The chimney cricket is placed on the high side of a brick chimney on a peaked roof. The actual cricket is a V-shaped, peaked piece of metal set into the small valley that invariably occurs above a square chimney. The cricket is designed to shed water to both sides, so moisture doesn't collect behind the chimney.
Also called a saddle, a cricket can be used in other roofing situations, besides a chimney. However, the purpose is always the same. It's used to divert roof runoff areas where water can collect. A cricket might also be placed where two sections of roof meet and form a valley.
Small crickets can be covered by sheet metal and flashing cement, but larger crickets are first constructed as part of the roof frame. They are then covered with wood sheathing, building paper and asphalt shingles. All crickets need to be designed, so water does collect at the edge of the cricket. This is especially important if ice build-up is possible during the winter.