Abstract: A combined ceiling, air distribution system, mechanical chasse and structural roof member for a building is formed by a plurality of parallel inverted U-shaped structural modules 15 which are supported in a common horizontal plane. The flanges 17 of adjacent modules are spaced apart and define the side walls of a duct 21, top and bottom walls 22, 24 for the duct also being supported between the adjacent modules. The duct may be of any suitable dimensions, and in particular may be sufficiently large as to handle air flow requirements for the heating, ventilating or air conditioning system for the building. The duct may also accommodate other services such as electrical, plumbing, compressed air, etc. This arrangement dispenses with the need for a suspended ceiling which has conventionally been provided beneath the duct work and service lines attached to the ceiling of a building.