The beam rotated roughly ninety degrees, from vertical to lying on its side, while the failure worked through the structure above the produce aisles. Announcements went out over the public address system. Approximately 600 customers and 307 employees left the building in under five minutes, with the mayor of Burnaby, Bill Copeland - who had come to preside over the opening ceremony and instead found himself running an evacuation - helping direct people to the doors. Then four structural bays of roof, 27 by 23 metres, some 6,400 square feet of it, dropped into the store, bringing the rooftop parking deck and twenty cars down with it. The displaced air knocked people off their feet. One employee was pinned in the debris and had to be freed by first responders using forklifts; he suffered a crushed pelvis. Twenty-one people were treated in hospital. Nobody died.

The mechanism has been studied ever since, and the short version is that the roof beams were not adequately designed and the lateral supports meant to keep them from rolling sideways under load were not all there. A steel beam carrying weight is strong in one direction and comparatively helpless in the other; take away the bracing that keeps it upright and enough load will persuade it to lie down. Reviews of the structure before the collapse had not caught the missing supports. It is worth being precise about who built what: Save-On-Foods, then part of the Overwaitea division of Jim Pattison's companies, was a tenant. The supermarket was a $5.4-million piece of a $100-million mixed-use development in Metrotown, put up by developers, a general contractor, architects, and two structural engineering firms - none of whom sold groceries.

The province convened a commission of inquiry on May 4, 1988, headed by Dan J. Closkey, and it reported on August 26 of the same year - four months, start to finish. Its most uncomfortable finding had nothing to do with steel. Looking at the fee proposals submitted by the engineering firms competing for the work, the commissioner concluded that at those prices none of them intended to provide the proper level of professional service. The bidding itself had priced out diligence. The report also punctured a widespread public assumption: building inspectors do not perform the engineering review. That responsibility sits with the professional engineers hired for the project, and always did.

The recommendations reshaped practice across British Columbia. Structural calculations and drawings should accompany building permit applications; a random sample should get detailed review; designs of concern should go to the engineers' governing body; the cost of all that should be carried by municipal levies on permits. In 1989 a five-member disciplinary panel found four engineers from the two firms guilty of incompetence, negligence, or professional misconduct. The findings were specific and damning: inadequate design of the roof structure and its beams, failure to inspect the roof assembly as it was built, failure to direct or check the work of the engineer-in-training who was actually managing the project day to day, and a partner who had applied his professional seal to plans he had neither prepared nor supervised. The engineers' association spent the years from 1991 to 1994 rebuilding its governance around those lessons; the Canadian Institute of Steel Construction took up its own. Lawsuits ran in every direction, including from the owners of twenty flattened cars.

Vancouver being Vancouver, the event acquired a nickname within days: Cave-on-Foods. It stuck for decades, which is what happens when a catastrophe produces no bodies and one very good pun. The store itself was repaired and went back to work, selling produce under a rebuilt roof until 2012. The whole complex came down in 2013, replaced by a development anchored by a thirty-five-storey tower of condominiums, offices, and ground-floor retail, and in late 2015 a Save-On-Foods opened again on the site at roughly half its former size. Shoppers in that store are standing more or less where the roof landed. They are also standing inside a regulatory system that exists in its current form largely because of what happened here on a Saturday morning in April, during a sale for seniors, when a pipe broke first.