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Masonry structures are classed as "Fire-resistant" or "Type I" construction, the highest rating possible. According to the National Institute of Standards and Technology (NIST), construction is considered to be fire-resistant if it has a 4-hour fire rating or can withstand complete combustion of contents without collapse. HOW RATINGS ARE DETERMINED Fire resistance ratings are based on standard ASTM or NFPA fire endurance tests. One side of a specimen is subjected to controlled heat for a maximum of 8 hours and 2,300 degrees Fahrenheit. Fire ratings, generally in 1- or 1/2-hour increments, are assigned according to the elapsed time at which the test is stopped. The test is stopped when any one of three conditions is met: a maximum rise of 250 degrees Fahrenheit is measured on the unexposed side of the wall; heat, flame, or gases escape to the unexposed side igniting cotton waste samples; the wall collapses under the design load (load-bearing construction only). RATING INFLUENCES Fire ratings for brick or clay tile walls depend to some extent on the percent of cored area in the individual units. The percent of core area affects the fire resistance of concrete masonry too. So does the unit thickness and the type of aggregate in the concrete. RATINGS DON'T TELL ALL Fire ratings of masonry walls almost always are based on heat transmission, not on structural collapse. Unlike many wall constructions, a masonry wall maintains its structural integrity far beyond the time indicated by the fire rating.