The structure he entered has a mechanical room in the bowels of the building. This "boiler room" contains a vast air duct that feeds into the air filters. The duct itself is more than strong enough to support the weight of a man. Indeed, inspectors are required to climb onto the duct from a catwalk on the floor above, in order to inspect one of the fire extinguishers.
Peter had just inspected that very device, and was standing on top of the air duct when he decided to save himself a few minutes of time. Oh, the Darwin Awards that have resulted from time-saving shortcuts. The nearby fire device was almost in range if he stretched!
A highly trained Fire and Safety Inspector--it's his or her job to know how to inspect a building safely. But sometimes the safe route is inconvenient. Instead of taking that safe route--traveling back down to the basement and climbing a ladder--Peter decided to: a) climb down the side of the air duct, b) in nearly complete darkness, c) despite being warned by his senior partner an hour earlier that he definitely should not climb on the ductwork.
Halfway down, he misjudged his footing and gravity performed its civic duty. Peter plummeted ten feet down to the cement ground in the carpentry shop, adjacent (in photo--w) to the boiler room below, punching a hole through the tile roof in the process.
Peter survived the accident with two broken ankles, but easily could have killed (impaled) himself if he had landed to either side, on the table saw or the tool bench. If you find yourself thinking "this precarious shortcut will save a little time..." you too might be headed toward a Darwin Award.
A picture of the aftermath of Peter's decent can be found here.