These requirements cover the design and construction of "tin-clad" fire doors which have demonstrated in fire tests, fire performance properties sufficient to warrant their use in closure assemblies having a fire protection rating of up to and including 3 h, when tested in accordance with the Standard Method for Fire Tests of Door Assemblies, CAN/ULC-S104. A door conforming to these specifications consists essentially of a core made up of layers of boards nailed to each other, encased in terne or zinc plates jointed together at their edges and nailed through the seams to the core. The sizes and ratings for three-ply and two-ply doors are designed to enable regulatory bodies to determine the suitability of the door assemblies described in this Standard for use in locations where a fire resistance of a specified duration is required.