Roof Truss Deflection Limits

See the table below.

Roof truss deflection limits.

Codes provide the maximum allowable deflection limits for floor and roof trusses which is based solely on the truss span. Mitek roof and floor truss manual for architects and engineers mitek industries inc. 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. Allowable spans for 2x4 top chord trusses using sheathing other than plywood e g.

Truss that would allow 2 5 16 total load deflection and 1 live load deflection. Limit the buckling length of members in compression although in a 2d truss the buckling length is only modified in one axis. For example the allowable deflection of a 12ft span floor joist with plaster l 360 is 0 4 12ft divided by 360. Note it gives the allowable deflection based on a fractional span quantity so a larger denominator will yield less deflection.

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. If that same joist had gypsum ceiling l 240 the allowable deflection is 0 6. 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. Spaced sheathing or 1x boards may be reduced slightly.

For floor trusses the deflection in inches due to live load cannot. 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. In calculating load the weight of the stone and setting bed must be considered. 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.

For a 35 ft. 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. Applied must be designed to have a deflection not exceeding l 720 of the span. Some truss design software permits the truss design technician to limit the truss deflection to a specified maximum amount.

A 48 foot span roof truss could have total load and live load deflections of almost 3 1 4 inches and 2 1 2 inches respectively and meet minimum code deflection criteria of l 180 and l 240. On the 70 ft.