking the roof—they are cheap, visible, and temporary. Chalk the basic layout, starting with the outline of the array from your system design, marking corners. Also mark your rail lines under the modules with a different color to avoid confusion. With the system drawn on the roof, you can begin to identify mounting points where rails or structural members will intersect with rafters.

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