Single pole-mounted arrays have the same basic advantages of ground-mounted systems, plus they can be more easily adjusted for increasing peak-sun hours captured seasonally and to shed snow. Compared to a ground mount, a pole mount releases more ground area for other uses. Tall pole mounts not only keep wires running along the backside of the array from being “readily accessible” (if the wiring is above the ground by 8 feet or more), but also could reduce theft potential. However, since tall pole mounts cannot be reached from the ground, they also require a tall ladder for array maintenance and for adjusting tilt angle.
A ground mount uses multiple, lighter-weight supports, but a single pole mount has one large steel support. Depending on the ground clearance and array size (along with wind loads and soil type), that pole can be a heavy beast, commonly 6 or 8 inches in diameter and either schedule 40 or 80 steel pipe. For example, a specific design for a 3-kilowatt array with 8 feet of clearance to the bottom edge of the modules (at a winter tilt of 55°) will require the pole to be almost 9 feet in the ground. That’s a total pole length of 22 feet of 8-inch, schedule 80 galvanized steel pipe, which will require a 9-foot-deep (3-foot-wide) hole and approximately 2.3 yards of concrete. Installing the modules on this top-of-pole mount also requires scaffolding or a scissor lift, or for the completed array to be lifted in place by a crane.
Roof-mounted arrays take advantage of space that is otherwise wasted, leaving the yard for other uses. They are less expensive to install, since no excavation or concrete work is required, and they naturally remove ready access to array wiring. (Wire management is still required; see “Array Wire Management” in HP154.)
But roof installations often take place on high and sometimes steep roofs (as does any future array maintenance) and fall arrest systems (i.e., a body harness attached to a lifeline cable and an anchor) should be used. All roof penetrations must be flashed and sealed. The combined weight of the modules and racks, along with wind loading on the building, must be appropriately engineered. Flush-mounted arrays, even with the recommended 4- to 6-inch spacing above the roof, will have less cooling airflow than a ground- or pole-mounted array, and array orientation and tilt is often less than optimal and not adjustable.
Roof mounting space is usually limited, constraining the array’s size. The array design must accommodate obstructions, such as vents, chimneys, and skylights. Often, setbacks around the array are required for firefighter access, pathways, and smoke ventilation. For example, California’s fire guidelines call for 3-foot setbacks from the top of the array to the ridge and along one (or both, depending on the roof type) sides of the array.
Working with a roofing contractor is a good idea, as they can properly secure attachments without compromising the roof structure, are familiar with the various code requirements for roofs, and can warranty the penetrations they work on. They can also evaluate the remaining life of the roof—an important consideration before installing a 20- to 25-year PV system on it.
Justine Sanchez is a Home Power technical editor and an instructor for Solar Energy International. She is certified by ISPQ as a PV Affiliated Master Trainer. As her house hunt continues, she’s contemplating potential PV sites and mounting methods.