What Goes Under a Metal Roof, and Why It Matters

Underlayment is the layer between your roof deck and the metal. Nobody sees it, most quotes barely mention it, and it is one of the few line items where the cheap choice reliably costs you later. Here is what it does and how to tell whether a quote has it right.

What it is for

Two jobs. First, it is the secondary water barrier. If wind drives rain past a seam or a flashing detail, the underlayment is what keeps that water off your decking. Second, it is a separation layer between the metal and the wood, which matters more on a metal roof than people expect.

That second job is where metal differs from asphalt. A metal roof moves, expanding and contracting across its full length, and it gets hot. A dark panel in July sun can run well past 150°F on the underside. Underlayment rated for shingles is not necessarily rated for that, and the failure is ugly: the asphalt in standard felt softens, sticks to the panel, and tears when the panel moves.

The three types, and where each one fails

Felt. Asphalt-saturated paper, #15 or #30. The traditional choice and the cheapest. It has decades of history, and under a metal roof it is the one I would argue against. It absorbs water if it sits exposed during the install, it wrinkles, and standard felt is not high-temperature rated. If you see #15 felt on a metal roof quote, ask why.

Synthetic. Woven polypropylene or polyethylene. Lighter, stronger, lies flat, does not absorb water. This is the sensible default for the field of a metal roof, but only the high-temperature versions. The rating is printed on the roll. Under metal you want one rated to at least 240°F, and not all of them are. A contractor who cannot tell you the rating is guessing.

Peel-and-stick. A self-adhered rubberized asphalt or butyl membrane that seals around fasteners. It is the best water barrier of the three and the most expensive. It also has the most specific requirements: under metal it must be a high-temp formulation, and it should not be run across a whole deck on an unvented assembly without someone thinking about vapor, because it is a vapor barrier and it will trap moisture in the wood if the assembly cannot dry.

Where each one goes

On most of our installs the answer is a hybrid, and I think that is the right answer for the Midwest:

• Eaves. High-temp peel-and-stick from the edge up past the interior wall line. This is the ice-dam zone. If backed-up water is going to reach the deck anywhere, it reaches it here.
• Valleys. Peel-and-stick, full width. Valleys carry two slopes' worth of water and are the second most common leak point after flashing.
• Penetrations and sidewalls. Peel-and-stick around anything that comes through the roof or meets a wall.
• The field. High-temp synthetic.

Full peel-and-stick over the whole deck is available, and some manufacturers require it for a low-slope warranty. It is a real upgrade and a real cost increase. It is not automatically better on every house.

The limit, stated plainly

Underlayment is a backup, not a fix. It buys time and handles incidental water. It does not make up for a badly detailed flashing or a seam that was never formed. I have pulled panels off roofs where the membrane was doing its job while water poured past a chimney counterflashing that had been caulked instead of built. The membrane was never going to be the answer there.

It also has a shelf life while exposed. Synthetics carry a UV exposure limit, often 90 to 180 days. If a job stalls with underlayment open to the sky past that, it should be replaced rather than covered.

Three questions for any quote

• What is the product, and what is its temperature rating?
• Where does peel-and-stick go, and how far up from the eave?
• Is the underlayment part of the manufacturer's system warranty, or a separate product the roofer chose?

That last one matters more than it sounds. A manufacturer-backed system warranty generally requires their approved underlayment. Mixing a cheaper roll into an otherwise certified assembly can quietly void the coverage you thought you were paying for.

Where this matters most, and least

It matters most on complex roofs, lots of valleys, dormers, low-slope sections, or a house that has already had ice-dam damage. On those, the underlayment spec is a genuine difference between quotes.

It matters least on a simple, steep, well-ventilated gable roof. There, high-temp synthetic across the field with membrane at the eaves and valleys is plenty, and paying for full-deck peel-and-stick is buying insurance against a risk you do not really have.

Now you have the context to read that line on the quote.