Roof Purlin Deflection

In architecture or structural engineering or building a purlin or purline is a horizontal structural member in a roof.

Roof purlin deflection.

In timber construction purlins are nailed to the rafter or supporting trusses while in steel roof construction they are welded or bolted to the rafters or trusses by the means of cleats. Butted purlin system butted connection roof sag system roof angle brace system mono pitch roofs steep pitch roofs shallow pitch roofs long roof slopes curved roofs tiled roofs cantilever purlins introduction zed and cee section range overview product range page 1 sections eaves beam details typical connections. Purlins support the loads from the roof deck or sheathing and are supported by the principal rafters and or the building walls steel beams etc. If purlins are undersized missing or poorly connected to the party wall to support the rafters it can result in deflection which often has a dipping or bulging appearance to the roof surface.

All structural members will deflect or flex under load. The use of purlins as opposed to closely spaced rafters is common in pre engineered metal building systems and both the ancient post and beam and newer pole building timber frame construction methodologies. Allowable deflection is generally expressed as a fraction of the span in inches. Another issue can arise when homeowners unknowingly remove struts and collars in their loft to create extra space for storage again this will cause roof deflection.

An example is air conditioning put on top of a roof this may well minimize wind uplift whilst increasing dead load downwards. Another clients on the odd occasion suspend ceilings to office complexes from roofing systems designed to have deflection ratios of 1 150 when most office ceilings are designed for around 1 300.