Rooftop solar is often treated as an afterthought. On a healthcare campus in Wisconsin, prefabricated concrete and thoughtful structural design made all the difference in creating a parking structure that was engineered to carry a state-record array before the first piece was cast, and the sequence made all the difference.
When a healthcare system commits to an ambitious renewable energy target, the surface that ultimately supports the solar array is often chosen long after the frame beneath it was designed. That sequence is backward. If you want a building to generate a meaningful share of its own power via solar power, the capacity to hold an array must be engineered into the structure from the outset, and few structures are better suited to the job than the parking garage.
At UW Health Eastpark Medical Center in Madison, Wisconsin, a five-level precast parking structure was topped with roughly 2,000 solar panels, the largest photovoltaic array in the state. The array generates approximately 1,260 megawatt-hours a year, close to 30 percent of the campus’s energy needs, and fully offsets the electricity used by the facility’s proton therapy center. This was not a retrofit. The structure was engineered to carry the array before the first component was cast. That kind of planning changes what an owner can expect from the building, and it matters for anyone weighing a similar investment.
The single decision that impacts the rest of the design is how the panels attach to the structure. On flat roofs, you generally have three options: rack-mounted, ballasted, or post-mounted. Each carries a different penalty.
Structures are designed to account for anticipated loads, typically expressed in pounds per square foot (psf). Rack-mounted systems are light, often adding 5 psf or less, but they penetrate the roof and restrict how you use the space beneath them. Ballasted systems avoid penetrations but replace them with weight, sometimes 20 to 30 psf of concrete counterweight, and they can slide in high wind or seismic events. Post-mounted systems have the biggest structural impact and, like rack systems, penetrate the roof and require an engineered structural interface, but they return the most usable space below the array, which is why they suit a parking canopy so well. Decide this early, because it sets the structure loads.
A rooftop array introduces three loads worth your attention: dead, snow, and wind.
Dead load is the easiest to plan for and usually the smallest, ranging from around 5 psf for rack systems to 10 to 15 psf for post-mounted ones. Snow is more subtle. In cold climates, panels act as obstructions that let drifts build, and sliding snow can accumulate in ways that exceed typical roof loading. In designing the array itself, that sliding effect is desirable, since it keeps the panels unobstructed through the winter.
Wind also plays a role when looking at panel loads. The array is angled to capture sunlight and can act like a sail. When wind hits the back of a sloped panel, it generates significant uplift, and that uplift determines how many anchorage points are needed or how much ballast it takes to hold the system down. Another loading consideration for larger arrays is thermal restraint. The support structure should be designed to allow for movement of the elements. Long, continuously anchored systems can restrain the shortening or expansion of those elements and overload the anchorage, leading to moisture penetration or system failures.
The top level of a parking garage is usually its least desirable real estate: uncovered, farthest from the entrance, and in cold climates sometimes closed seasonally to avoid snow removal. Those levels are usually left uncovered to hold down structural cost, even though much of the structural system already continues above the roof deck to form a vehicle crash barrier. Putting an array overhead turns that liability into an advantage. The result is covered, more comfortable parking, more usable spaces during winter closures, and a large, unobstructed platform for energy generation.
Parking structures also tend to be open-air, without an insulated roofing system to protect the top level. That removes most of the waterproofing and maintenance risk that comes with mounting panels over conditioned space. At Eastpark Medical Center, the array does double duty: it generates power and shades the top deck, improving comfort and extending the life of the driving surface.
Precast concrete directly addresses the wind problem. Its mass counteracts uplift without added ballast, and it offers flexibility in where to place anchorage points. For the Eastpark Medical Center’s garage, the array’s support trusses imposed significant lateral forces on the concrete walls and framing. Those loads were carried by high-capacity embed plates set at connection points around the perimeter and along the interior ramp walls. These were engineered into the design before fabrication, not configured in the field.
That last point is the catch. Prefabricated components are cast in a manufacturing facility and delivered finished, so a coordination miss is expensive to fix once on site. The projects that go well are the ones where structural, energy, and construction teams coordinate early and together, with everyone aware of how their systems affect others. The best solution often requires compromises among the systems. Coordination breaks down when teams work in isolation and discover conflicts after the concrete is placed. That upfront coordination pays for itself in the field: because components arrive finished, erection moves quickly, which shortens the overall construction schedule and limits disruption to a busy, operating campus.
Two things matter most if you are planning a sustainable-driven project that is solar-ready. First, understanding that roughly 96% of a building’s lifetime carbon comes from the operational stages, and an array adds its own load demands, so it's important to get the building envelope and structure right before anything else. If those do not perform, the facility will not be sustainable, no matter how much power it generates. Second, build in flexibility for technology you cannot yet specify, especially energy storage. Given how long a building lasts and how fast storage is evolving, the cost of leaving room to grow is almost always lower than the cost of retrofitting later.
Designing solar from day one is less about the array than the timing. Make structural decisions early enough that the renewable energy goal and the building support each other for decades. Treat the parking structure as energy infrastructure rather than just a place to leave a car, and it will repay the foresight.
Brent Kriha is an Engineering Manager at Wells, a nationwide prefabricated building systems provider. He leads structural design teams on complex healthcare, commercial, and parking projects, focusing on long-term performance in prefabricated building systems.
