Canning and Purlin Creasing

Oil canning, panning, or quilting, is the term used to describe visible waviness of the pan of a metal roof. 

Oil canning is one of the most controversial aspects of tray roofing. Some people accept it as an innate feature of a tray product, others want a flat tray with no visible waviness. It may not be obvious, but it is always present in tray roofing to some extent. The visibility of canning is affected as much by the lighting, line of sight, cleanliness, and gloss levels of the roof, as it is by the degree of canning present in the product.

Clients expecting no canning should be informed of the reality, particularly if the roof runs at an acute angle to one’s line of sight. Canning can also be induced by stretching the material or excessive foot traffic. The substrate must be true to plane and not convex.

The most effective ways to minimise canning in a highly visible situation is to use a profile with a narrower pan and use material with a low-gloss or textured surface. Alternatively, 10 mm closed-cell, high-density, self-adhesive PE foam can be used placed down the centre of te pan, or strips of geotextile mat or other resilient material, to give the pan a slight convex shape.

With self-support tray roofs, excessive foot traffic will accentuate purlin lines, because of the ductility of the metal (grade G300) and the wide flat pan.