Using the Leibler-Ohta-Kawasaki (LOK) phase-field model of block copolymers (BCPs), we characterize how line-width roughness (LWR) in the chemoepitaxial template affects the BCP microdomain shape. Specifically, we derive formulas for the monomer density and the microdomain interface profile of periodic, lamellar BCP melts whose template lines widths oscillate with frequency k. The key idea behind our approach is to identify variations in the microdomain interface positions (relative to their averages) as corresponding to a local excess of one of the monomer types. For lamellar systems, our analysis (i) shows that the BCP morphology arises from the constraint that the total mass in a microdomain is fixed, independent of LWR, and (ii) determines a length scale A over which template LWR affects the polymer melt.