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When dealing with fire retardants, there are typically two standards - time ratings and class ratings. Class ratings are usually Class A, B or C, with 'A' being the best. This is usually measured with the test for ASTM E84, which measure Flame Spread and Smoke Development. It most often applies to NON-structural building materials, like plywood, fascia, wall coverings, and things like that. Time ratings are usually 1, 2 or 3 hours, and apply to STRUCTURAL things like beams, joists, flo …