What is a purlin?

• A. Horizontal member between trusses that support the roof
• B. Upright post in the framework of a wall
• C. Inclined beam that supports a roof
• D. The ridge line at the top of a roof

Answer: (A)

A purlin is a horizontal beam that runs between roof trusses to support the roof decking or sheathing. It carries the roof load from the covering down to the trusses, helping to keep the roof span stable. This makes it different from a vertical wall post, which bears loads upward; an inclined beam like a rafter, which slopes from the ridge to the wall; or the ridge line at the top of the roof, which is the peak where roof slopes meet. So the horizontal member between trusses that supports the roof is the purlin.