Commercial Building Inspector B2Roof Ceiling ConstructionEasy
An inspector observes a steel-framed roof structure under construction. The roof purlins are bolted to the top chords of the steel trusses. What is the primary purpose of these purlins in the roof assembly?
- ATo provide lateral bracing for the truss top chords.
- BTo support the roof decking and transfer loads to the trusses.
- CTo act as the primary structural members resisting gravity loads.
- DTo transfer wind uplift forces directly to the foundation.
Show answer & explanationAnswer & explanation
Correct answer: B. To support the roof decking and transfer loads to the trusses.
Purlins are horizontal structural members in a roof that span between trusses or rafters. Their primary function is to support the roof decking (sheathing or panels) and transfer the loads from the decking (dead, live, snow, wind) to the main supporting members like trusses or beams.
Why the other options are wrong
- A. While purlins can offer some bracing, their primary role is load transfer, not lateral bracing for the top chords (which is often handled by other elements or truss design).
- C. Trusses are the primary structural members for gravity loads; purlins are secondary members supporting the deck.
- D. Wind uplift is transferred through the entire roof system, not directly by purlins to the foundation.
Roof Purlins
Horizontal structural members in a roof that span between main supports (like trusses or beams) to support the roof decking.
- Transfer loads from roof deck to primary structural members.
- Typically run perpendicular to trusses/rafters.
- Can be made of wood, steel, or cold-formed steel.
Memory trick: Roof components work together like puzzle pieces to hold up the sky.