Surface relief gratings have been generated in an azobenzene side chain liquid-crystal polymer using an intensity interference pattern from two coherent laser pulses (532 nm and 4 ns). We elucidate whether peaks or trenches correspond to the high intensity regions of the interference pattern. For low-energy pulses, peaks are generated in the irradiated areas while trenches appear for energies above a threshold (around 100 mJ/cm2 per beam). This phenomenology, together with the lack of relief when polarization patterns are used, indicates that in this polymer surface relief has a thermal origin.