A metal roof is only as dry as the layers beneath it. Under the panels you want a synthetic or high-temperature self-adhered underlayment over a sound deck, and above the ceiling you want attic ventilation that lets moisture out. Metal roof underlayment and ventilation are the two line items where low bids cut corners, and they are where most call-backs start.
Why the underlayment matters more under metal
Underlayment is the water-resistant sheet between the roof deck and the roof covering. On any roof it is the backup barrier for wind-driven rain, ice dams and the occasional seam that opens up. Under metal it does three more jobs. It handles condensation that forms on the cold underside of the panels on clear nights. It keeps the metal from touching materials that could scratch it or react with it. And it has to survive more heat, because a metal roof in full sun gets hotter at the surface than shingles do and passes that heat straight down.
That heat is why the word high-temperature matters. A standard self-adhered ice-and-water membrane can soften and flow under a dark metal roof in July, and it can bond to the back of the panel so hard that the roof can never be repaired cleanly. High-temperature versions are made to stay put. Their data sheets list the rated temperature, and most panel manufacturers require them wherever a self-adhered membrane is used under metal.
The common types
Synthetic underlayment is a woven or spun plastic sheet, mechanically fastened with cap nails or cap staples. It is light, strong, lies flat and tolerates heat and sun exposure far better than felt. It is the standard field underlayment under metal today. High-temperature self-adhered membrane is a peel-and-stick sheet that bonds to the deck and seals around fasteners, used at the eaves, valleys, penetrations and low slopes, and over the entire roof in some climates and some specifications. Asphalt felt is the old standby; codes often still allow it, but it dries out, tears and sticks under metal, and few metal specialists use it. A slip sheet, often rosin paper, goes under copper and zinc on certain substrates so the metal can move freely and stay separated from materials that would stain or corrode it.
Where the self-adhered membrane goes
At minimum, expect self-adhered membrane along the eaves in any climate that sees ice, running from the edge to a point past the inside face of the exterior wall; in every valley; around chimneys, skylights and pipes; and under any low-slope section. In high-wind coastal zones and on roofs with long, low runs, many installers and manufacturers call for full coverage. Full-coverage high-temp membrane costs more, but it turns the deck into a sealed roof on its own, which is comforting when the panels are the only other layer.
| Underlayment | What it is | Where it belongs | Notes |
|---|---|---|---|
| Synthetic | Woven or spun plastic sheet, cap-fastened | Field of the roof over solid deck | Standard under metal; check the sun-exposure and temperature ratings |
| High-temp self-adhered | Peel-and-stick membrane rated for heat under metal | Eaves, valleys, penetrations, low slopes; full coverage in some specs | Seals around fasteners; required by most panel makers where membrane is used |
| Standard self-adhered | Peel-and-stick membrane for shingle roofs | Not recommended under metal | Can soften, flow and bond to panels |
| Asphalt felt | Asphalt-saturated paper | Allowed by many codes, rarely chosen for metal | Dries, cracks and sticks under heat |
| Slip sheet (rosin paper) | Thin separation sheet | Under copper and zinc on certain decks and membranes | Lets the metal move; prevents staining and reaction |
The deck under it all
On a house, metal panels and their clips are fastened to solid sheathing, plywood or OSB, and the underlayment lies on that deck. Older homes sometimes have spaced boards or skip sheathing that was built for wood shingles; standing seam clips need solid wood to bite, so those roofs usually get new sheathing or furring first. Barns and sheds often have panels screwed directly to purlins with no deck and no underlayment, which is fine for a tractor and not for a bedroom. If your installer finds soft or rotten sheathing during tear-off, it has to be replaced; ask for the price per sheet in the contract before work starts. Our guide to metal roofing over shingles covers the special case of leaving the old roof in place.
Ventilation: why a metal roof still needs it
Every day a household pushes moisture into the air from cooking, showers, laundry and breathing, and some of it drifts into the attic. In summer the attic also fills with heat. Ventilation moves that air out so it cannot condense on cold surfaces in winter or cook the roof in summer. A metal roof does not change those facts. It reflects more sun and sheds snow faster, but the attic underneath still needs intake low at the soffits and exhaust high at the ridge.
How a vented metal roof is built
Standing seam and exposed-fastener panels use a vented ridge: the panels stop short of the peak, a breathable vent material or profile closure fills the gap, and a ridge cap covers it. Intake comes from continuous soffit vents or edge vents, and baffles keep insulation from blocking the path. Codes commonly call for about one square foot of net free vent area for every 150 square feet of attic floor, or one per 300 when intake and exhaust are balanced and other conditions are met. Installers who do metal regularly calculate this; the rest guess. A balanced system also fights ice dams, which form when attic heat melts snow that then refreezes at the cold eave.
Cathedral ceilings and unvented roofs
Some homes have no attic to vent: the insulation sits against the roof deck in a cathedral ceiling or a spray-foamed assembly. Those roofs follow different rules, and the roofer and the insulation contractor need to agree on them. One option that pairs well with metal is above-sheathing ventilation: furring strips create an air channel between the deck and the panels, open at the eave and the ridge. It carries heat and moisture away from the deck and makes the roof cooler in summer, a benefit explained in our guide to metal roof energy savings.
Condensation on the metal itself
When warm, damp air touches a metal panel that has cooled below the dew point, water forms on the underside. On a house with underlayment over a solid deck, that moisture is held by the underlayment and dries out as the day warms. On an open porch, carport or pole barn with panels over bare framing, it drips. The fixes are the same ones above: a deck and underlayment, a vented air space, or a condensation-control fabric that comes bonded to the back of the panel and holds the moisture until it evaporates. Snow country adds one more reason to get this right, which our guide to snow retention and gutters explains.
What a good bid looks like
A complete metal roofing bid names the underlayment product, says where the self-adhered membrane goes, prices deck replacement per sheet, and describes the ridge vent and intake plan. It reads like a system, not a pile of panels. Those few lines are the cheapest insurance you can buy on a roof meant to last 40 to 70 years. Our guide to standing seam metal roofing covers the rest of the system, and the metal roofer directory is where to start comparing the people who will build it.
Frequently asked questions
Does a metal roof need underlayment?
Yes, on a house. The underlayment is the second line of defense against wind-driven rain and condensation, it protects the deck during installation, and it separates the metal from anything beneath it that could abrade or react with it. Open-frame barns and sheds are sometimes built without it, which is why they drip in cold weather. Every residential metal roof should have a synthetic or self-adhered underlayment over a solid deck.
What is high-temp underlayment and why does metal need it?
A metal roof in full sun gets hotter than a shingle roof surface, and that heat passes straight to whatever is under the panel. Standard self-adhered ice-and-water membranes can soften, flow or stick to the metal at those temperatures. High-temperature versions are formulated to stay stable under metal, and their data sheets list the rated temperature. Most panel manufacturers call for high-temp products wherever self-adhered membrane is used under metal.
Can you use regular felt paper under a metal roof?
Codes often still allow asphalt felt, but it is a poor match for metal. Felt dries out and cracks under the heat, tears easily during installation, and can stick to the back of the panels. Synthetic underlayment is lighter, stronger, lies flatter and tolerates heat better, and it has become the standard under metal. If a bid lists felt, ask for a synthetic or high-temp self-adhered product instead.
Why is my metal roof dripping inside when it is not raining?
That is condensation, not a leak. Warm, moist air from inside the building rises, touches the cold underside of the metal and turns to water, which then drips. It is common on porches, sheds and pole barns built with panels straight over framing. The cures are a proper underlayment over a solid deck, a vented air space under the panels, better attic ventilation, or a condensation-control fabric bonded to the panel underside.
How much ventilation does an attic under a metal roof need?
The same as any attic. Codes commonly call for about one square foot of net free vent area for every 150 square feet of attic floor, or one per 300 when intake and exhaust are balanced and other conditions are met. In practice that means continuous soffit intake paired with a ridge vent sized for the roof. Metal panels use a vented ridge cap with closures made for the profile, and the installer should calculate the area rather than guess.