Recent studies have examined the possibility of growing honeycomb silicene and germanene, the silicon and germanium analogues of graphene. Here we use first-principles calculations to examine the relative stability of a number of other single-layer structures that are derived from prominent surface reconstructions of group-IV semiconductors. We find that Si single-layers with the geometry of the reconstruction are more stable than honeycomb silicene. Likewise, honeycomb germanene is less stable than germanium ultrathin films with a or 7 × 7 arrangement. Si and Ge single layers are metals with strong peaks at the Fermi level, and, in the case of Ge, they bear a magnetic moment. Overall, the results demonstrate that free-standing Si and Ge nano-sheets differ in key structural and electronic properties from graphene, and may thus provide systems with alternative functionalities.