A modular perimeter frame system (10) is described for forming a perimeter frame (11) used in the construction of floors, walls and roofs of buildings. The modular perimeter frame system has a first modular sub-frame (16) having one or two blunt end portions (26, 28), and a second modular sub-frame (18) having one or two overhang end portions (36, 38). The blunt and overhang end portions are so dimensioned and shaped as to facilitate a continuous abutting engagement between at least two surfaces which meet at a corner of the blunt end portion and at least two surfaces which meet at a corner of the overhang end portion.