In architecture, structural engineering or building, a purlin (or historically purline, purloyne, purling, perling) is a horizontal beam or bar used for structural support in buildings, most commonly in a roof. Purlins are supported either by rafters or the walls of the building. They are most commonly used in metal buildings.

The purlins of a roof support the weight of the roof deck. The roof deck is the wood panel, ply board, or metal sheeting that creates the surface of the roof.

Several kinds of purlins exist. They are divided into categories based on the material from which they are made and their shape. Different purlins are used for different purposes, including structural support of walls or floors. Purlin is important because without it, there’s no frame for the sheeting on the roof to rest on, making purlins critical to the structure of the roof.

Steel Purlin are light weight, dimensionally stable, accurate and straight. They expand and contract reasonably in extreme temperature changes.

Steel purlin is usually made of cold-formed steel that is thin enough to put screws through. Cold-formed steel is made by rolling or pressing thin sheets of steel into the desired shape. It is less expensive for the manufacturer than hot-rolled steel and is also easier to work with. Though cold-formed steel is stronger than hot-formed steel, it is more likely to break when under pressure rather than bend.

Purlins are manufactured from hot dipped galvanized steel with a coating, in line with other common lightweight steel structural building products. This gives good protection in most exposed internal environments. Run off from, or contact with, materials which are incompatible with zinc should be avoided.

To protect the purlin, they also apply a layer of paint outside them. Zinc and paint in combination (synergistic effect) produce a corrosion protection approximately 2X the sum of the corrosion protection that each alone would provide.