Condensation Under a Metal Roof: Why It Drips and How It Is Fixed
Why Metal Roofs Get Condensation
Condensation forms, in Fabral's words, "when warm, moisture laden air comes in contact with anything that is below the dew point temperature." Think of a glass of iced tea on a humid day. The glass is not leaking; the air around it is dropping its moisture on a cold surface.
Metal roofing is especially prone to it for two reasons that Fabral's technical bulletin points out. Metal conducts heat easily, so it "will get down to the outside temperature quicker" than the wood around it. And metal does not absorb moisture, so instead of soaking in, condensation "runs off it and can drip down onto the ceiling." Wood framing and decking will absorb a little moisture and hide it. Bare metal shows every drop.
That is why it tends to appear when the outside temperature drops, on cool nights and in cold snaps, while the air under the roof stays warm and humid.
Condensation or a Roof Problem?
Fabral notes that some homeowners report condensation as a roof leak "even though it may not have rained for many days." A few clues help tell them apart:
| Points to condensation | Points to a roof problem |
|---|---|
| Dripping on dry, cool mornings or after a cold night | Dripping only during or right after rain |
| Droplets spread across the underside of many panels | Water at one spot, often near a fastener, seam or penetration |
| Worse when the house or building is humid inside | Same spot every storm regardless of indoor conditions |
| Frost or beads of water on exposed metal or nail tips in the attic | Staining that tracks from a single point downhill |
If the water only appears during rain at one location, it is likely a fastener, seam or flashing issue instead. Our metal roof repair page covers those repairs.
Where the Moisture Comes From
Fabral lists the usual sources of the moisture that ends up condensing on a metal roof: showers, laundry, humidifiers, unvented gas fireplaces and kerosene heaters, damp crawlspaces, plumbing leaks, green lumber in new construction, and people. A bathroom or kitchen exhaust fan or a dryer that dumps into the attic instead of outdoors is one of the biggest contributors, and Fabral says these must discharge "to the outside of the house; not into the attic."
Buildings with a lot of moisture inside need extra attention. McElroy Metal lists agricultural buildings with high internal moisture loads, and buildings with elevated interior humidity, among the uses for its anti-condensation membrane.
Fixes on a House: Deck, Underlayment and Vapor Control
- Put the metal over a solid deck with underlayment. Fabral's recommendation is to "always install residential metal roofs over a solid deck with an underlayment under the metal." Any condensation then forms on the deck side, where the wood absorbs some of it, and the underlayment carries moisture down and out at the eave. The 2024 International Residential Code requires underlayment under metal roof panels (R905.1.1), except for structural panels that do not need a substrate.
- Stop humid air at the ceiling. Fabral calls for a continuous vapor barrier between the living space and the attic. A taped, painted drywall ceiling is normally enough. Wood plank ceilings need a membrane above them.
- Seal the shortcuts. Unsealed recessed lights, ceiling fan boxes and pipe openings let moist air bypass the ceiling. Sealing them matters as much as the ceiling itself.
- Send exhaust outdoors. Route bath fans, kitchen hoods and dryers outside.
Fabral also lists attic air movement as part of the picture; that is a broader attic topic, so here we only note it.
Fixes on Barns, Carports and Shops
Open-framed buildings, where panels sit on purlins with no deck beneath them, have nothing to catch or absorb condensation. The options are:
- Anti-condensation membrane. A felt-like backing applied to the underside of the panels at the factory. McElroy Metal describes its version (sold as MoistureLok, its name for the DripStop line) as forming "small pockets beneath the panel substrate to manage condensation before it drips," absorbing moisture at dew point and releasing it as conditions change. McElroy notes the membrane's R-value is nominal and that it is not insulation.
- Insulation with a facing. Insulation keeps the underside of the panel warmer, and a vapor-retarder facing on the warm side keeps humid air off the metal.
- Less moisture inside. Fabral lists unvented heaters among the moisture sources; cutting what goes into the air reduces what condenses.
A membrane has to be ordered with the panels, so it is easiest to solve at installation or during a metal roof replacement.
What Condensation Can Damage
Left alone, repeated condensation can stain ceilings, soak insulation, encourage mold on framing and decking, and corrode metal. U.S. Steel's guidance on Galvalume and galvanized steel lists contact with wet insulation, and metal kept wet for long periods, among the conditions that break down the coating's protective film.
Sources: Fabral Technical Bulletin No. 404, "Condensation and Ice Damming on Residential Roofing" (11/30/2005); McElroy Metal, MoistureLok anti-condensation membrane product information; International Residential Code (2024) R905.1.1; U.S. Steel Technical Bulletin TBP 2005.18, "Conditions Corrosive to Hot-Dip Galvanized and GALVALUME Coated Steel Sheet".
Frequently Asked Questions
Why is water dripping from my metal roof when it is not raining?
Most likely condensation. Warm, humid air touches metal panels that are colder than its dew point, and the moisture condenses and drips because metal does not absorb it.
Do metal roofs sweat?
No. As Fabral puts it, metal roofing doesn’t sweat. The water is moisture from the air condensing on the cold metal.
What stops condensation under a metal roof on a house?
A solid deck with underlayment under the metal, a sealed ceiling that keeps indoor humidity out of the attic, and exhaust fans and dryers that discharge outdoors.
What is a condensation barrier for a metal roof?
Usually an anti-condensation membrane, a felt-like backing applied to the underside of the panels that holds condensation until it can evaporate. It is common on barns and shops and is not insulation.
Can condensation damage a metal roof?
Repeated wetting can stain ceilings, soak insulation, cause mold and contribute to corrosion. U.S. Steel lists wet insulation and long periods of wetness among conditions that harm Galvalume and galvanized coatings.
