We propose a directed self-assembly mechanism toward obtaining defect-free perpendicular lamellar phases of diblock copolymer (BCP) thin films. In our numerical study, a thin BCP film having a flat top surface is cast on a unidirectional corrugated solid substrate. The substrate is treated chemically and has a weak preference toward one of the two BCP components. Employing self-consistent field theory (SCFT), we find that there is an enhanced synergy between two substrate characteristics: its topography (geometrical roughness) combined with a weak surface preference. This synergy produces the desired perpendicular lamellar phase with perfect in-plane ordering. Defect-free BCP lamellar phases are reproducible for several random initial states and are obtained for a range of substrate roughness and chemical characteristics, even for a unidirectional multimode substrate roughness. Our theoretical study suggests possible experiments that will explore the interplay between unidirectional substrate corrugation and chemical surface treatment. It may lead to viable and economical ways of obtaining BCP films with defect-free lateral alignment.