In order to evaluate the scattering coefficients of architectural surfaces, a numerical technique based on Mommertz’s definition is developed with employing a 3-dimensional boundary element method. Numerical examination on the setting of parameters in computation and the conditions of samples is performed so as to ensure accurate calculation with this technique. As a result, criteria for the numerical parameters and for the test arrangement are clarified, and additionally, illustrating the behavior of directional and random-incidence scattering coefficients. Furthermore, in comparison between the values with the present method and those for an infinite periodic surface, general correspondence is confirmed although some differences appear due to the edge diffraction by the finite sample.