Roof Purlin Metal Building Air Gap

Purlins are 2 by 4 feet and are installed much like metal roofing.

Roof purlin metal building air gap.

Warm air is actually lighter than cold air known as avogadro s law essentially condensation occurs at night when the temperature inside a building is warmer than outside. Research has confirmed that creating an air space under a metal roofing system will increase energy savings during both summer and winter months. Warm air holds more moisture than cold air. Do all roofing projects require purlins.

They are installed on top of the roof rafters with a felt underlayment or vapor barrier installed on top. Ventilation air follows exactly the opposite path. This lets any condensation on the underside of the metal roof evaporate. The colder the climate the more essential the air gap.

The metal will be attached to at least 3 4 purlin and 1 2 plywood instead of just 1 2 plywood which should make the overall roofing system stronger. 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. Getting a perfectly sealed vapor barrier under purlins would be nearly impossible to achieve. Metal roof coatings with solar reflectance can help building owners save substantially in annual cooling costs.

In the case of sips it s essential in my view to have an air channel between the roofing underlayment and the metal roofing. As it cools it can t hold as much moisture. Warm wet air rises and meets the cold roof and cools. If i had to choose between a 1 2 inch gap and a 1 inch gap i would be inclined to go with the 1 inch gap if i were building in climate zone 5 or anywhere colder.

On a sloped metal roof there is a vent at the bottom to allow air in and natural convection takes air at the top out notes wingfield. Purlins at a 45 degree angle still provide sufficient support for installation of the metal roof panels. You would have to seal the ridge vent it isn t working anyhow because your building does not have an air intake from enclosed vented soffits. 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 purpose of the purlins is to provide a solid foundation for the metal roofing panels and trim to be screwed to. You have no way to achieve this without a major remodel. The air space allows natural ventilation when there is a vent at the low part of the roof structure and another at the high part of it. They give added support to the roof and also provide a nailing surface for the end panels and drip edge.

You don t even want to go there.