Two-Way Action of Pitched Roofs

A 12-foot-square half-scale model of a house roof was constructed using 11 plywood-gusseted 4-1/2 in 12 pitched trusses spanning a clear 12 feet and spaced at 12-inch centers. Plywood sheathing, 1/4 inch thick, was applied conventionally over the trusses and on the two gable ends, the size of the sheathing nails used being modeled to give load-slip characteristics equivalent to full size. At the eaves the trusses were blocked and rested on a double plate, the plates being connected to the blocking by a 3-1/4-inch-wide strip of 1/4-inch plywood. The roof system was set on supports located 12 inches apart under the truss heels and gable walls. A load of 2 tons of concrete blocks, giving a uniform load of 40 psf, was applied. The supports were then systematically removed until the loaded structure was supported at the four corners only. The structure remained intact until the load was removed 14 days later. Deflection of the system spanning 24 inches (supports to one truss removed) was approximately one-fifth that of a truss supporting load on its own over a span of 12 feet. In practice, the longitudinal strength contributed by the plywood sheathing would not be brought into action be removal of walls but by variations in truss strength and stiffness. The situation may be such that one would be justified in considering the average strength of the trusses. It is apparent that the longitudinal strength depends upon the pitch of the roof and that sheathing would have no strengthening effect on a roof supported by flat trusses.

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