Rafters And Roof Ridges Are Normally Under Deflection

A ridge board is a non structural member that serves as a prop for opposing rafters to rest against and connect to.

Rafters and roof ridges are normally under deflection.

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. The route taken by the weight of the roof through the framing members to the soil is called the load path. This roof is composed of rafters which meet a center ridge beam. Allowable deflection is generally expressed as a fraction of the span in inches.

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. All structural members will deflect or flex under load. Ridge beam double span. Ridge beams are required by code if the roof slope is less than 3 in 12 irc section r802 3.

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. You have probably seen this and it is most likely due to roof deflection sag and creep. 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. 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.

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. Typically for better performing floors minimal defection is desired. The center beam is probably not terribly thick. Ridge beam sizing is based on the span of the beam between supports and the amount of roof load supported by the beam.

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. Some of these can support the roof and prevent ridge sagging and wall spreading. What you see is the result of weight and time and gravity. Ridge beam single span.

Roof beam double span. Collar ties rafter ties tension beams structural ridge beams. Deflection criteria default deflection criteria for the following members. Deflection criteria for rafters roof beams joists and floor beams.