Building codes dictate the fire ratings required for walls, partitions, floors, roofs, and other structural elements in buildings. The requirements depend on the function of the elements and the occupancy of the spaces they enclose. The ratings are based on ASTM E 119 tests, in which specimens of the assemblies are subjected to controlled heat. The test is terminated when any one of three endpoint criteria is reached: 1. An average temperature rise of 250 degrees Fahrenheit is measured on the unexposed side of the specimen 2. Heat, flame, or gases escape to the unexposed side, igniting cotton waste samples 3. The assembly fails under the design load (loadbearing construction only) Because masonry, of course, is noncombustible, masonry assemblies usually reach the temperature increase criterion first. CALCULATING EQUIVALENT THICKNESS How long a clay masonry assembly will resist the transfer of heat from a fire is based on its mass. The greater the wall's mass, the higher its heat storage capacity, and the higher its fire resistance rating. Because the type of aggregate used in concrete masonry units affects their fire resistance, ratings are based on both aggregate type and thickness for solid units, or on aggregate type and equivalent solid thickness for hollow units. MULTIWYTHE CALCULATIONS If a wall of hollow units is grouted solid, the increase in fire resistance is greater than the simple increase in equivalent solid thickness. The same is true for multiwythe walls- the whole is greater than the sum of the parts. ADDING PLASTER Applying plaster to one or both sides of a clay or concrete masonry wall increases the fire rating of the assembly. Calculating the fire resistance of masonry walls is a relatively simple and widely accepted procedure. The building permit process will probably be expedited if the code references, equations, and calculations are shown on the drawings for the building official's review.