A mechanistic model is presented for predicting the tensile stiffness and strength of 2 by 4 lumber boards containing a single major face knot. The primary inputs to the model are local grain angle maps for each wide face of a board and estimates of average clear-wood properties. The grain angle maps were obtained through electrical scanning of board surfaces and represented the in-plane orientation of the wood fibers. Out-of plane dive angles were not considered. The model simulates the failure process that occurs within lumber subject to tension and thereby provides insight to, and understanding of, the failure of wood with defects. The model is devoid of empirical adjustment factors, and it has produced tensile strength predictions that correlate with measured strengths by a correlation coefficient of 0.86 and an average absolute error of 12per thousand. It is hoped that insight gained through use of this model will provide a foundation for improvements in lumber grading.