How thick does a 2-hour masonry wall have to be?
This depends on the type of wall and the type of framing members that frame into the wall. For example, a nominal 4-inch clay masonry wall has a rating of 1 1/4 hours, but it can achieve a 2 1/2-hour rating if plaster is applied to both sides and no combustible members are framed into the wall. With combustible members framed into the wall, the wall must be 8 inches of solid masonry with plaster on the exposed side. Without plaster, the rating of this same wall system is reduced to 2 hours. The Brick Institute of America Technical Note 16 provides fire-resistance values of several different loadbearing clay brick walls. Technical Note 16B explains the formulas used to calculate fire-resistance ratings. The fire-resistance rating of concrete masonry walls depends on the type of aggregate used in the units. A 2-hour rating can be achieved by a wall with an equivalent thickness of as little as 3 1/5 inches when expanded slag or pumice is used or 4 1/2 inches when siliceous gravel is used as the aggregate. For cored units, the equivalent wall thickness is calculated by multiplying the percent solid by the wall thickness. Thus, an 8-inch unit that is 55% solid would have the equivalent thickness of just over 4 inches. NCMA Tek 7-4 discusses the use of core fillers to increase the fire resistance of concrete masonry. Cells that are solidly filled with grout, approved insulation, or dry granular material can in some cases allow 4-inch hollow units to have an equivalent thickness of 4 inches.