The corrosion problem was so bad, the police-commissioned report states, it's a mystery it took as long as it did before the rooftop parking deck in Elliot Lake, Ont., caved in.
"The rates (of corrosion) observed in the Algo Centre Mall are comparable to those found in a 'marine environment'," the report states.
"It is in fact somewhat surprising that failure did not happen earlier."
Criminal investigators with Ontario's provincial police asked Norr — an architectural and engineering company — to do the forensic engineering report into the collapse that killed two women and injured several other people.
The 142 pages of findings, made public on Tuesday, amount to a depressing account of a tragedy long in the making that could have been averted.
According to the report, the collapse occurred when a weld between a support column and beam failed in two stages because of corrosion caused by years of water and road-salt penetration.
The collapse may have been unprecedented in the developed world.
"One is hard-pressed to find a similar example where a carbon-steel framed building in North America or Europe continued to corrode to the point of failure, when no extreme loading is present," the report states.
As the judicial inquiry into the collapse has already heard, leaking occurred because an "intrinsically flawed" waterproofing system installed in 1980 failed from the start, prompting years of complaints.
Inquiry documents show some residents dubbed the shopping centre the "Algo Falls." Others jokingly advised taking an umbrella when shopping there.
Still, despite some efforts — one of which saw workers fill cracks and joints in the cement with colourful pool noodles — the leaking continued.
"The fact that the roof was allowed to leak for 32 years is perplexing," the report states.
"A number of actions could have been taken to avoid collapse had the critical condition been identified in time."
Even more perplexing, perhaps, is that none of the engineers and others who inspected the mall over the years foresaw the looming catastrophe.
Two companies, Pinchin and MRW, issued "unequivocal" reports attesting to the soundness of the structure, while an MRW report — released just two months before the collapse — deemed the corrosion "not of structural significance."
"The clean bill of health given to the structure by a number of consultants in the past few years before collapse is quite alarming," the report states.
Norr also makes the new allegation that Coreslab, the company that supplied the precast concrete for the roof deck, and John Kadlec, the structural engineer, misled the mall's original owner about the capabilities of the product, likely in an "aggressive effort" to win a competitive tender.
While the sub-strength hollow-core slabs did not play a direct role in the collapse, the deficiency later thwarted proper waterproofing solutions.
The report also notes that authorities appeared more focused on fireproofing than on dealing with the severe corrosion problems.
At the inquiry Tuesday, Commissioner Paul Belanger said he was discontinuing contempt proceedings against the mall's last owner, Bob Nazarian.
Belanger said Nazarian, his wife Irene and son Levon, had now provided 85,000 emails and access to tax records in response to a commission order.
The case was due to be heard Wednesday in Divisional Court in Toronto.