To extend the large-eddy simulation (LES) technique to high Reynolds-number flows, the use of approximate boundary conditions is required to bypass the extremely stringent resolution requirements in the near-wall region. In this investigation, the two-layer wall model [Balaras et al., AIAA J., vol. 34, 1111, (1996)] is used for the LES of a backward-facing step. The filtered Navier-Stokes ... [Show full abstract] equations are solved up to the first grid point, and from this point to the wall an embedded grid is set and the thin-layer Navier-Stokes equations are solved with the Spalart-Allmaras one-equation model [La Recherche Aerospatiale, vol. 1, 5 (1994)]. An iterative technique to assign the boundary conditions for the Spalart-Allmaras eddy viscosity at the inner/outer-layer interface, which minimizes the discontinuity between the stresses at the interface itself, has been devised. This method results in improved agreement with the experimental and direct simulation (DNS) data in a variety of cases. The backward-facing step calculations were carried out at two Reynolds numbers, to match resolved LES and experiments, as well as previous LES calculations that also used approximate conditions. The present technique results in accurate prediction of the mean flow and turbulence statistics. The re-attachment point is predicted accurately, with a grid that uses a small percentage of the points required by resolved LES.