COP v3.0:fitness-purpose; purlin-creasing

11.2 Purlin Creasing 

Due to improvements in colour coating technology, the level of reflection of new pre-painted roof sheeting is now considered to be higher. Overdriven nails or screws can produce visible distortion on the purlin line in the pan of trapezoidal profiles that cannot be easily remedied.

Trapezoidal profiles with a wide pan manufactured from 0.4 mm steel and 0.7 mm aluminium are particularly susceptible to purlin creasing, and although it does not affect performance, their appearance can be aesthetically unacceptable

It is the responsibility of the roofing contractor to ensure that nails are not overdriven. A nail or screw should only be driven into the purlin to produce a 50% compression of the sealing washer or until the roof is firm. Using too many fasteners should also be avoided, and nails should always be fixed at right angles to the roof.

Before fixing the roof cladding, the contractor should check the alignment of the purlins or girts. Purlins should be aligned within 5 mm tolerance of each other to avoid purlin creasing.

Purlins should be accurately positioned with their top face parallel to the rafter and should be fixed to a straight line.

When appearance is important or where wide pan trapezoidal cladding is close to eye level, heavier gauge cladding should be specified because light gauges such as 0.4 mm steel and 0.7 mm aluminium are likely to show distortion. Purlin creasing will happen on both concave and convex curved roofs if the recommended purlin spacings are exceeded, and great care should be taken to align purlins on such roofs.

Purlin creasing can be exacerbated by roof traffic. 12.6 Walking On Roofs