Concrete tiles are made of portland cement, sand and water in varying proportions. The material is mixed and extruded on molds under high pressure. The exposed surface of a tile may be finished with cementitious material colored with synthetic oxide additives. The tiles are cured to reach the required strength. They generally have lugs on their undersides for anchoring to batten strips. There are additional waterlocks or interlocking ribs on the longitudinal edges that impede movement and prevent water infiltration.