Severe local fiber deviation in timber boards can lead to reduced strength values. As grading machines do traditionally not detect these deviations, such boards are typically assigned to higher strength values than they actually possess. Based on statistics, these cases are taken into account in machine grading. Slope of grain can be measured on the surface by laser diffraction. The aim of this study is to estimate the reducing effect of local slope of grain on tensile strength, which is caused by top ruptures and the impact on the grading results of machine strength grading. This is investigated in full-scale tensile tests. In total, 192 test specimens of Norway spruce (Picea abies) with more or less pronounced local slope of grain were selected, measured and strength-tested to show the influence of local slope of grain on tensile strength. The utilized timber was sawn from relatively small diameter logs. The grain angle was determined on all four longitudinal sides of the boards with the help of a dense matrix. A model is presented for estimation of the grain deviation over the cross section of the sawn timber. These values are converted into a strength-reducing factor taking into account the ratio of tensile strength parallel and perpendicular to the grain. The introduced strength prediction algorithm is tested for the investigated sample. Improvement of the reliability and economy of timber grading using grain angle as an additional parameter will be possible. Furthermore, it is shown that different strength-reducing features such as top ruptures in sawn timber can be located on the specimens, and the strength-reducing effect can be estimated. Local slope of grain around knots is not part of this study.