Metal Architecture Home
Features

Is Your Roof Solar Ready?

Here are a few things to consider and prepare for before installing solar

More and more buildings in America are realizing the benefits of solar photovoltaic (PV) systems. According to the "U.S. Solar Market Insight" a quarterly publication of Washington, D.C.-based Solar Energy Industries Association (SEIA) and GTM Research, the United States began 2012 with the second highest quarter for PV installations ever. More than 18,000 photovoltaic systems totaling 506 megawatts came online in the first three months of the year. Reduced electricity bills, solar rebates, tax credits and net metering are the contributors to this solar boom.

MCN_ProdFeature_Sep12_4What can be done to a metal roof to prep it for successful PV installation? Not a lot is really necessary. One reason is metal roofs are the most compatible roof format for PV installation. A modern standing seam roof system is easier, faster and less expensive to mount solar on than any other roof type. Not only can it be "direct-attached" and penetration-free using seam clamps and PV kits, but metal is the only commercial roof type that offers a service life in excess of the solar PV.

"All other commercial roof types will require replacement before PV life expires-a very expensive proposition when the roof is covered with a solar array," says Jerry Heininger, environmental products coordinator for Englert Inc., Perth Amboy, N.J. "In the residential market, slate and clay tile may offer similar service lives to standing seam, but the array cannot be installed without some roof disassembly and/or penetration of the roof. So the preferred 'solar ready' roof is clearly standing seam metal. This advantage is so compelling that often asphalt shingles are replaced with standing seam prior to solar installation."

Before installation

Before installation, every PV project begins with a site and roof evaluation to determine factors such as solar orientation, wind loads, roof warranties, useful life remaining in the roof system, usable roof area and location of the building's electrical service. A review of a building's energy consumption will show how much energy can be saved. Prior to installation, the correct permits and a plan check can finalize an installation plan that includes the number of panels, positioning and the overall PV design.

"Prior to any solar installation, we will determine the roof type, age, warranties remaining, manufacturer and condition," says Kate Riedo, solar development manager at Tecta America Co., Rosemont, Ill. "We recommend that solar be installed on roofs with at least 15 years of life remaining. If necessary, Tecta will do repairs/replacements to bring the roof up to 15 years. [We] always follow the original roof manufacturer's guidelines for installing solar PV and file the necessary warranty continuation paperwork on the customers' behalf."

MCN_ProdFeature_Sep12_3The roof should be walked and a report prepared showing any deficiencies such as cracks, open seams, ponding water or punctures. "This is especially true if the roof is older than 10 years," says Monique Hanis, spokesperson at SEIA. "Prior to installation, open seams should be sealed and any damage repaired in areas within the solar footprint to maintain the roof's integrity. Inspect the roof for sags and other abnormalities." A sag or deep depression may indicate a structural weakness in the support system that may require correction.

"Tecta hires an outside structural engineer to evaluate the roof and determine the excess load capacity," Riedo says. "Prior to any work on the site, Tecta will develop a site specific safety plan and set up appropriate safety measures including safety flags, OSHA-approved cable systems and/or guard rails. Tecta also designates a staging area(s) on the roof, keeping safety and weight considerations in mind."

Load limitations

A structural analysis must be performed on a metal roof to ensure its load limitations will not be exceeded. Crystalline solar modules weigh about 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 fixtures, 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-engineered 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 accommodate a PV system of any appreciable size. In the northern hemisphere, southern exposure to the sun is ideal to harness its full force. However, panels can be oriented to the southeast or southwest without substantially decreasing performance.

Exposure to the panels should be free of obstructions 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.

MCN_ProdFeature_Sep12_5

PV system sizes are usually referenced in kilowatts 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 fit 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. How large a system will satisfy a significant portion of the occupant's power demand without waste and payback his investment in the shortest time possible? We have seen high-energy-use commercial facilities where every square foot of usable roof space was employed, and other cases where only 20 or 30 percent of available space was used."

PV installation and preparation is easier for large, flat, unadorned roof areas. Roof vents, antennas, satellite dishes or any other roof fitting may need to be relocated. "If the roof area is cut up, there is typically more installation time needed," says Alan E. Burnett, PE, associate at Gales Associates Inc. in Mountain View, Calif. "Open roofs with low slopes and simple shapes like a rectangular outline provide more efficient installation. Access and staging are typically easier for low‐slope roofs."

High-sloped roofs will typically require a penetrating PV racking system that meets wind and snow requirements of the loads for its region. To determine wind loads, Hanis recommends checking local building and safety departments for specific requirements. International building code wind speed charts can determine basic wind speeds for U.S. locations.

MCN_ProdFeature_Sep12-2

[sidebar]

Solar prep: new versus existing roofs

Solar panel installation preparation is different for existing building roofs than it is for new building roofs. For existing roofs, the contractor should inspect the roof to confirm that existing features will not be affected by the new photovoltaics.

Also, the contractor should verify that the remaining roof service life is similar to the PV service life. "If the PV system will be mounted to the roof structure, the penetrations should be flashed in a watertight manner and the roof warranty should not be compromised, if the roof is under warranty," says Alan E. Burnett, PE, associate, Gales Associates Inc., Mountain View, Calif. "If under warranty, the contractor should ensure the roof manufacturer is notified and approves of the roof repairs for warranty compliance."

For new building roofs, the PV layout needs to be coordinated with other roof‐mounted items such as mechanical units and electrical conduits. If the PV will be connected to the roof structure, the curbs or stanchions should be installed prior to the roof installation to avoid having to repair the recently‐installed roof. "These PV support items will need to be 8-inch minimum above the finished roof surface and should not obstruct drainage," Burnett says. "For some projects, the PV system can be a separate design‐build item and the contractor will need to coordinate his work with the PV designer and contractor."