Deep rooting can be inhibited by strong layers, although there is evidence for species and cultivar (cv.) differences in their penetration ability. Here, the availability of near-isogenic lines (NILs) in rice (Oryza sativa L.) was exploited to test the hypothesis that increased root diameter is associated with greater root bending stiffness, which leads to greater root penetration of strong layers. Wax/petrolatum discs (80% strong wax) were used as the strong layer, so that strength can be manipulated independently of water status. It was found that good root penetration was consistently associated with greater root diameter and bending stiffness, whether comparisons were made between cvs or between NILs. With NILs, this effect was seen with ‘research’ lines bred from recombinant inbred lines of a cross between cvs Bala and Azucena and also in improved lines developed from cv. Kalinga III by introgression of parts of the genome from Azucena. Much of the bending behaviour of roots could be explained by treating them as a simple cylinder of material. In both wax disc and sand culture systems, roots that had encountered a strong layer had lower bending stiffness than roots that had not encountered a strong layer which is a novel result and not previously reported.