A perimeter frame used in a construction of floors or roofs of buildings, comprising: (a) a pair of first modular sub-frames being symmetrically opposite each other across a first axis, each of the first modular sub-frames having a blunt end portion (26, 28), the blunt end portion formed at one or both ends of the first modular sub-frame which has a pair of parallel, spaced apart, linear beam members (20, 22) interconnected by a plurality of cross-beam members, the linear beam members being symmetrically opposite each other at the, or each, blunt end portion, (b) a pair of second modular sub-frames being symmetrically opposite each other across a second axis perpendicular to the first axis, each of the second modular sub-frames having an overhang end portion (36, 38), the overhang end portion formed at one or both ends of the second modular sub-frame which has a pair of parallel, spaced apart, linear beam members (30, 32) interconnected by a plurality_of cross-beam members, a first of the linear beam members having a length which extends further at the, or each, overhang end portion than a length of a second of the linear beam members to form a linear extension of the first linear beam member at the, or each, overhang end portion, wherein the linear extension has a length which is substantially equal to a distance separating the linear beam members of the first modular sub-frame, wherein the linear beam members of the first and second modular sub-frames (16, 18) are perpendicularly interconnected at respective end portions (26, 28, 36, 38) of the first and second modular sub-frames, and wherein each of the symmetrically opposite first modular sub-frames has an internal linear beam member located along an internal perimeter of the perimeter frame, and each of the symmetrically opposite second modular sub-frames has an internal linear beam member located along the internal perimeter of the perimeter frame, and the internal linear beam members of the first modular sub-frames are spaced apart from each other by a distance equal to a length of the internal linear beam members of the second modular sub-frame, characterised in that the perpendicular interconnection between said overhang end portion (36, 38) of said second modular subframe and said blunt end portion (26, 28) of said first modular subframe comprises an end cross-beam member of said second modular subframe arranged parallel to and contacting alongside an end section of said internal linear beam member of said first modular sub-frame. (a) a pair of first modular sub-frames being symmetrically opposite each other across a first axis, each of the first modular sub-frames having a blunt end portion (26, 28), the blunt end portion formed at one or both ends of the first modular sub-frame which has a pair of parallel, spaced apart, linear beam members (20, 22) interconnected by a plurality of cross-beam members, the linear beam members being symmetrically opposite each other at the, or each, blunt end portion, (b) a pair of second modular sub-frames being symmetrically opposite each other across a second axis perpendicular to the first axis, each of the second modular sub-frames having an overhang end portion (36, 38), the overhang end portion formed at one or both ends of the second modular sub-frame which has a pair of parallel, spaced apart, linear beam members (30, 32) interconnected by a plurality_of cross-beam members, a first of the linear beam members having a length which extends further at the, or each, overhang end portion than a length of a second of the linear beam members to form a linear extension of the first linear beam member at the, or each, overhang end portion, wherein the linear extension has a length which is substantially equal to a distance separating the linear beam members of the first modular sub-frame, wherein the linear beam members of the first and second modular sub-frames (16, 18) are perpendicularly interconnected at respective end portions (26, 28, 36, 38) of the first and second modular sub-frames, and wherein each of the symmetrically opposite first modular sub-frames has an internal linear beam member located along an internal perimeter of the perimeter frame, and each of the symmetrically opposite second modular sub-frames has an internal linear beam member located along the internal perimeter of the perimeter frame, and the internal linear beam members of the first modular sub-frames are spaced apart from each other by a distance equal to a length of the internal linear beam members of the second modular sub-frame, characterised in that the perpendicular interconnection between said overhang end portion (36, 38) of said second modular subframe and said blunt end portion (26, 28) of said first modular subframe comprises an end cross-beam member of said second modular subframe arranged parallel to and contacting alongside an end section of said internal linear beam member of said first modular sub-frame.