Rafters And Roof Ridges Are Normally Under Deflection

Ridge beam double span.

Rafters and roof ridges are normally under deflection.

This roof is composed of rafters which meet a center ridge beam. Ridge beam single span. Ridge beam sizing is based on the span of the beam between supports and the amount of roof load supported by the beam. Deflection criteria default deflection criteria for the following members.

The route taken by the weight of the roof through the framing members to the soil is called the load path. Duration of load is 1 15 snow tributary length 12 0 exclude outer 6 ft of rafters which bear on outside walls the ridge beam. Supporting roof and floor. Removal of the lateral bracing necessary for a roof rafters that are framed without a ridge beam will make the ridge dip noticeably as in the photo below.

A ridge board is a non structural member that serves as a prop for opposing rafters to rest against and connect to. Deflection criteria for rafters roof beams joists and floor beams. Collar ties rafter ties tension beams structural ridge beams. This article describes and illustrates the different types of support that prevents roof sagging and wall bulging at buildings including definitions of collar ties rafter ties and structural ridge beams.

In homes with conventional ridges the rafters support the weight of the roof and transmit the roof load down through the walls to the foundation and finally to the soil. Allowable deflection is generally expressed as a fraction of the span in inches. All structural members will deflect or flex under load. Structural defects undersize roof rafters or trusses that have had their center web members removed to make room for attic storage can cause a roof to sag along its span.

The center beam is probably not terribly thick. Roof beam double span. Ridge beams are required by code if the roof slope is less than 3 in 12 irc section r802 3. You have probably seen this and it is most likely due to roof deflection sag and creep.

Live load 30 psf dead load 14 psf dead load manually increased for roof slope total load 44 psf span is 16 0 to center of posts supporting ridge beam. What you see is the result of weight and time and gravity. The amount of flex depends on the magnitude of the load applied span of the member and stiffness of the member. If the sagging appears in the plane of the roof that is up and down between the ridge and the eave and from side to side the sagging is probably due to deflection and creep of the rafters themselves figures 8 and 9.