Abstract: Building frame elongate structural elements are provided having at least cavity which in transverse cross section has a mouth permitting access to said cavity, defined by a pair of opposed flanges which have first portions extending toward each other across the mouth and second portions extending inwardly of the cavity from said first portions and terminating at free edges so as to leave to each side of the mouth an undercut space which is behind the first portion and laterally outside the second portion, the said second portions of the flanges being deformable. Fixing members for combining said structural elements are also provided. The fixing members and structural elements are useful in making building frames from a plurality of the structural elements.