What is the common size for LWP Purlins?

• A. 6 x 12 inch
• B. 4 x 12 inch
• C. 3 x 9 inch
• D. 2 x 8 inch

Answer: (B)

LWP purlins are horizontal members that carry the roof sheathing and transfer loads to the roof framing, so their size is chosen to resist bending over the typical span and load they must carry. A 4 by 12 inch section provides enough depth to resist bending and maintain stiffness for common roof spans and coverings, making it a standard, widely used size. Smaller dimensions like 2 by 8 are often not adequate for the same spans without closer spacing, while much larger sizes (such as 6 by 12) are heavier and more costly for routine applications. For these reasons, 4 by 12 inch is the common choice for LWP purlins.