In materials that have bulk inversion symmetry, optical second harmonic generation (SHG) is sensitive to regions when the inversion symmetry is broken, i.e., a surface or interface. We measure SHG from the interface between Si(100) and thin layers of SiO2. Measurements on a series vicinal samples (0 degrees-5 degrees off axis) show that one-and threefold symmetries in the SHG signal increase with increasing off-axis angle. Comparison to x-ray scattering measurements of the interface roughness, for a set of on-axis samples, demonstrates the sensitivity of SHG to interface roughness.