Metal roofing underlayment is the moisture barrier laid between the roof deck and the metal panels, and the single rule that separates it from ordinary underlayment is heat. A metal panel in direct summer sun radiates surface temperatures near 170 degrees Fahrenheit, and that heat drives straight into whatever sits beneath it. Standard felt softens, sticks, and degrades at those temperatures, so metal roofs call for a high-temperature synthetic or a high-temp self-adhered membrane rated to 240 degrees or higher. Which one you use depends on the panel system and the slope.
This guide covers the metal-specific requirements that general underlayment articles skip: why the high-temp rating is non-negotiable, why a smooth surface matters under standing-seam clips, and how the underlayment choice changes between standing seam and exposed-fastener panels. For the broader material comparison, see our guide to synthetic roofing paper and the full breakdown of roof underlayment types.
What underlayment does a metal roof actually need?
A metal roof needs underlayment rated for sustained high heat, because the assembly runs far hotter than a shingle roof. The two workable categories are high-temperature synthetic (mechanically fastened across the field) and high-temp self-adhered membrane (peel-and-stick, used at vulnerable areas or full-deck on low slope). Standard 15-pound or 30-pound asphalt felt is the wrong choice on most metal roofs because it breaks down under the panel heat.
The reason is the temperature swing. A dark metal panel can hit surface temperatures around 160 to 170 degrees Fahrenheit under summer sun, then drop below freezing overnight. That daily thermal cycle moves every panel and fastener, and the underlayment beneath has to hold up to both the heat and the mechanical rubbing without tearing, bunching, or bonding uncontrollably to the panel above.
Why high-temperature rating is non-negotiable under metal
High-temperature rating matters because ordinary underlayment fails from the top down under a metal panel. Standard asphalt felt softens as it heats, then the organic saturant degrades. In a self-adhered membrane, a low-temp adhesive can flow and bond to the underside of the panel. When that happens, the underlayment fuses to the metal above and the deck below, and there is no clean repair short of a full tear-off.
For field synthetic, look for a product rated to at least 240 degrees Fahrenheit. Sheffield Metals notes that Sharkskin synthetic products withstand up to 260 degrees. For self-adhered membranes at penetrations, valleys, and eaves, the two named benchmarks are Grace Ice and Water Shield HT (rated to 260 degrees) and Atlas WeatherMaster Flexible Ice and Water (stable to 250 degrees). A membrane labeled only for shingle use, with no high-temp rating printed on the roll, does not belong under metal.
| Underlayment type | Typical temp rating | Where it fits on metal | Approx. cost per square |
|---|---|---|---|
| Standard asphalt felt (15/30 lb) | Not rated for sustained high heat | Avoid on metal; degrades under panel heat | $5 to $10 |
| Standard synthetic | Around 220 degrees F | Marginal; low-slope and dark panels push past it | $10 to $20 |
| High-temp synthetic | 240 to 260 degrees F | Field underlayment for most metal roofs | $25 to $50 |
| High-temp self-adhered (peel and stick) | 240 to 260 degrees F | Eaves, valleys, penetrations, or full low-slope deck | $45 to $90 |
Ranges reflect material only and vary by region, brand, and roll size. Full-deck self-adhered on a large roof is the top of the range.
Synthetic vs felt under a metal roof: which wins?
Under metal, high-temp synthetic wins over felt on nearly every metric that matters. Synthetic resists the panel heat, stays dimensionally stable through thermal cycling, sheds water without wrinkling, and weighs a fraction of felt. Felt absorbs moisture, buckles, and degrades faster under the heat load of a metal assembly. The one thing felt has going for it is price, and on a metal roof that saving is a poor trade.
Synthetic also covers more per roll. A synthetic roll typically covers about 10 squares (1,000 square feet) at 2 to 4 pounds per square, while a felt roll covers 2 to 4 squares at 15 to 30 pounds per square. That is fewer seams, faster installation, and less weight carried up the ladder.
| Factor | High-temp synthetic | Asphalt felt (15/30 lb) |
|---|---|---|
| Heat tolerance under metal | 240 to 260 degrees F | Softens and degrades |
| Water absorption | Sheds water, will not wrinkle | Absorbs, buckles when wet |
| Weight per square | 2 to 4 lb | 15 to 30 lb |
| Coverage per roll | About 10 squares | 2 to 4 squares |
| Tear strength | High | Low, tears at fasteners |
| Relative cost | 2 to 5 times felt | Cheapest |
For the same comparison on shingle roofs, see our felt vs synthetic underlayment guide. On metal, the heat rating tips the decision harder toward synthetic than it does on asphalt.
The slick-surface caution most guides skip
On standing seam, the underlayment surface texture matters as much as its heat rating. Standing-seam panels attach with clips that let the metal expand and contract, and those clips ride directly on the underlayment. A textured or granular surface abrades the clip and the panel bottom during thermal movement, and it can catch and bunch. A smooth, slick-film synthetic reduces that abrasion and lets the panel float the way the system is designed to.
This cuts against a habit carried over from shingle work, where a walkable, high-traction surface is a selling point. Under standing seam you want the opposite in the field: a low-friction top film so the panel slides. That trades away some walkability during installation, which is a real safety consideration. Crews manage it with staging and fall protection rather than by picking a grippier underlayment that fights the panel movement. For the panel system itself, see standing seam metal roof cost.
Standing seam vs exposed-fastener: does the underlayment change?
Yes. The panel attachment method changes what the underlayment has to tolerate. Standing-seam clips create concentrated movement and abrasion points, so a smooth, high-temp synthetic (or self-adhered on low slope) is the standard. Exposed-fastener panels are screwed straight through the panel and underlayment into the deck or purlins, so the underlayment is pinned in place and sees less sliding, but every screw is a puncture the membrane must seal around.
- Standing seam: smooth-surface high-temp synthetic in the field; the panel floats on clips, so low friction and heat stability rule.
- Exposed fastener: high-temp synthetic still preferred; because fasteners puncture the sheet, tear strength and the ability to seal around screws matter most.
- Low-slope metal (below 3:12): a fully adhered high-temp membrane over the whole deck is often specified, because mechanical seams and clips leak more readily at shallow pitch. See low-slope metal roofing.
Where self-adhered (peel and stick) is required vs optional
Self-adhered high-temp membrane is required at the roof’s leak-prone details and optional across the open field. Manufacturers and most contractors run peel-and-stick at eaves, valleys, hips, and around every penetration, then cover the field with high-temp synthetic. Running self-adhered across the entire deck is generally not recommended on standard-slope metal because it is costly and can trap moisture, but it becomes the norm on low-slope metal where water sits longer.
- Prime the deck if the membrane calls for it, and let the primer dry to tack-free, usually at least one hour.
- Apply self-adhered membrane at eaves first, running it up past the interior wall line where ice dams form in cold climates.
- Line valleys, hips, and all penetrations with the same high-temp membrane.
- Cover the remaining field with high-temp synthetic, lapping over the self-adhered edges so water always sheds downhill.
In snow-load regions, code often requires an ice-barrier membrane at the eaves regardless of roof type. Our guides to peel-and-stick underlayment and ice and water shield cover where that line falls and current per-square pricing.
What metal roof underlayment costs
Metal roof underlayment runs roughly $25 to $50 per square for high-temp synthetic and $45 to $90 per square for high-temp self-adhered membrane, material only. On a typical roof that mixes self-adhered at the details with synthetic in the field, underlayment lands as a small line item next to the panels but a meaningful one against a shingle budget. High-temp synthetic costs about $15 to $30 more per square than standard synthetic, and that premium buys the heat rating the assembly needs.
The cost trap is spec substitution. A bid that quotes standard synthetic or plain felt under metal panels is cutting a corner that fails from heat, not from water, and the failure shows up years later as a fused, un-repairable layer. Confirm the printed temperature rating on the product, not just the word “synthetic,” before you sign. For the full panel-and-labor picture, see metal roof cost and how to read a metal roof estimate.
Frequently asked questions
Do you need underlayment under a metal roof?
Yes. Underlayment is a secondary water barrier and a slip layer that protects the deck if a panel seam or fastener ever leaks. On metal, it also separates the hot panel from the deck and dampens condensation. Most codes and manufacturer warranties require it, and skipping it can void the panel warranty. Use a high-temp product rated for the heat a metal assembly generates.
What is the best underlayment for a metal roof?
For most metal roofs, a high-temperature synthetic rated to 240 degrees Fahrenheit or higher is the best field underlayment, paired with a high-temp self-adhered membrane at eaves, valleys, and penetrations. Named high-temp self-adhered products include Grace Ice and Water Shield HT and Atlas WeatherMaster. The right choice depends on panel type and slope, with fully adhered membrane favored on low-slope metal.
Can you use felt paper under a metal roof?
Felt is a poor choice under metal because the panel heat softens the asphalt and degrades the organic fibers, causing the felt to buckle, absorb moisture, and break down years before the metal fails. Some low-budget or agricultural jobs still use 30-pound felt, but high-temp synthetic outperforms it on heat tolerance, weight, and tear strength for a modest cost increase.
What temperature rating do you need for metal roof underlayment?
Look for at least 240 degrees Fahrenheit. Metal panel surfaces reach roughly 160 to 170 degrees in summer sun, and self-adhered adhesives can flow at lower ratings, bonding the membrane to the panel underside. High-temp synthetics are commonly rated to 260 degrees, and self-adhered HT membranes to 250 to 260 degrees, giving margin above the assembly’s peak temperature.
Do standing seam and exposed-fastener roofs use different underlayment?
They use the same high-temp material class but with different priorities. Standing seam favors a smooth, slick-surface synthetic so the clip-mounted panels can float and expand without abrading the membrane. Exposed-fastener panels pin the underlayment with screws, so tear strength and sealing around punctures matter more. Both should be high-temp rated, and both use self-adhered membrane at leak-prone details.
Does underlayment stop metal roof condensation?
Underlayment alone does not stop condensation, but the right choice helps manage it. Metal roofs sweat when warm interior air meets the cold panel underside, so ventilation and a vapor strategy matter more than the underlayment by itself. Some assemblies pair a permeable synthetic with proper attic ventilation, while others use an anti-condensation membrane. See our metal roof condensation guide for the full fix.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.