Cross-polymerization in alkylsilane self-assembled monolayers (SAMs) has been thought to be an essential ingredient of their stability. For alkylsilane SAMS at full coverage, it is shown here that cross-polymerization cannot occur due to steric effects. Thus, to form dense, fully covered monolayers, cross-polymerization must be avoided. Such monolayers should be as stable as alkanethiol SAMs, which also do not cross-polymerize. The structure of the headgroup plays a limiting role in possible monolayer structures. Due to large steric hindrances, the dimethylmonochlorosilanes cannot cover a silica surface with a density equal to the density of surface hydroxyls. In contrast, the trichlorosilanes can achieve such densities. To satisfy the steric constraints, the aliphatic tails must form a herringbone structure.