The design of a solar array on a shingled or tiled pitched roof is influenced by whether the system will be attached to the roof’s rafters or decking, and there isn’t exactly a straight answer on which is best. It really depends on the entire building’s structural support and the mounting system. The go-to option, said S-5! CEO Rob Haddock, is to attach to the rafters.
“When attaching to almost any roof that is supported by wood rafters or trusses, and then covered with plywood or OSB deck, the easiest and safest approach is to attach rail or racking anchorages to the truss/rafter, but attach to every one, not every second or third one, thus distributing loads evenly over all trusses,” he said. “Of course, there are other considerations that could save a lot of money and potential failures.”
A roof surface transfers loads onto a building’s structure. Positive loads push down on the roof (snow loads) and negative loads pull up (wind loads). When a solar system is mounted to a roof, more components are added to the load chain. Positive and negative loads are now carried through the solar system and mounting components to the building structure. Attaching solar to the rafters eliminates a few middle steps of load transfers and gets right to the core building components. But, that isn’t to say deck-attached systems are an inferior choice.
“Attachment [only] to the deck is convenient for many reasons, not the least of which is that we don’t have to find the truss for our attachment,” Haddock said. “When attaching to a truss or rafter, not only do we have to find something that we cannot see, but we have to find the center of it to run a lag into, we have to predrill it so that it doesn’t split, and so on. When we attach to deck only, we don’t have to do any of that, but we do have to be sure that the attachment is adequate to withstand the point loads that we are introducing to it.”
Mounting manufacturer Preformed Line Products leaves the option of rafter- or deck-attached systems up to the installer, although global market manager John Markiewicz prefers using the deck.
“It’s better to offer whatever the customer prefers, and they can choose,” he said. “But direct-to-decking has so many more benefits: simplified layout, getting around roof obstructions, hitting the module where it needs to be mounted.”
PLP product manager Dustin Graef further explained the benefits of deck-attached systems.
“When attaching to rafters, you have the possibility of splintering the rafters. You also don’t know for sure if you are in the rafter,” he said. “It can end up shifting the entire system in either direction if you have to hit that rafter. With our POWER DISK [rail-less system], you can put it anywhere you want. If you have an obstruction like a vent, you move it over a foot and it doesn’t affect the layout of the array.”
Fifteen years ago, PLP released its Easy Foot mounting bracket that could be installed on the decking or the rafters. Markiewicz said when PLP developed the rail-less POWER DISK system, the company again went with the “mount anywhere” option.
“When we came out with the rail-less, we decided to use the same concept that has been proven for 15 years that lets you attach anywhere,” he said. “People are worried about the strength of the decking, but we have not experienced any issues with the Easy Foot for 15 years.”
Ecolibrium Solar is firmly on the other side. The company’s EcoX rail-less system is primarily attached to the rafters, but it does have the option for deck mounting. Jonah Coles, Ecolibrium’s product solutions manager, said the company suggests rafter attachment for the most secure connections and resistance to wind and snow loads. Coles said rafter-attached systems connect directly to the building’s structure, whereas deck-attached systems rely on the decking’s connection to the structure, and that could be unknown.
“Rafter connections are generally more efficient from a design approval perspective,” he said. “Taking conservative assumptions, it is relatively straightforward to check that a rafter attachment will support the load of solar on a roof. It is more challenging to confirm that the decking on a given site has the needed strength for a deck-mounted array.”
Ecolibirium offers secure deck-mounting options when the decking thickness, material, condition and connection to the structure can be easily verified, often only on new building construction.
“Typically, a rafter-attachment is stronger than a deck-attachment which means that fewer attachments are required,” Coles said. “Our EcoX base is about three-times stronger when attached to a rafter, compared to attachment to OSB decking.”
Whether deck- or rafter-attached, sealing roof penetrations is still important. Flashing materials are the same in either situation, but the type of fasteners or screws differ. PLP includes 1 1/2-in. deck screws for deck-attached systems and 2 1/2-in. lag screws for rafter-attached systems.
“When racking companies sell their mounting feet they don’t necessarily include their fastener,” Markiewicz said. “We provide a specialty screw for direct-to-decking. We didn’t leave it to chance. For rafter-attached systems, we do also include the right fastener.”
Attaching a solar array to the rafters is the most straightforward, trusted option, but attaching to the decking isn’t wrong. It just takes a little more engineering to ensure a successful installation.
“In either case (truss/rafter or deck-only), the attachment and transfer of point load through the load chain and into the structure is a very critical step that should never be overlooked. It should not be guessed at, but tested and engineered to prove adequacy,” Haddock said.
Of course, if one wants to avoid this debate entirely, install solar on a metal roof. When using the correct engineered components, a solar array is attached to either the seams (standing-seam metal roof) or ribs (face-attached metal roof)–never to the rafters and rarely to the deck.