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.

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. Fasten the purlin with 16d common nails into each vertical rafter. Numbers 20 and 22 u s.

Ventilation air follows exactly the opposite path. The photo above shows battens at a tile roof installation in progress. The air gap that battens create between the roof sheathing. 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.

The spacing of purlins on roofs and girts on wall is usually 4 to 6 ft. 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. 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. Load tables specifying safe loads for different spans are available from metal deck manufacturers.

Placing batts between purlins is probably not a top twenty best answer for several reasons. Standard gage are generally used for roofing. Lay the first purlin at the ridge of the roof down to the chalk line beginning at either corner. Purlins at a 45 degree angle still provide sufficient support for installation of the metal roof panels.