Roof Purlin Air Gap

Load tables specifying safe loads for different spans are available from metal deck manufacturers.

Roof purlin air gap.

The spacing of purlins on roofs and girts on wall is usually 4 to 6 ft. Standard gage are generally used for roofing. The map at right which is based on table r806 4 of the irc lists the minimum r values required to prevent condensation in unvented assemblies in various climate zones. Rising along the underside of the angled purlin until it reaches a gap then rising up to the next purlin and so on to exit at the ridge or gable.

Lay the first purlin at the ridge of the roof down to the chalk line beginning at either corner. Snap a chalk line horizontally across the roof two feet down from the top. As the metal roof heats up in the summer time the added air gap in between will act as a buffer instead of the the metal heating up the below shingle roof. But probably the best benefit that comes from using purlins is the air gap it creates between the metal roof and the existing shingle roof.

Ventilation air follows exactly the opposite path. Numbers 20 and 22 u s. An unvented roof assembly is possible only if you keep the roof sheathing warm enough to prevent conditioned air from condensing against it. Insert two equally spaced nails into the rafter.

But a purlin is applied directly onto the roof rafters running perpendicular between them while a batten is nailed to roof sheathing or applied over an existing roof for installation of a new roof. Fasten the purlin with 16d common nails into each vertical rafter. The photo above shows battens at a tile roof installation in progress. The air gap that battens create between the roof sheathing.