Roof Truss Deflection Limits

Some truss design software permits the truss design technician to limit the truss deflection to a specified maximum amount.

Roof truss deflection limits.

Generally for roof trusses the deflection in inches due to live load cannot exceed the span in inches divided by 240 l 240 and due to total load l 180. See the table below. Spacing a live load deflection limited to l 240 maximum and use lumber properties as follows. 3 8 3 1 strongbacks cross bridging or other reinforcement shall be used to limit differential deflection between adjacent framing members.

Truss the limit is 4 5 8 total load deflection and 3 1 2 live load deflection unless the building designer specifies more restrictive criteria. Codes provide the maximum allowable deflection limits for floor and roof trusses which is based solely on the truss span. For the pratt truss and any of the types of truss mentioned below it is possible to provide either a single or a double slope to the upper chord of a roof supporting truss. Per ansi tpi 1 a scissor truss can have up to 1 25 of total horizontal deflection in the absence of stricter limits from the building designer.

Applied must be designed to have a deflection not exceeding l 720 of the span. For floor trusses the deflection in inches due to live load cannot. If that same joist had gypsum ceiling l 240 the allowable deflection is 0 6. 2x4f 2000psif 1100psie 1 8x10 2x6f 1750 psi f 950 psi f 1900 psi e 1 8x10.

Spaced sheathing or 1x boards may be reduced slightly. On a roof with a slope greater than 4 to 12 the live load limit is typically adjusted downward from 20 psf to 15 psf to allow for the relatively greater dead load on the steeper roof. For a 35 ft. Limit the buckling length of members in compression although in a 2d truss the buckling length is only modified in one axis.

On the 70 ft. Scissor trusses may even be designed with more than this amount of horizontal deflection along with a warning that special provisions for lateral movement may be required. Note it gives the allowable deflection based on a fractional span quantity so a larger denominator will yield less deflection. In calculating load the weight of the stone and setting bed must be considered.

Mitek roof and floor truss manual for architects and engineers mitek industries inc. For example the allowable deflection of a 12ft span floor joist with plaster l 360 is 0 4 12ft divided by 360.