Purlin
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A purlin is a longitudinal, horizontal, structural member in a roof. In traditional timber framing there are three basic types of purlin: purlin plate, principal purlin, and common purlin.
A view of a roof using common purlin framing. The purlins are marked in red. This view is from the inside of the building, below the roof. The rafters are the beams of wood angled upward from the ground. They meet at the top of the gable at a ridge beam (also ridge purlin or roof-tree), which has extra bracing to attach it to the rafters. The purlins are the large beams perpendicular to the rafters; from this shot, it appears that there are three purlins on either side of the roof. The sheathing boards are sometimes called the
Roll-formed metal roof purlins, also called roof battens. They are cropped to the angle of the purlin top cuts and can be lapped for joining.
Metal purlins or roof battens screwed to roof gang-nail-type trusses
Hardwood purlins fixed to steel supports on a skillion roof and main roof. House under construction, tropical North Australia.
Rafter
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A rafter is one of a series of sloped structural members such as steel beams that extend from the ridge or hip to the wall plate, downslope perimeter or eave, and that are designed to support the roof
Purlin
…purlin", and "principal purlin". The term plate means a major, horizontal, supporting timber. Purlin plates are beams which support the mid-span of rafters and are supported by posts. By supporting the rafters they allow longer spans than the rafters alone could span, thus allowing a wider building. Purlin…
Common rafters without collar beams form most of this roof. There is not always a ridge board or beam where the rafter tops meet. Under the midsections of the rafters are purlins which support the common rafters and are supported by principal rafters. This roof ends in an octagonal hip.
Rafter and tie-beam joints (Carpentry and Joinery, 1925)