Abstract: A riddle assembly for the grading of particulate materials has a pair of screens forming respective screening stages and disposed one above another. Each of the stages is formed by a pair of longitudinally extending rails interconnected by longitudinally spaced transverse rails having multispan configuration with a pair of outer spans projecting laterally outwardly beyond the longitudinal rails and an inner span interconnecting the two rails and disposed between them. The multispan transverse beams are of nonuniform cross-section corresponding to the distribution of stresses whereby each beam is of uniform strength.