Main Difference
The main difference between Rafter and Purlin is that the Rafter is a structural members in architecture and Purlin is a structural member in a roof
Rafter
A rafter is one of a series of sloped structural members that extend from the ridge or hip to the wall plate, downslope perimeter or eave, and that are designed to support the roof deck and its associated loads. A pair of rafters is called a couple. In home construction, rafters are normally made of wood. Exposed rafters are a feature of some traditional roof styles.
Purlin
In architecture, structural engineering or building, a purlin (or historically purline, purloyne, purling, perling) is any longitudinal, horizontal, structural member in a roof except a type of framing with what is called a crown plate. In traditional timber framing there are three basic types of purlin: purlin plate, principal purlin, and common purlin.
Rafter (noun)
One of a series of sloped beams that extend from the ridge or hip to the downslope perimeter or eave, designed to support the roof deck and its associated loads.
Rafter (noun)
flock of turkeys
Rafter (noun)
A raftsman.
Rafter (verb)
To make (timber, etc.) into rafters.
Rafter (verb)
To furnish (a building) with rafters.
Rafter (verb)
To plough so as to turn the grass side of each furrow upon an unploughed ridge; to ridge.
Purlin (noun)
A bridging two or more rafters of a roof.