Collar Purlin: Horizontal longitudinal beam running under the collar beams and usually supported by crown posts.
Common Purlin/Principal Purlin: In a roof frame, lengthwise member regularly spaced in sets, connecting principal rafters and carrying the roof sheathing.
Principal Purlin: In a roof frame, lengthwise timber connection principal rafters and carrying common rafters.
Purlins: Horizontal timbers that connect rafter trusses.
Purlin Plate: Timbers used to support common rafters near the centre of their span and itself supported by posts or struts.
Ridge Purlin: In a roof frame, a ridge member, continuous or interrupted by rafter apexes, lying in notches or trenches on one side of the roof; if continuous, sometimes itself trenches where it crosses a principal rafter.