In light weight panelized roof configurations, Purlins spacing is defined as which of the following?

• A. Beams: 12'-40' Apart
• B. Ceiling Joists - 24" on Center
• C. Purlins: 8' on Center
• D. Purlins: 12' on Center

Answer: (C)

In light-weight panelized roof systems, purlins are the secondary members that the roof panels tie into, carrying the panel loads and transferring them to the main framing. The spacing is chosen to provide solid edge support for the panels, keep deflection low, and align with typical panel widths and fastening patterns. Eight feet on center is the standard because it gives enough support along the length of the panels to prevent sagging and to provide reliable nailing or screw points, while still keeping materials and labor economical.

Why the other options aren’t the right fit: those describe spacing for different components or use a spacing that would leave the panels insufficiently supported. Beams spacing refers to the primary structural members, not purlins. Ceiling joists pertain to interior ceiling framing, not roof panel support. A twelve-foot on-center purlin spacing would create longer spans between supports, increasing the risk of deflection and inadequate panel backing, which is why eight feet on center is preferred.