Roof Truss Deflection

Allowable deflection is generally expressed as a fraction of the span in inches.

Roof truss deflection.

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. Increased deflection can lead to a reduction of free height under the bottom chord which might prevent or upset the anticipated usage. 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.

Truss that would allow 2 5 16 total load deflection and 1 live load deflection. Spaced sheathing or 1x boards may be reduced slightly. A trusses are spaced less than or equal to 24 in. The weight of non bearing partitions shall be permitted to be ignored for truss design purposes given the following conditions.

Deflection is the bending or sag caused by loading. 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. 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. Differential deflection tpi 1 6 2 2 1 non bearing partitions.

If a design is known to be good but has excessive deflection it points to another cause. Dead load live load and in service creep deflection criteria for roofs subject to ponding loads. Codes provide the maximum allowable deflection limits for floor and roof trusses which is based solely on the truss span. 610 mm on center.

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. 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. 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. All structural members will deflect or flex under load.

Allowable spans for 2x4 top chord trusses using sheathing other than plywood e g. The amount of flex depends on the magnitude of the load applied span of the member and stiffness of the member. Criteria for differential deflection from truss to truss or truss to adjacent. For a 35 ft.

Spacing a live load deflection limited to l 240 maximum and use lumber properties as follows.