In architecture or structural engineering or building, a purlin (or purline) is a horizontal structural member in a roof.

In architecture or structural engineering or building, a purlin (or purline) is a horizontal structural member in a roof. Purlins support the loads from the roof deck or sheathing and are supported by the principal rafters and/or the building walls, steel beams etc. The use of purlins, as opposed to closely spaced rafters, is common in pre-engineered metal building systems and some timber frame construction.

In lightweight timber roof construction under purlins were used to support rafters over longer spans than the rafters alone could span. Under purlins were typically propped off internal walls. For example, an 8 x 4 under purlin would support the center of a row of 6 x 2 rafters that in turn would support 3 x 2 roof purlins to which the roof cladding was fixed.

In traditional timber truss construction, purlins rest on the principal rafters of the truss.

In all metal or mixed building roof systems, purlin members are frequently constructed from cold-formed steel, (or roll formed) C or Z sections. The Z sections can be lapped and nested at the supports which creates a continuous beam configuration between the bays.

When C and Z sections are used in wall construction, it is normal to call them girts.

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