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A purlin 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, though they sometimes replace closely spaced rafters in wood frame structures.
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. When made of wood, it is usually covered with some sort of weatherproofing and sometimes an insulation material.
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. Though solid beams of wood are sometimes used as purlins, the purlin most commonly used in roofing is the steel purlin, which 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.
There are three types of steel purlins. Rectangular hollow section (RHS) is a type of purlin often used in roofs where the support structure will be visible when construction is complete. For example, decks and covered patios often use this type of purlin. As the bar is hollow, caps are welded to the ends of the purlin to keep moisture from getting inside and corroding the metal. The rectangular shape gives the roof the same aesthetic quality of a roof supported by wooden beams.
Cee or C purlins are shaped like a squared-off letter C. They are often used for structural support in walls and as floor joists in addition to roofs. Cee purlins may also be used to form braces, ties, or columns in sheds.
The zed or Z purlin is shaped like a letter Z. Its shape allows the purlin to overlap with others at the joints. This gives zed purlins the potential to be much stronger than C purlins. They are mainly used in walls or for large roofing projects.