Are Purlins Necessary for Proper Installation of Metal Roofing?

The Purpose of Purlins

• Purlins are wood or metal strips that are perpendicular to the roof rafters. In agricultural and some commercial buildings, like warehouses, purlins take the place of solid roof decking. In a barn, which is open to the elements, it’s not feasible to install roof decking that can swell and warp with humidity and temperature fluctuations. Purlins give the roofer a base to which he can attach the metal roofing panels.

Corrugated Metal Roofing

• Metal roofing comes in long panels that run vertically from the ridge of the roof to the gutters. Corrugated metal roofing, the kind that looks like waves, is attached through the lowest part of its indentations. The indentations run vertically, however, so it’s not feasible to install corrugated panels directly to the rafters, which are also vertical, because the rafters are too far apart to secure the panels. Standard residential rafters may be 24 inches apart, and on agricultural buildings, they can be 4 to 6 feet and even farther apart. By installing purlins horizontally over the rafters, the roofer can insert screws as close together as desired.

Standing Seam and Other Types

• Not all metal roofing is suitable for use with purlins. Newer types of residential roofing, such as standing-seam metal, are installed just as traditional shingles are installed, on the top of standard roof sheathing. Metal roofing manufacturers design this type of roofing to enhance curb appeal, as well as to provide a strong roof that can last for decades. A metal roof on a home must meet local building codes, which frequently call for roof decking on the rafters and a moisture barrier. Many types of residential metal roofing make use of hidden fasteners, snap-lock panels or hydrostatic seam-sealing techniques.

Retrofitting Metal Roofs

• Purlins are beneficial when you want to install a new metal roof over an existing metal roof without tearing off the old metal. The purlins are installed horizontally over the existing roof, and the new panels are attached directly to the purlins. This is feasible because metal roofing is lightweight and does not add a substantial stress to the roof structure, as more than one layer of asphalt shingles can. The space between the two metal roofs can be filled with rigid foam insulation board, if desired. Check with your local building codes before retrofitting in this manner.