Sun Light & Power (No. 223 on the 2021 Top Solar Contractors list) has installed solar on many different roof types in the San Francisco Bay Area. Tile roofs, shingled roofs, flat roofs, metal roofs — the Berkeley-based installer has seen them all in the last 45 years. But none were such a tight squeeze as the company’s 2020 installation at the University of California San Francisco’s Medical Center at Mission Bay.

The plan was to install a 244-kW solar array using a custom-designed steel structure as the system’s foundation that would span across an elevated roof section at the UCSF Medical Center. Everything was laid out in the design documents, but what SLP installers immediately found was they only had 1/4 in. of wiggle room with the attachments.

“There’s a lot of play with rail, and then with L-feet there’s a lot of vertical adjustment,” said Scott Hosterman, senior project engineer at SLP. “I mean, we rarely get into this tight of a situation because we’re working with rafters or concrete and we have plus-or-minus 5 in. When something comes up, we can shove things, we can push the cantilever and give ourselves more room.”

At first touch, they were off by one-sixteenth of an inch. That measurement spread out over 30 module rows meant the system would walk out of place and not fit. SLP worked alongside general contractors TCB Builders to drill accurate attachment points into the steel foundation.

The steel beams were set about 20 ft off the roof, so to get installers up to that height, SLP placed plywood atop wood planks stacked in the slots of its I-beams to create a surface for contractors to walk along and set modules.

SLP installers worked this way from one end to the other, placing 678 SunPower modules. Given the limited working footprint of the building and wanting to maximize power output, modules were placed in an east-west orientation.

“East-west lends itself to zero loss of space,” Hosterman said. “You can stack east-west right on top of each other, making an almost continuous envelope of solar.”

The SLP crew was working through the early months of the pandemic at Mission Bay when shutdowns due to COVID-19 restrictions were common. The hospital had established windows of time for when the solar contractors could work. Everyone had to carry IDs and were temperature-checked each time they entered the building. While running electrical into the building, contractors worked inside an “airtight cart” as a precaution.

On top of that, air quality in California was poor, and the installers and steelworkers were working atop a six-story building in unrelenting heat. And the project was finished in July, just before wildfire ash started falling on San Francisco.

“Even almost a year after, we haven’t worked on another project like this,” said Xavier Garcia, project manager at SLP. “We have challenging clients, challenging locations, challenging weather, challenging installation, challenging AHJs, but not the project itself.”

He added that with the many parties involved in the construction of this array, and the circumstances it was built under, SLP walked away with some lessons learned from this project.

“One is that we know we can make it happen because we already did it once in the worst-case scenario — and we did it well,” Garcia said.

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This story was featured exclusively in our 2021 Top Solar Contractors issue. See the issue and full list of top U.S. solar installers here.