Abstract: For the building of roomed structures there are provided two kinds of modular units which respectively form walls, and floors or ceilings. Two of each kind of unit can be bolted together to form an annular assembly. A plurality of annular assemblies can be bolted end to end to form a tube. The external faces of the units are shaped such as, when opposed to another similar unit, to define channels for air conditioning, electrical services, and the like, and other spaces to receive reinforcement and poured concrete for horizontal cross beams, and vertical pillars to support the cross beams.