3 pounds per square foot (psf). A metal roof must be capable of taking this added collateral load. Most pre-manufactured trusses are designed with 7 to 9 psf of collateral load designed into the truss (in addition to wind and snow loads). “After drywall and light fi xtures, there is still a
range of 4 to 5 psf collateral load that can be added,” Heininger says. “With most conventionally framed (bar joist) commercial structures there is also some ‘wiggle room’ in structural design. With pre-engi- neered metal buildings this may or not be the case. In any event, on existing construction it is a good idea to check out the structural capacity with a licensed engineer or the original manufacturer of the structural
system. On new construction, be sure the structural design can accommodate the required 3 psf.”
Roof size matters
Roof size must be considered during PV solar evaluation and preparation. Roofs need a certain amount of south-facing roof space to accommo- date a PV system of any appreciable size. In the northern hemisphere, southern exposure to the sun is ideal to harness its full force. However, pan- els can be oriented to the southeast or southwest without substantially decreasing performance. Exposure to the panels should be free of ob- structions and shading. A shade analysis examining
roof equipment, overhanging trees or surrounding buildings can determine if a roof is suitable for a productive PV array. PV system sizes are usually referenced in kilo-
watts like 1,000 watts of rated DC power. A typical crystalline module is about 240 watts—although they vary—and takes about 18 square feet of gross roof space, so approximate about 80 square feet of gross “usable” roof area for each kilowatt of system size. One kilowatt of panels can be fi t on about 100 square feet. Systems are usually sized in accordance with the
power consumed by the occupant. “The objective is normally driven by economics,” says Heininger.
This 541.8-kilowatt-peak rooftop solar PV system for Shop Solar LLC, Whitinsville, Mass., is projected to generate 627-megawatt hours of energy in its fi rst year of operation, equivalent to powering 86 average Massachusetts’s homes for a year. (Image courtesy of Tecta America Corp.)