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We report a detailed experimental and theoretical investigation of the effect of residual strain, and strain relaxation, which manifests itself at the Si/SiO$_{2}$ interfaces in commercial silicon-on-insulator (SOI) wafers. SOI material is made of a single-crystal silicon overlayer (SOL) on top of an insulator (buried SiO$_{2}$ layer) sitting on a handle silicon wafer. Infrared reflectivity spectra show that the buried SiO$_{2}$ layer relaxes continuously when thinning the SOL. At the same time the SOL surface roughness and the linewidth of optical phonons in Si near the Si/SiO$_2$ interface (probed by micro-Raman specroscopy) increase. In the as-delivered wafers, this comes from a slight expansion of Si on both sides of the buried SiO$_{2}$ layer which, conversely, is compressed. Thinning the SOL modifies these initial equilibrium conditions. To get quantitative results, we have modeled all our Raman spectra using a theory of inhomogeneous shift and broadening for optical phonons, which takes into account the phonon interaction with the static strain fluctuations. From the variation of linewidth versus interface distance, we have found that the mean squared strain continues to relax in the bulk of the wafer through a depth on the order of several $\mu m$. We also show that the SOL surface roughness is related to strain fluctuations near the Si/SiO$_2$ interfaces., Comment: 28 pages, 11 figure, to be published in Phys. Rev. B (Nov 15 2000)