Abstract: A structural unit for forming a building, which structural unit comprises at least three elongate members which are connected to each other to define an area, a covering of a flexible material which is secured to the elongate members and which covers the area defined by the elongate members, and a locking device which connects at least one pair of the elongate members, the locking device being such that it operates with a wedging action whereby tightening of the locking device causes the two elongate members to move with respect to the locking device such that the two elongate members move apart to increase the surface area of the structural unit and thereby tighten the covering of the flexible material.