How should attic ventilation work with a metal roof?
Why ventilation still matters under metal
Metal reflects more sun than most dark shingles, especially with a light or coated finish, so attic heat can be lower. But moisture does not care what the roof is made of. Warm, humid indoor air that reaches the cold underside of the deck or panels on a clear night can condense. Ventilation, a good underlayment, and air sealing the ceiling are what keep that water from soaking the deck.
How metal roof ventilation is built
- Ridge: the deck is slotted as for any ridge vent, and vented closures or vent material sit under the metal ridge cap. Closures must match the panel profile so they block wind driven rain and insects while leaving open area.
- Intake: standard soffit vents work. Where there is no overhang, vented eave closures or edge intake products can be used.
- Panels over purlins: when metal is installed over battens on an old roof, the air space under the panels can be vented too, which helps drying.
- Penetrations: any box vents or pipes need flashing designed for metal, such as flexible pipe flashings and curb style vent bases, sealed with compatible sealant.
Underlayment and condensation
Most metal manufacturers call for a high temperature synthetic or self adhered underlayment. Exposed fastener panels laid over open purlins without a deck are more prone to dripping condensation, which is one reason residential metal in Georgia is usually installed over solid decking with underlayment. Learn more in metal roofing in Georgia and synthetic underlayment vs felt.