Roof Purlin Deflection

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.

Roof purlin deflection.

In architecture or structural engineering or building a purlin or purline is a horizontal structural member in a roof. Allowable deflection is generally expressed as a fraction of the span in inches. An example is air conditioning put on top of a roof this may well minimize wind uplift whilst increasing dead load downwards. The amount of flex depends on the magnitude of the load applied span of the member and stiffness of the member.

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. Deflection is the bending or sag caused by loading. Image copyright belongs to albion sections limited uk roof purlin are members used to directly support roof sheeting materials and could be made of timber or steel. 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.

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. 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.