This paper aims at estimating the stiffness of Glued Laminate Timber (GLT) beams on the basis of an accurate analysis of the board's mechanical properties and a simple composite beam theory. Modern grading technologies, like laser scanning, have the capability to accurately detect grain orientation on the surface of timber boards, allowing for an accurate analysis of local mechanical properties. Conversely, this information is only partially exploited in the estimation of strength and stiffness of GLT structural elements since, also nowadays, the mechanical properties of GLT beams are evaluated only on the basis of boards grading, leading to extremely cautious but also coarse estimations. A recent paper exploits information on wood's mechanical properties and high resolution data on grain orientation in order to estimate GLT beam stiffness obtaining promising results. Unfortunately, the approach needs highly refined 2D Finite Element (FE) analysis resulting quite expensive from the computational point of view. Aiming mainly at reducing the computational cost, this paper exploits high resolution information for the estimation of homogenized mechanical properties of boards. Thereafter, it uses the so called composite beam theory for the direct estimation of the GLT beam stiffness. The comparison with experimental data highlights that the proposed approach provides estimations with an accuracy similar to 2D FE analysis, but, since it is significantly cheaper, it turns out to be more convenient. Finally, the simplicity of the beam model allows for an easier understanding of relations between input and output, facilitating a more rigorous interpretation of experimental data.