Whether roof-, ground-, or pole-mounted, PV rack and its associated parts, such as structural attachments, grounding, and wire management systems, are available in hundreds of styles and varieties. There has been a recent explosion of companies that offer PV racks.
Just as when you’re selecting PV modules or inverters, cost shouldn’t be the primary factor in choosing PV racks. Company sustainability and longevity, product warranties, aesthetics, the level of customer and engineering support, and Code-compliance—all of these factors should play a role in deciding what rack system to rely on.
No two solar sites are precisely alike; both the built and natural environments affect an installation’s specifics. A thorough site survey quantifies these factors, and a quality system requires tailoring the design to the site specifics (see “Solar Survey” and “Optimal PV” in HP130). Working with PV rack companies can take much of the guesswork out of the process, as they will provide engineered designs to meet wind uplift forces, snow load, and soil or roofing material types.
Because of space limitations, ground-level shading, and the excavating and trenching required for pole and ground mounts, the least expensive and most frequent location for PV arrays is on a roof. (For a general survey of rack types, see “Rack and Stack” in HP124).
Roofs can be classified as either low- or steep-sloped—low slope generally means a roof with a pitch of less than 3:12 (less than 14°). Low-sloped roofs are often mistakenly referred to as flat roofs, but no roof is ever really flat, as a pitch is needed for shedding water. Even a roof that appears flat will have a pitch of at least 0.5:12.
On steep-sloped roofs, modules are almost always flush-mounted—mounted parallel to the roof plane. The most common technique for flush-mounting steep roof arrays is “top-down mounting.” Anodized aluminum rails are used to support modules, and stainless steel or aluminum compression clips hold the modules onto the rails, usually with a bolt and nut captured by slots in the rails. This speeds up installation, eliminating bolting through the mounting holes on the back of module frames as was once common. Now, installation is easily accomplished with the modules in position on the rails from above—thus, the description “top-down” (see “Modern PV Roof Mounting” in HP137).
Recent design improvements in top-down mounting decrease materials and reduce labor. They include automatic grounding (module bonding via the mounting clips—see the “New Criteria, Listings & Techniques” sidebar) and one-tool installation (all of the bolts have the same size head).
SnapNrack, a pitched-roof mounting system developed by solar installers, was designed so one wrench fits all bolts. Snap-in nuts attach standoffs and top-down clips, the rails are height-adjustable, and there’s a built-in channel for wire management. In addition, both the mid- and end-clips have a universal design, meaning that regardless of module-frame dimensions, a single clip works with any module and the clips don’t have to be specified in advance.
Structural attachments from the array to the roof are a critical part of the installation. The attachment type and method will vary based on the roofing type (shingle, metal, tile, etc.) and with the roof’s structural design (wood trusses, structural insulated panels, metal purlins, etc.).
Preventing roof leaks and meeting building codes for live and dead loads (including wind uplift, rack and array weight, and snow loads) are primary concerns. A properly installed array will meet these concerns and maintain the roof warranty. In nearly all installations, every roof penetration needs to be flashed for waterproofing. On a composition (asphalt) shingle roof, the metal flashing fits underneath higher rows of shingles, so water runs over the top of the flashing and around the roof penetration. For years, many installers relied solely on sealant for penetrations, but new structural attachments make installing flashed penetrations simple and quick.
Quick Mount PV manufactures flashing-integrated brackets for a wide range of racks and roofing materials. Their newest products, the Quick Hook curved and flat tile mounts, are the first flashed tile hooks available. Quick Mount PV has also partnered with Schletter to create the Gator Mount—an aluminum-flashed mounting block that attaches to a composition roof and to the Schletter Gator Clamp via a single hanger bolt. Other companies offering flashing-integrated mounts include EcoFasten, SnapNrack, ThomsonTech, Unirac, and Zep Solar.Whether roof-, ground-, or pole-mounted, PV rack and its associated parts, such as structural attachments, grounding, and wire management systems, are available in hundreds of styles and varieties. There has been a recent explosion of companies that offer PV racks.