Purlin, or purline, is a horizontal part of the support system in a roof. Purlins offer support for the load of the roof deck or sheathing. They, in turn, are supported by the main rafters or by the building walls themselves.
The use of purlins instead of closely-placed rafters is common in metal buildings and in wood-framed construction. Before there were trusses, purlins were used to extend the support rafters for greater distances than the rafters alone could support.
When used in conjunction with tresses, purlins are attached perpendicular to the truss top chord for further support. A purlin is made of steel.
Sub-purlins (also called roof huggers) are manufactured to connect existing roofing to new roofing. Made in ten-foot increments, sub-parlins are made by a computer- controlled punch press and can be made to clear the profile of any existing roof panel.
A purlin can also be made to any height to allow for any additional insulation between the existing roof and the new roof.