Timber is machine graded by passing it between rollers and measuring either the load required to give a fixed deflection or the deflection resulting from a given fixed load. A local modulus of elasticity is calculated, from which the bending strength of the timber, as a joist, is predicted. The timber is assigned to a particular strength class or grade based upon the weakest section of the measured length. There are three UK-approved machines: the MPC Computermatic; the Cook-Bolinder; and the Raute Timgrader. A single grade or strength class will include samples with a range of strengths above the grade minimum. Machines are often set to grade two strength classes in one pass and the ranges of actual strength for the two classes may overlap. The retesting of rejects to obtain more graded timber is not permitted, because the statistical methods used to determine the sample strength would be invalidated by the removal of the stronger samples approved in the first pass. 11 refs.