The EE was preparing a catwalk in order to begin fastening roof decking on a flat roof (total roof area 800 sq ft - 20ft x 40ft). It is unknown how or why the fall occurred, however, the EE was not wearing fall protection equipment as there was no means of tying off. He fell between two rafters that are 2' apart, through the insulation and drywall to the floor below (approx 9 ft).