Low-energy electron diffraction (LEED) was used to investigate the relaxation of strain in thin epitaxial films of FeSi2 grown on Si(100) surfaces. The Si(100) substrates were both unstrained and strained by an underlying SiGe alloy. Analysis of the LEED spot sharpness as a function of film thickness and silicide-substrate lattice mismatch indicates that elastic stress in the films is partially relieved prior to the formation of misfit dislocations at critical thickness. We examine the role of roughening and the formation of rotational domains of the orthorhombic FeSi, unit cell as mechanisms for the relief of this strain, and conclude that roughening due to the formation of islands is the dominant strain relief mechanism below critical thickness. We apply a recent analysis by Freund and Jonsdottir (J. Mech. Phys. Solids, 41(7) (1993) 1245) to correctly account for the scaling of the lateral size of terraces produced by the roughness as a function of the film thickness and lattice mismatch. The critical thickness, h(c). for films on unstrained Si substrates was found to be approximately 30 Angstrom.