What to know about installing solar on metal rooftops

By Billy Ludt | August 23, 2023

Every type of rooftop has its own quirks that contractors must consider when installing a solar project. Metal roofs come in a variety of profiles and material makeup that require specialized mounts to match them, but installing solar on these specialized roofs doesn’t have to be difficult.

A solar contractor securing modules to a standing seam metal roof. S-5!

Metal rooftops are a common covering choice for commercial buildings with a top built at a slight pitch, and they’re growing in popularity in the residential market too. The Dodge Construction Network, a construction industry analyst, released a report claiming residential metal roof adoption increased from 12% in 2019 to 17% in 2021 in the United States.

Perhaps metal rooftops are a little louder during a hailstorm, but their durability contributes to a 70-year lifespan. Meanwhile, asphalt shingle roofs have an even shorter lifespan (15-30 years) than solar panels (25+ years).

“The metal roof is the only roof that will outlast the solar. You install solar on any other roof type — TPO, PVC, EPDM — if the roof is new when you’re installing the solar, it might last 15 or 20 years,” said Rob Haddock, CEO and founder of S-5!, a metal roof attachment manufacturer. “You’re going to have to dismantle the solar to replace the roof and that just shatters the financial pro forma of the solar.”

Installing metal is more expensive than a composite shingle roof but could make more economic sense for a building in the long-term. The three types of metal roof coverings are corrugated, standing-seam and stone-coated steel:

Mounting to metal

Each of these roof types require different mounting techniques for solar projects. Mounting solar panels to corrugated roofing most closely resembles installing on comp shingle, since securing through penetrations is still necessary. On a corrugated roof, a lag bolt is driven through the sides of a trapezoid or elevated portion of the roof, or the mount is secured straight down to the building structure.

Solar mounts for corrugated roofs are designed to follow their contours. S-5! manufactures a line of attachments for corrugated roofs that use fasteners with gaskets to waterproof each roof penetration.

Solar mounts for corrugated roofs can attach to the raised portion of the roof. S-5!

For a standing-seam roof, penetrations are rarely necessary. Solar mounts secure to the top of the seams using angled fasteners that drive into the surface of the vertical metal plane to create indents that hold a mount in place. These raised seams can also act as structural rails often seen on pitched-roof solar projects.

“You basically have rails on the roof that you can just grab, clip around and install,” said Mark Gies, director of product management at S-5! “You don’t need as much hardware because the hardware is kind of integral in the roof.”

Stone-coated steel roofing resembles clay tiles not only in shape but also in how solar is installed onto it. On a tile roof, installers must either remove a section of tiles or cut through the tile to reach the underlayment and attach a hook to the roof surface that protrudes through the gap between tiles.

“Usually, they’ll grind or chip out the tile material so that it can rest on top of the other tile like it’s supposed to, and the hook can come through it,” said Mike Wiener, marketing manager at QuickBOLT, a solar mounting manufacturer. “You don’t really worry about that with stone-coated steel because it’s metal and it’s overlapping. By design, it’s meant to have some wiggle room in between.”

With stone-coated steel, installers can bend and lift that metal shingle without removing or breaking it, and similarly install a hook that extends past the shingle. QuickBOLT recently developed roof hooks that are designed for stone-coated steel roofs. The hook is shaped to work around the wooden batten that each row of stone-coated steel roofing is attached to.

Chemical and electrical considerations

Metal rooftops are primarily composed of steel, aluminum or copper. At a chemical level, there are metals that are incompatible when they touch each other, causing what is called a galvanic reaction that can spur corrosion or oxidation. For example, crossing steel or copper with aluminum can cause a galvanic reaction. Luckily, steel rooftops are sealed, so installers can use aluminum mounts, while there are brass mounts on the market that are compatible with copper.

“Aluminum will pock up, rust away and disappear,” Gies said. “Uncoated steel, just the environment will rust that. Whereas, you can have bare aluminum, because aluminum protects itself with an anodization layer.”

Running wires on a metal rooftop solar project shares most of the same tenets as wiring on other roof types. However, it’s more critical to keep wires from touching metal roofs, Gies said.

QuickBOLT developed a roof hook that works with the profile of stone coated steel roofing. QuickBOLT

Wiring a rail-based system follows the same steps as other roof types, where installers can use the rail to clip wires or as a channel for wire runs. With rail-less projects on standing-seam roofs, installers must clip strings to the module frames. Gies recommends determining string routes and clipping wires before solar modules even reach the roof.

“There’s more emphasis on preparation and the design of where the string’s jumpers are when you do rail-less on a metal roof,” he said. “It’s important to prepare the module ahead of time — that everything’s clipped and out of the way and nothing’s hanging. That’s good practice anyway, because then when you’re on the roof, installation is so much easier.”

Running conduit lines functions the same on metal rooftops as well. If wires are running inside, there’s a single penetration at the top of a roof with a junction box that runs wiring to the designated load point indoors. Or if inverters are placed on the outside wall of the building, the wires can be directed there instead.

Despite metal being a conductive material, grounding a solar project on a metal roof is the same as any other style on the market.

“The roof is kind of out-of-scope,” Gies said. “You still have to bond and ground that system like you normally would, no matter if you’re on asphalt or whatever. Just do that the same way and don’t think about the fact that you’re on a metal roof.”

Installer perspective

For building owners, the appeal of a metal roof is a long operating lifespan with a material built to withstand the elements. Solar installers building projects on these roofs have some material advantages over comp shingle and tile, but can encounter conditional risks.

The granules in comp shingles and even stone-coated steel make it easier to walk and grip on these roofs. Corrugated and standing-seam roofs are smoother, and with rain or snow can be slick. That slip risk increases as a roof’s pitch gets steeper. Proper rooftop fall protection and anchor systems should be used when working on these special roofs.

Metal is also an inherently heavier material than comp shingle and, especially in commercial scenarios with larger roof spans, buildings cannot always handle additional weight atop them.

“That’s part of the challenge in that these steel buildings, they’re not built to take a lot of weight up top sometimes,” said Alex Deeter, senior sales and marketing engineer at SunGreen Systems, a commercial solar contractor from Pasadena, California. “So, depending on when it was constructed or what purpose it was constructed for, it’s figuring out what’s the lightest weight solution or how can we spread it out across the building.”

Despite these potential challenges, installers will no doubt encounter more solar projects on metal rooftops as more people choose this material for its durability and lifespan. With its unique considerations in mind, contractors can hone their installation practices like sharpened steel.