Interior bearing walls are reinforced concrete. The walls of the covered dock and shop located at the eastern end of the structure are constructed of reinforced concrete. Four feet of the walls are above-grade. Depending on the location and function, four different roof types are present. These roof types include reinforced concrete slab supported by concrete beams, structural steel roof frame covered with corrugated asbestos, open-web steel with metal decking, and reinforced cement.
Ceilings consist mostly of the undersides of the various roofs.
Doors in the building are either hollow metal or wood. Building 991 is the main structure of the 991 complex. Tunnels and vaults in the complex are maintained at a slight negative air pressure by the building heating, ventilating, and air conditioning system. Support operations located in Building 991 included laboratories, utilities, and maintenance operations. A modular laboratory was located in Room 143.