A slab model of a completely dehydroxylated silica surface free from structural strain and highly reactive defects is proposed as a model of a fully dehydroxylated silica material. Its structural rigidity and symmetry imply a low computational cost which renders the model suitable for quantum mechanical calculations. The hydrophobic character of the fully dehydroxylated silica is assessed through this model by simulating the adsorption of water at increasing loading. Structural, energetic, and vibrational features of the resulting systems have been compared with recent experimental measurements on a thin dehydroxylated silica slab grown on Mo(112) surface, and all data concur to define this model as highly hydrophobic. The present data are of relevance for the understanding of adsorption phenomena on silica surfaces with various degrees of hydroxylation.