The underside of an S-tile includes two parallel ridges designed to lock into the birdstop metal. When installing flat tiles, we use an eaves riser instead of birdstop metal. An eaves riser has a straight edge, instead of a wavy edge, to match the flat tiles.
Some roofers elevate the first course of tile with a raised fascia board rather than an eaves riser.
Using an eaves riser or birdstop metal is simpler and eliminates the need for anti-ponding foam. Laying out the tiles. After the birdstop metal is installed, the first course of tiles is laid out and nailed off. On subsequent courses, the top of each tile is aligned with a chalk line. S-tiles line up vertically like squares on a chessboard, rather than being staggered like courses of brick.