Current lateral torsional buckling design provisions in the Canadian wood design standard CSA O86:19 stipulate that the lateral buckling capacity of built-up beams consisting of multiple plies is equivalent to that of a solid beam with the full width of the built-up system, provided that individual plies are securely connected at intervals that do not exceed four times the depth. Within this context, the present study numerically examined the lateral torsional buckling behaviour of built-up wooden beams consisting of two lumber plies of identical depth connected together through mechanical fasteners such as nails. The analysis was conducted using 3D finite element simulation that enabled the relative slip between plies. The study investigated the effect of fastener stiffness and spacing on the elastic lateral torsional buckling resistance and corresponding mode shapes for the built-up system. Comparisons were made against the buckling resistance of a single ply and that of a solid beam with a width of both plies. The study shows that the critical moment of the built-up system is found to be significantly lower than that implied by present design provisions based on a solid beam with full width.