What causes condensation on a metal roof?
Short answer. Condensation forms when the underside of the metal cools below the dew point of the air touching it, which happens on clear nights when the panels radiate heat to the sky while humid air sits beneath them. It is most common on porches, sheds, barns and garages with metal over open framing, and on any metal roof without proper underlayment and ventilation.
The physics in one minute
Metal is thin and sheds heat fast. After sunset, a metal roof facing a clear sky can drop below the outside air temperature. Georgia air is often close to saturated on summer and fall nights, so its dew point is high and easy to reach. When the panel's underside gets colder than the dew point, moisture condenses on it and drips. Morning sun dries it again, which is why the problem is often discovered as rust spots or water stains rather than seen happening.
Where it shows up
- Open-framed buildings: metal on purlins over a barn, carport or shop, with the underside exposed to humid air.
- Spaces with moisture sources: a garage where wet cars park, an enclosed porch with a hot tub, a workshop with a fuel-burning heater.
- Houses with poor attic ventilation: metal over a deck can still sweat if moist attic air reaches it and there is no proper underlayment.
Prevention
- On homes, install solid decking with a synthetic or high temperature underlayment, so any condensation meets a waterproof layer instead of wood.
- On open framing, use panels with a factory anti-condensation fleece, or add insulation with a vapor retarder.
- Ventilate with eave intake and a vented ridge closure.
- Cut down moisture sources inside the space.
For the attic side of the story see roof ventilation basics; for underlayment choices see synthetic underlayment vs felt.