A d.p.c. is mainly used at Ground Floor Level, at a minimum of 150 mm above ground level, to prevent moisture from rising. It is laid as a horizontal layer of impervious materials such as bituminous felt (various grades); asphalt, or two courses of slate. Two courses of Blue Engineering Bricks make an effective d.p.c. for walls subject to high compressive loads. It is also necessary to form an impervious barrier in cavity walls when it is bridged at, for example, door and window openings.