In this paper, a wire mesh metallic photonic crystal is modelized as a stack of gratings of period d made of very thin infinitely metallic rods (the conductivity is assumed to be infinite) of radius a (a << d). This structure is illuminated by a s-polarized plane wave of wavelength lambda. First, we give the diffracted field in closed form for very large wavelengths compared to the period (lambda >> d). We derive a very accurate formula for the cut wavelength and we show that the plasma frequency in wire photonic crystals depends upon the Bloch vector. This latest formula is checked numerically.