Abstract: A hollow building element of a plastics material comprises an upper wall, a lower wall, a pair of opposite side walls interconnecting the lateral edges of the upper and lower walls, a joining member in the form of a cylindrical, slotted, downwardly open flange provided at the upper edges of each side wall, the lower end of a side wall being integrally connected to an anchoring element for anchoring the building element to a support and the lower end of the opposite side wall being integrally connected to a guide member adapted to engage the anchoring member of an adjacent building element so as to maintain the lower ends of two adjacent side walls in spaced relationship and so as to form a tight connection between such elements.