Thread: Angle of Purlins

Are these purlins angled too much? What's the rule of thumb? Looks like a post-fire retrofit due to the paint/charred rafters.

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The purlin (the board running perpendicular to the rafters) needs to be sized no less than the required minimum size of the rafters they support. The one in your photo may be too small. It should also be installed with its edge bearing against the rafters, not flat as in the pic.

The braces can be 2x4s, must go from the purlin to a bearing wall, can't be more than 4 ft on center, if longer than 8 feet they need to be braced, and the angle between the brace and the ceiling joists must not be less than 45 degrees.

Refer to IRC 802.5.1

I call them out if they are not close to perpindicular with the rafter.

No rule of thumb, but code complying:
IRC-R802.5.1 Purlins: Installation of purlins to reduce the span of rafters is permitted as shown in Figure R802.5.1. Purlins shall be sized no less than the required size of the rafters that they support. Purlins shall be continuous and shall be supported by 2-inch by 4-inch braces installed to bearing walls at a slope not less than 45degrees from the horizontal. The braces shall be spaced not more than 4feet on center and the unbraced length of braces shall not exceed 8 feet.

Unless the UBC says something different than the IRC, I've corrected that drawing.

Caught redhanded! The 2x6 purlin brace is history in the CBC 2007.