Roof Collapse of a Large Retail Building

On a rainy morning just before opening, a large section of roof collapsed onto the checkout stands in a large retail store. Had the collapse occurred when the store was open, numerous people would have been injured. The building sat vacant for several years before it was purchased and renovated. As part of the renovation, the building was reroofed, which included replacement of over 100 sheets of decayed or damaged plywood roof sheathing. Third party inspectors were present during the sheathing replacement and reroofing. Aerial images showed that after the reroofing, a low spot formed over the collapse area and got larger within a few years. On site investigation after the collapse showed that the top laminations of the glulam beam supporting the collapsed area was heavily decayed. New roof sheathing had been nailed into the decayed portions of this glulam beam. Investigation of areas beyond the collapse found significant decay of other roof framing elements. Besides reconstructing the collapsed portion of the roof, the remaining roof framing had to be strengthened because of this decay. The case study calls into question the lack of adequate investigation into the existing condition of the roof framing system prior to and after the purchase of the building, as well as during subsequent renovation work. Decayed joists and beams were not directly visible from above because of the roof sheathing, and not directly visible from below because of the ceiling tiles and the roof insulation, which was sandwiched between the roof joists. Still, the fact that over 100 sheets of plywood sheathing had to be replaced because of decay and damage, and that there were visible signs of numerous leaks in the roof, should have caused the owner and its consultants to further investigate the condition of the roof framing. Had that been done, and the decay in the glulam beam been addressed, the collapse would not have occurred.