Roof Purlin Air Gap

If you do not completely fill purlin cavities code requires airflow from eave to ridge over top of the insulation.

Roof purlin air gap.

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. Lay the first purlin at the ridge of the roof down to the chalk line beginning at either corner. Ventilation air follows exactly the opposite path. 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.

An unvented roof assembly is possible only if you keep the roof sheathing warm enough to prevent conditioned air from condensing against it. 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. Snap a chalk line horizontally across the roof two feet down from the top. The spacing of purlins on roofs and girts on wall is usually 4 to 6 ft.

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. Numbers 20 and 22 u s. Standard gage are generally used for roofing. The air gap that battens create between the roof sheathing.

Load tables specifying safe loads for different spans are available from metal deck manufacturers. Fasten the purlin with 16d common nails into each vertical rafter. Purlins at a 45 degree angle still provide sufficient support for installation of the metal roof panels. The photo above shows battens at a tile roof installation in progress.