Abstract: Employee #1 and two coworkers were working 45 ft above the ground on the elevated platform of a Hek manually propelled mobile lifting platform, model MS 3000. The motorized unit raises and lowers the platform by electric motors. Employee #1 and the coworkers were carrying a 50 in. long by 12 in. wide 150 lb limestone window header and were about to raise it into position. The platform was fully planked with standard 2 in. by 10 in. planking, except for an area about 19 in. long by 18 1/2 in. wide in front of the platform's supporting masts. This opening was covered by a scrap piece of 12 in. by 53 3/4 in. plywood. While Employee #1 was holding the end of the header, he stepped onto the scrap piece, which broke under his weight. He fell through the opening to the ground and was hospitalized. The planking used to cover the opening was not scaffold grade or the equivalent.