Abstract: A plate roofing material is divided into an exterior portion which is exposed when a roof is covered with a plate roofing material and an overlap portion which is concealed under an exterior portion of a plate roofing material superimposed on the overlap portion of the plate roofing material. At least one longitudinal groove is formed in the surface of the exterior portion. The longitudinal groove has wide side surface parts alternating with narrow side surface parts with sidewalls formed of a plurality of stepped portions. The longitudinal groove has a linear bottom part having an approximately constant depth and is approximately V-shaped in cross section.