Figure 1 shows a generally rectangular mould polymeric screen element 1 having complementary opposed edge formations 52, 54. Channels 56 in the latter have outer parts 60 open to a side of the elements and a restricted entry to inner parts 58 for accommodating reinforcing elements (not shown), of wedge-or squaresection wire form, snap-fitting with them. An alternative oblong section reinforcement with retention lips is also shown in another figure (figure 6). A preferred fabricated strip metal reinforcement 50 saves edge 10, 12, 14, 16 and division 18, 20 widths but allows a supplying degree of curving.