In a second day of testimony, the man whose company tried to waterproof the roof-top parking deck testified he now believes the designers failed to take into account the heavy traffic — including buses and snow plows — that would use the exposed garage.
"The primary source of our problems was the choice of the hollow-core structural deck and the way it was constructed," Dave Monroe testified.
"There were too many slabs of hollow-core precast concrete loosely tied together and they moved more than we would have expected and in ways we didn't expect."
Monroe, former vice-president of Michigan-based Harry S. Peterson Co., said the leaking from the parking deck into the stores below proved to be more trouble than the company had ever experienced.
"We realized we had problems we did not understand," Monroe said. "It was very disheartening."
He described a series of remedial measures — some after a controlled flooding of the parking deck — including replacing seals and caulking.
At one point, he said, they thought they had fixed the problem that began immediately after the mall construction was completed in 1980.
"We felt like, 'Eureka!'" he said.
The euphoria didn't last long, with serious leaks re-emerging within a few weeks.
"The mystery continued."
It would be a decade later — in 1992 — that an engineering-inspection report stated: "The design used for this roof slab is inappropriate in achieving a water-tight condition."
Yet even a seemingly simple solution, such as restricting access to the garage, was never properly implemented and there were no other parking options for shoppers, the inquiry heard.
After years of water and salt penetration, the rooftop garage collapsed June 23, 2012, killing two women and injuring several others in the mall below.
Monroe said never in his wildest thoughts did he ever imagine the roof deck would create a safety hazard.
"When I heard (about the collapse) I was shocked. I was very sad."
Lawyer Douglas Elliott, who represents citizens of Elliot Lake, praised Monroe for his candid admission that his company's waterproofing system failed.
"I don't want you to think your company was the only one who made mistakes," said Elliott, who called Monroe's testimony a "healing moment" for the community.
"The people of Elliot Lake are good people and I know they will find it in their hearts to forgive you."
In later testimony Friday, Henry Jasskelainen, a former Peterson employee, said installation of the waterproofing began late because of project delays.
The inquiry has heard how the temperatures and moisture adversely affected curing of the sealant materials and how they bond to the concrete.
"We were basically under the gun to put the system in," Jasskelainen said.
"It seemed to always be wet here and we had an early winter. It created a very difficult environment."
The inquiry, under Commissioner Paul Belanger, is looking into the reasons for the collapse and the emergency response to the disaster.
On Monday, the mall's architect is expected to testify.
Note to readers: This is a corrected story. An earlier version incorrectly said the mall collapsed on June 23, 2013.