The structural integrity of a solar array is only as good as its mounts, and that becomes especially imperative when installing solar carports or canopies. The material composition of a canopy structure far outweighs that of arrays with similar wattage in rooftop and typical ground-mount applications.
The type of foundation used on a carport depends on whether it’s mounted to an existing parking deck structure or in the ground, most often in parking lots. The popular solution in ground-mount carport applications is concrete piers, dug at varying depths depending on soil quality. Parking deck projects, on the other hand, often don’t even have to move dirt. Instead, solar canopies are fastened to anchors or saddles attached directly to the existing structure.
“Once you get out of the ground, carports are easy,” said Brad Fey, senior project manager at RBI Solar. “It’s basically a giant erector set. You’re putting steel together, it’s all pre-punched, everything bolts together, it’s pretty simple. It’s the foundations that are the hardest part of any carport project.”
Establishing a foundation
Carport system design hinges on proper foundation placement, and that relies on accurate geotechnical reports in ground-mount projects. Hiring a proper geotechnical surveyor will give project planners information on soil quality, underground utility locations and watershed heights, all necessary factors in selecting and placing carport foundations.
“The more you can preplan and get that information up front to know where you can place foundations, what type of foundation is going to work, what type of soil conditions you’re getting into, it’s really going to go a long way to figure out what foundations work best for that site,” Fey said.
RBI Solar designs and fabricates carports on a per-project basis working from five base models with differing panel layouts. The standard concrete pier footing on ground-mount carport projects is cured on-site, poured in a cored hole around rebar, with the footing extending above ground level, providing the structure with stability to withstand vehicle collision. The depth and foundation type depends on soil quality.
Concrete piers are the standard, but there are other options like spread footing, a concrete foundation with a wider bottom segment for when a structure needs extra stability; helical piles, which are metal pipes with helix attachments to resist upward tension when drilled into the ground; or proprietary solutions, like Quest Renewables’ hybrid pier.
Then, placement depends on the canopy structure brand, but a typical carport has foundations about every 30 ft, with holes dug between 10- to 14-ft deep in average soil conditions.
“With everything else, foundations are basically the key to any canopy project, so there’s a lot of nuances to put them in there. It’s also making sure that you’re laid out correctly, making sure you have the right spacing, because all of your steel comes pre-punched,” Fey said. “So, if you have a foundation that’s out of place, if it’s off by two inches or it’s moved in the wrong direction, you don’t have anchor rods that are in correctly, now you’re slowing down the steel install process too.”
The equipment required to core holes for foundations also depends on the soil conditions. In normal soil conditions, it’s common to use a pressure digger, a truck-mounted auger. For sandy or loose soils, temporary casing can be used to maintain the shape of the holes being cored. Hammer drills can break up the rocks present in soil. Sump, suction or any other number of pumps are needed to dewater water tables. And concrete saws or carbine teeth attachments for augers can be used to cut through the parking surface.
Parking garage projects, and to a lesser degree parking lots, will also require load bearing approvals on the carport itself and also the heavy equipment being used to install them. System designers aim to place carport foundations to prevent any loss of parking spots. Ideal placement is in grassy medians or the corner where four parking spaces meet, still leaving enough for vehicles room to park.
The same concern of maintaining the number of parking spaces also falls on designing parking deck carports but comes with a whole different set of considerations depending on the condition of the existing structure.
“If the deck is not new, then you’ve got to find plans for how it was built and you have to go and do some forensics — scanning for where rebar is in the columns, how much is there and what the capacity for the deck is,” said Finn Findley, CEO of Quest Renewables.
Quest Renewables creates saddle attachments as the foundation for carport projects on parking decks. The saddle is placed over a column or wall and bolted in place. The carport racking attaches to an extension placed atop the saddle.
“It’s very seldom that someone designs a parking deck to accommodate a canopy. When you’re working with solar, you need to have a system that’s able to adapt to a really wide range of widths and columns,” Findley said. “Loads fall between columns, and it’s a lot of work to get very efficient at solving that problem for parking decks.”
RBI Solar drills an epoxy anchor onto the existing garage structure and the canopy racking is attached to it. Adding anchors to the columns stiffens them, because they cannot handle the lateral force exerted by the canopy structure without reinforcement. Unlike structures with similar foundations found in these settings, like lighting, solar carports have greater potential for resistance from natural forces like wind uplift.
“That canopy wants to rotate on us — it wants to turn back and forth, so we’re putting a lot of forces down those foundations,” Fey said. “Imagine a giant pivot arm. You push at the top, the bottom wants to move out, so you put a lot of weight at the bottom of that arm to keep it from moving.”
Ensuring greater vertical stability with reinforcement from anchors and saddles will prevent the carport structure from bowing when deployed in parking decks.
The cost of a carport
Solar carport structures physically rely on the stability of their foundations. The price of a project can increase if it’s on improper footing.
“A lot of the cost is in contingencies — labor, foundations — those are really big ‘gotchas’ that can come in carports,” Findley said. “You’ll know what the system costs from a material standpoint. But when it comes to foundations, one of the things that happens is in the industry, there’s a habit of folks doing initial quotes. They undersize the foundations, then there’s a big adder that comes in.”
Quest Renewables tries to give customers safe estimates on what foundations could cost in the total project. The company was born from a Department of Energy-funded project at the Georgia Tech Research Institute, developing economically feasible solar carports. In that pursuit, one of Quest’s design traits is trying to minimize the number of required foundations in its solar canopies.
Varying soil reports could lead to different types and numbers of foundations and site preparation requirements, which increases project cost.
“In the long run, you’re just going to make your customer angry,” Findley said. “You need to be realistic about what the foundations are.”
That’s where companies like Foundation & Shoring Service come in. F&SS is a Maryland-based geotechnical contractor that installs foundations for a range of industries, including solar carports, and does follow-up on geotechnical reports.
Foundation work is commonly hired out to geotechnical contractors, while solar installers will handle erecting the carport itself. The geotechnical contractors will re-test a site and deliver another system design better suited to its conditions if needed — often at a lower cost.
“What happens is, especially when someone that’s a developer in the solar industry, a lot of this is economic risk factor because it’s an investment tool, almost like buying a commodity on a market,” said Chris DeBlauw, project engineer and manager with F&SS. “Inaccurate info leads to a design that might be OK or appropriate, but is by no means the most economical solution.”
Putting solar carports on proper footing might be the first piece in the construction process, but it’s arguably the most important. Project developers should take extra care when analyzing the soils and structures where they’re sited.