Since the primary ingredient in brick is clay which is fired to around 20008 F, it is a non-combustible material. As such, it is an excellent cladding choice to resist or confine fires. A brick wall's fire resistance rating can be calculated. This is done by extrapolating from known fire resistance periods for a given thickness of wall. Refer to Technical Notes 16B for further information on how to calculate fire resistance ratings for specific brick walls. Currently, there are four Underwriters Laboratories tests which have assigned fire resistant ratings to specific wall assemblies. They include U302, U418, U425 and U902. U302 yields a 2 hour rating and involves a brick veneer drainage wall with wood studs and gypsum sheathing and wallboard. U418 and U425 vary in rating between æ to 2 hours and are brick veneer drainage walls with steel studs and gypsum sheathing and wallboard. U902 consists of a drainage wall with brick veneer and concrete masonry units and has a 4 hour rating. For further information on fire resistance of brick, see Technical Notes 16 and 16B.