Homogenization theory is used to derive the effective properties of gratings with complex subwavelength structures. Going beyond the effect of the filling fraction, geometrical effects are analyzed using a two-step homogenization process. An explicit expression for the transmission spectrum is derived, able to predict the Fabry-Perot resonances and the Brewster angle realizing broadband extraordinary transmission. With the same filling fraction, one expects from this analytical expression that gratings with different geometries may display very different transmission properties. This sensitivity to the microstructure geometry is exemplified in the case of gratings made of hard material and made of dielectric material. The analytical results are shown to be within a few percentage points as compared to full-wave numerical simulations, paving the way for transmission properties tuned by structural and geometrical manipulations.