Adsorption of ultrathin epitaxial layers of Cu on Ru(1 0 (1) over bar 0) was investigated by ab initio density functional theory (DFT) simulations combined with directional elastic peak electron spectroscopy (DEPES) measurements. In the low-coverage regime, energetics of the adsorption process indicates that the deposited Cu atoms form instantly a bilayer (BL) and leave some substrate uncovered rather than we the substrate with a single monolayer. The bilayer-by-bilayer growth is energetically favorable up to 3 BLs, but the third BL already occurs thermodynamically metastable. The DFT-predicted optimal arrangement of Cu adatoms in substrate-latticecontinuation sites was validated by DEPES measurements. This confirms the formation of a stable ultrathin film of hcp-like copper on the Ru(1 0 (1) over bar 0) substrate.