They are installed on top of the roof rafters with a felt underlayment or vapor barrier installed on top.
Purlin roof support.
The purlins of a roof support the weight of the roof deck.
E purlin is an engineered lvl roof underpurlin to support stick roofs for tile and metal sheet cladding.
Purlin plates are beams which support the mid span of rafters and are supported by posts.
They give added support to the roof and also provide a nailing surface for the end panels and drip edge.
Purlin plates in wooden construction.
A common arrangement is a rafter and purlin system which combines purlins and rafters.
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.
By supporting the rafters they allow longer spans than the rafters alone.
The use of purlins as opposed to closely spaced rafters is common in pre engineered metal building systems and both the ancient post and beam and newer pole building timber frame construction.
The term plate means a major horizontal supporting timber.
Several kinds of purlins exist.
Metal roof purlins are installed horizontally under metal roofs.
A purlin plate in wood construction is also called an arcade plate in european english under purlin and principal purlin.
The roof deck is the wood panel ply board or metal sheeting that creates the surface of the roof.