One of the many beautiful things about photovoltaic technology is the ability to transform underutilized space—the residential roof—into a clean, renewable, local power production plant. Today’s roof-mounting systems can make installations faster and more streamlined than before.
Whether because of limited ground space, better solar access with less shading from structures and trees, or lower installed cost, roofs are the default location for residential solar-electric systems. While the vagaries of residential construction lead to roofs of myriad shapes and sizes, most roofs are designed with a pitch (slope). For the PV system and installer, the pitch is important for ease of installation and for system efficiency, since modules produce greater power the more perpendicular they are to the sun’s rays.
Manufacturers' mounting systems for pitched roofs tend to follow a top-down rail mounting scheme. Aluminum or stainless steel L-feet or standoff posts, usually lag-bolted into a rafter, support an aluminum rail at regular intervals. Two rails run under each row of PV modules, and the modules are secured to the rail by means of top-mounted clips. These are bolted down to the rail between modules, with one clip connecting the sides of two adjacent modules.
Parts & Pieces: Feet
Mount feet come in many varieties, and this is one of the places where great innovation has been happening in the industry. Examples include Quick Mount PV’s Composition Mount, Thompson Technology’s Flat Jack, and Direct Power & Water’s (DP&W) Easy Feet. The mount you choose to hold your rail depends on a few factors: roof type, climate characteristics, price, and personal preference.
L-feet are lag-screwed to rafters through the existing roofing layers and depend solely on sealant to remain watertight during the life of the roof. PV mounting manufacturers don’t recommend a brand of sealant, but refer you to roofing manufacturers. While they are the fastest, least-expensive footing option, L-foot installations may void roof warranties when not flashed (see “Protecting the Roof with Flashing” sidebar). Some types of sealant are designed to work only with certain roofing materials—and will not work properly with others. Leaking mount feet may cause damage to the underlying roof structure, which could lead to mold problems and, in some cases, litigation. When in doubt, consult a professional roofer.
Flashed standoffs resist water infiltration more reliably than a sealed L-foot. Standoffs, or stanchions, are specialized bolts or posts with a solid base that is lag-screwed into the roof rafters. They are the best choice when working with a new roof installation where the decking is exposed and accessible. Roofers can flash around them just like any other vent pipe or duct as the roofing material is installed. Standoffs can also be retrofitted onto an existing roof, although this is a slightly more time-consuming process than using L-feet. Some newer standoffs, such as the Flat Jack and Quick Mount PV’s Composition Mount, come with flashing integrated into the mount, and can be slid under existing asphalt shingles, shakes, and tiles. Other standoffs rely on roofing industry standards like Oatey flashings, which slide over the standoff post.
With the exception of only a few products on the market, rail supports attach to rafters with stainless-steel lag screws. The screws must be sized based on uplift point loads and wood strength charts like those provided by the American Wood Council (see Access). Racking manufacturers should provide engineering recommendations. Exceptions to lagging into a rafter include the Easy Feet and SunPower’s Smart Mount. Easy Feet come with butyl mastic backing and are held down with five screws, which are secured through the roofing material into the decking or sheathing. These are good options in cases where the rafters are inaccessible (as with standing-seam metal roofs, when rafters are inaccessibly located under a V), or when retrofitting a structural insulated panel (SIP) roof, which has no rafters. Note that Easy Feet depend on structurally sound roof decking and should not be used on roofs with compromised decking.
Parts & Pieces: Rails
Nearly every rail on the market is made of extruded aluminum. In the extrusion process, hot aluminum is pushed through a die to create the desired profile. Many manufacturers use anodizing, which creates a hard, nonreactive surface film to improve corrosion- and wear-resistance. Generally, an anodized rail retains its silver color , though other anodized rail colors (such as bronze for SunEarth’s CompRail system) may be available. The other commonly available rail color is black, which usually means a protective powder coating has been applied. Powder coating is a dry coating that’s cured under heat to form a “skin.” It can create a hard finish tougher than conventional paint.
Some manufacturers sell both light and heavy rail. Heavy rail is thicker and stronger, and can span longer distances between supports and carry heavier loads. Light rail is considered adequate for the majority of residential pitched roofs. Rail can typically be ordered in lengths up to 27 feet, and splice kits are used to tie lengths together. Rail can run horizontally or vertically on a roof.
Spacing of rail supports, referred to as “rail span” or “foot spacing,” depends upon rafter spacing (commonly 16 or 24 inches on center), manufacturer’s recommendations, and loading estimates, such as dead load, wind load, and snow load (see “Dealing with the Forces of Nature,” at right). Each manufacturer has its own recommendations for maximum recommended rail span wind speeds—some to 90 mph, some to 125 mph. Typically, rail span will vary between 48 and 96 inches when using light rail.
Another facet of rail span is the cantilever or rail overhang—the distance between the rail end and the first support. Manufacturer’s instructions will vary: For example, Unirac recommends no more than 25% of module width as an acceptable overhang. DP&W’s Power Rail allows an overhang of 32 inches for 125 mph wind load and 36 inches for 90 mph wind load. Of course, the spans and cantilever can be greater for heavy rail. Be sure to follow manufacturer’s recommendations.
Parts & Pieces: Clips & Bolts
Clips secure the modules to the rail. Two types of clips are used in top-down mounting: mid-clips, which fit between modules, and end-clips, for the outside edge of the end modules. Clips are either aluminum or stainless steel, and may be powder-coated or anodized. Because modules come in a variety of thicknesses, the clip height must match the module frame height.
Clips are attached to rails with stainless steel bolts and nuts. There are a few basic styles, some with bolt heads that are secured (captive) within the rail. One, such as DP&W offers, uses an oval head bolt that must be slid into position from one end of the rail. (Watch out for rail splices, which can stop your bolt!) Another, such as used with Unirac’s SolarMount, uses a T-head bolt, which can be placed in the rail at the appropriate spot, and then twisted to grip the inside of the rail. Care must be taken to make sure it engages properly. There are other approaches as well, such as Conergy’s mounting system, which uses specialized nuts (“Quickstones”) that can be slid down the channel and into position.
A few systems (such as SunEarth’s CompRail and Unirac’s SunFrame) forego mid- and end-clips. Instead, two rows of modules share a single, central rail between them. The modules sit on a shelf on either side of the rail, then are secured with a cap that bolts into the rail.
Beyond Composition Roofs
PV mounts for tile or wood shakes are available from Quick Mount PV and Professional Solar Products. Quick Mount PV’s mounts for curved or flat tile replace the tile itself, integrating the tile-shaped flashing and aluminum standoff into one bolt-together unit. The mount is attached to the decking with four 1/4-inch-diameter lag screws. Professional Solar Products’s Tile Trac takes a different approach by drilling, bolting, and sealing through the crown of the existing tile to the decking. The Tile Trac system lag-screws a metal track into the rafter, with a long bolt or standoff running up and through the crown of the sealed tile to hold the rail.
For installations on new seamless metal roofs, flashed standoffs can be used. However, retrofitting standoffs onto an existing roof is more challenging. On these roofs, securing a lag screw for an L-foot into a rafter can be difficult or impossible, as a raised metal V often lands over the rafter line and should not be squashed down by a mount. There are two possible solutions: adding spacers or sisters between the rafters to screw into or using a mount, such as DP&W’s Easy Feet, screwed into the existing sheathing. Spacers are boards in an attic nailed between and perpendicular to two rafters, and held tightly against the roof decking’s underside to provide a secure attachment point from above the roof. A sistered board is nailed against a rafter to extend the rafter’s width.
The rare metal shingle roof or raised profile (shingle-appearance) metal roof should be approached with caution. Regular flashing might work with some smaller metal shingles, but others are in large sheets that resist attempts at flashing. L-feet won’t usually work with this type of metal roof either, as the profile will get crimped. The best solutions are to mount standoffs and flashing during initial roofing or, for a retrofit, to remove the metal and reinstall after standoffs are placed.
Adjustable Array Angle. Using adjustable legs beneath the rail to optimize the tilt of a roof-mounted array is becoming less common, since the production gained is moderate (usually 1% to 5% for fixed-tilt arrays), and most homeowners don’t want to climb on the roof to make adjustments. Tilting an array can create a number of difficulties, including more complex wind loading, more hardware needed, more difficult installation, and a perceived lack of aesthetics. Using the National Renewable Energy Laboratory’s PVWatts calculator (see Access) to model power production at different tilt angles can be a useful cost–benefit analysis tool (see “PV Performance at Different Pitches,” below).
What is easily configurable in flush-mounted roof arrays on rails is the height of the modules above the roof. L-feet and standoffs can be purchased in varying heights. The taller the standoffs, the more airflow beneath the modules, which results in cooler modules and more power output. On the minus side, the farther an array is from the roof, the more it will stand out.
Grounding. Rail and modules must be grounded using UL- or ETL-listed equipment. Wiley Electronics’ WEEB (washer, electrical equipment bond) and Unirac’s grounding clips are listed to meet UL Standard 467. These clips lay in the rail between module corners, and are faster and less expensive to install than the old standby: the Ilsco lay-in lug. The Ilsco lug is made of tin-plated solid copper, with a stainless steel screw to hold the wire. It screws into the dedicated ground spot on the module frame, and requires a solid copper wire run from lug to lug. (For more on grounding, see Code Corner in HP128 and Independent Power Providers in HP121.)
Pitched Roof Performance
Along with the growth in the solar industry have come many advances in PV installation methods. Manufacturers and engineers are trailblazing options that make installations more durable, dependable, and cost-effective. To uphold our part of the bargain as end users, we must install components correctly, and use them only for the jobs they are meant to do. Being part of a vibrant, burgeoning industry means taking responsibility to continually learn on the job and being flexible enough to adapt to improvements in methods when they come our way.