The physical properties of irradiated materials must relate to their structure that can be studied by x-ray diffraction. These diffraction effects can be very subtle. The details obtainable by undertaking a careful analysis include strain relaxation in the layers, lattice distortion, change of the mosaic spread, crystal fragmentation, interfacial roughness, etc. All this information can help in ensuring the most fundamental understanding of the structure of irradiated materials and it is not easy or even impossible to achieve this knowledge by other analytical techniques. If the irradiated layer is very thin, then grazing-incidence geometry, where the x-ray penetration is close to the total external reflection condition, can significantly enhance the layer scattering. Recently, the effects of finite size on the physical properties of condensed matter have attracted a growing interest because several results suggest the properties of matter confined to nanometer scale differ from those of the bulk. Irradiation may produce structural changes at this scale that may depend on the implantation parameters, leading to unusual conditions characteristic of restricted or confined geometries. We will discuss some recent diffraction experiments on SiC, showing how a careful analysis of the changes in the diffraction patterns near the Bragg peaks may help to split between size, strain and orientation effects. Monitoring these structural changes is attractive for a fundamental understanding of their influence on the electronic structure and the thermal properties.