Purlin Bracing Question

Hey Brothers, I had a roof today about 60 ft across. Purlins spanned the entire way but only half the structure had the purlin bracing. The home is 1985 split level. Doesn’t look like anything has been purposely removed. Any thoughts?

R802.4.5 Purlins. Purlins shall be sized not 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 (or beams) at a slope not less than 45 degrees from the horizontal. The braces shall be spaced not more than 4 feet on center and the unbraced length of braces shall not exceed 8 feet.


Brian A. MacNeish, a non InterNACHI member I unfortunately teased too often, posted some thoughtful references regarding purlins and knee walls. Well worth the read IMO. MB 2008: In architecture or structural engineering, a purlin (or purline ) is a horizontal structural member in a roof.

appears to be over a garage. someone probably removed the braces to add storage up there. as there is some sort of flooring.

perlins and rafters seem to be bowing in also.

And like so many things, code says 2" by 4" which sounds like you must find some dimensional lumber, but standard 2X4s are allowed.
I love the graphic, BTW. I hear confusion over which piece is the purlin (the brace or the horizontal board). I see this done poorly all the time.

Hi Randy,
The second graphic seems to be misleading.The graphic shows the beam bearing on the ceiling joist.Is this acceptable?
Thank You, Joe

The beam should be supported at the ends on load bearing walls and blocked up above the ceiling joists to allow for the beam deflection under design loading.

Thank you .I did a job many years ago similar to that graphic and we did exactly what you described.