Deft Planning and Skillful Moves Play Out on the Reroof of the World’s Largest Water Filtration Plant

Calculated Movements

Situated atop rubble from the 1871 Great Chicago Fire and underground vaults, the structure had weight restrictions. To adhere to load requirements, Trinity purchased a crane and added custom-made crane mats on its outriggers to distribute its weight. When the brutal cold and ice came, the six other trades onsite started small fires under their equipment every morning to get their machines running. “We couldn’t afford to take any chances with our custom-tailored crane, so we had to drive it to our warehouse to keep it warm at night and drive it back the next morning,” Cronin says. “We’d always hit rush hour in afternoon. That round-trip added four extra hours a day to our schedule, but it had to be done.”

Workers placed 712,000 board feet of 2-inch cellular glass insulation over concrete roof channels, followed by 1,086 rolls of 90-mil membrane in hot asphalt. The fleece-backed membrane with air-welded seams produced an integrated roof system backed by a 30-year warranty.

Workers placed 712,000 board feet of 2-inch cellular glass insulation over concrete roof channels, followed by 1,086 rolls of 90-mil membrane in hot asphalt. The fleece-backed membrane with air-welded seams produced an
integrated roof system backed by a 30-year warranty.

During set up of the job, crews put down grading stone over sod and built two scaffolding mobilization sites at the roof height for loading.

Trinity had to remove old concrete deck slabs and install 30,000 new precast 2- by 7-foot concrete roof channels averaging 275 pounds each—and the 80 that housed drainage systems weighed up to 600 pounds. As slabs arrived, they had to be inventoried with respect to their installation so crews could easily pull needed ones first, since it wasn’t possible to put any extra load on the roof.

The heft and awkward concrete forms inspired the crew to build a plywood road on the new roof for transport. Because they could never drive on the old roof, Trinity strategically planned phasing to prevent painting themselves into a corner.

For the plywood road, the crew laid down 2- by 6-inch parallel “sleeper” boards, then cut 4- by 8-foot boards lengthwise (so crew members could carry them) and laid those across the sleepers so they wouldn’t crush the roof when driving on it. Gale-force
winds could kick up over Lake Michigan quickly, which had the ability to carry boards far afield and create serious hazards. To prevent this, the team had to break down and remove the plywood road each evening, then, rebuild it each morning.

Given the roof’s surface area and weight restrictions, it was common for crew members to travel up to 10 miles per day back and forth over the roof.

Given the roof’s surface area and weight restrictions, it was common for crew members to travel up to 10 miles per day back and forth over the roof.

Given the roof’s surface area and weight restrictions, it was common for crew members to travel up to 10 miles per day back and forth over the roof.

“Everyone on the team learned to combine tasks. No power carts traveled without a load. If a pallet of asphalt or insulation was delivered, they brought it to the scaffolding, broke the pallet down and drove [the broken-down pallet] back across the roof,” Cronin recalls.

Winning Strategies

The project was conducted in eight phases. At times, there would be 14 holes in the roof at once as Trinity waited for steel replacements to be fabricated, installed and painted before they could install new concrete slabs and roofing over them. Because the job couldn’t wait if someone was ill, Trinity had to ensure that two people qualified to run the necessary equipment were available each day.

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About KJ Fields

KJ Fields writes about design, sustainability and health from Portland,
Ore.

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