Silver was sputter deposited on a glass with a thin film thickness ranging from 10 to 80 nm. Scanning tunnelling microscopy was used to study the morphology of the obtained Ag-glass surfaces and to estimate the surface roughness. An equation for the surface roughness of the thin film was evaluated using parameters related to the thin film features: the surface roughness of the substrate, the compressibility of the thin film and the film thickness. The experimental results were fitted using the evaluated equation, and the conditions favouring lower or higher surface roughness were analyzed.