Roof Truss Deflection

The important point to keep in mind when you use your truss calculator is that every truss calculation is completely unique and is based on the size of your roof and its specific dimensions.

Roof truss deflection.

Spaced sheathing or 1x boards may be reduced slightly. Dead load live load and in service creep deflection criteria for roofs subject to ponding loads. 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. Deflection is the bending or sag caused by loading.

Codes provide the maximum allowable deflection limits for floor and roof trusses which is based solely on the truss span. Spacing a live load deflection limited to l 240 maximum and use lumber properties as follows. Vertical deflection criteria for a roof truss supporting drywall is l 180 for total load and l 240 for live load from section 7 6 1 of the truss plate institute s ansi tpi 1 and ibc 1604 3. Criteria for differential deflection from truss to truss or truss to adjacent.

A trusses are spaced less than or equal to 24 in. The truss is a framework consisting of rafters posts and struts which supports your roof. Differential deflection tpi 1 6 2 2 1 non bearing partitions. Because of this change most trusses designed using tpi 1 2014 will have a lower d l deflection than trusses designed using tpi 1 2007.

610 mm on center. 2x4f 2000psif 1100psie 1 8x10 2x6f 1750 psi f 950 psi f 1900 psi e 1 8x10. Allowable spans for 2x4 top chord trusses using sheathing other than plywood e g. The weight of non bearing partitions shall be permitted to be ignored for truss design purposes given the following conditions.

Allowable deflection is generally expressed as a fraction of the span in inches. For a 35 ft. This change is due to a new deflection criterion added to ansi tpi 1 2014 to correspond to the limits established in the ibc for deflection due to dead load plus live load d l. Allowable vertical horizontal or other required deflection criteria.

The amount of flex depends on the magnitude of the load applied span of the member and stiffness of the member. If a design is known to be good but has excessive deflection it points to another cause. Deflection provides critical feedback on truss design. There might be a defect in materials or manufacturing or an excessive load may be placed on the truss.

For example the additional deflection of a truss holding doors suspended in a gable of an aeroplane hangar could prevent the smooth operation of the doors.