Purlin braces

[Gary Mohel]

Do purlin braces always have to be at least the same size as the rafter it supports? I commonly see 2X6 rafters on one side of a ridge board and 2X8 rafters on the other side and both will have 2X6 purlins. If not do you have to calculate the required rafter size and determine the purlin from that calculation?

[Jerry Peck]

Do purlin braces always have to be at least the same size as the rafter it supports?

Yes ... sort of.

From the 2006 IRC.
- R802.5 Allowable rafter spans. Spans for rafters shall be in accordance with Tables R802.5.1(1) through R802.5.1(8). For other grades and species and for other loading conditions, refer to the AF&PA Span Tables for Joists and Rafters. The span of each rafter shall be measured along the horizontal projection of the rafter.
- - 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 (51 mm by 102 mm) braces installed to bearing walls at a slope not less than 45 degrees from the horizontal. The braces shall be spaced not more than 4 feet (1219 mm) on center and the unbraced length of braces shall not exceed 8 feet (2438 mm).

The purlins are to be sized no less than the "REQUIRED" size of the rafter.

If you see 2x6 rafters it would be a safe bet that the purlins should also be 2x6.

If you see larger rafters, maybe or maybe not.