A method of constructing a frame (10) for a filtering screen from a polymer material is described in which a wire frame reinforcement is wholly encapsulated during the frame moulding process so that two parallel spaced apart wires extend through each of the intersecting orthogonal ribs (14, 16) which define the open area of the frame (10) across which the wirecloths (158, 159) are stretched. The surfaces of the frame (10) in which the wirecloths (158, 159) are embedded are formed with ridges (22, 30, 32) and the crests of the latter extend to different heights. The crests are softened by heating during the manufacturing process and the wirecloths (158, 159) stretched over the frame (10) are forced into the softened crests after which the assembly is allowed to cool and cure. The frame (10) is re-usable by stripping wirecloths (158, 159) from the top of the frame (10) and fitting fresh wirecloths over the plastics material and heating the regions which are to encapsulate the fresh wirecloth.