Metal Roof Underlayment: Why Synthetic Beats Felt

Underlayment is the waterproof layer we install directly on the roof deck, beneath the metal panels, and it is the last line of defense that keeps water and fire off the wood deck. Even though the metal sheds almost all the water, in case of wind-driven rain and condensation, the underlayment seals the deck underneath. The underlayment we use is also fire resistant, which allows our metal roof system to be designated as a Class A fire rated roof, the highest fire rating a roof can get.

The best underlayment for a metal roof is a self-adhered, high-temperature, fire-resistant membrane, not old-fashioned felt which soaks up water and is not fire resistant. We install Polyglass XFR, a self-adhered Class A fire-rated underlayment that sticks down to the deck, and seals around every fastener. It gives you a watertight, fire-resistant deck for the full life of the roof.

"Felt" is the old asphalt-saturated paper that makes it look like felt. It absorbs water, wrinkles, tears, and dries out over the years, and it is not fire resistant. Synthetic underlayment is a stronger plastic-based sheet that resists tearing and lasts longer.

Even a nail-down synthetic underlayment leaves thousands of staple holes in the deck. A self-adhered, peel-and-stick membrane is better because it bonds to the entire roof deck instead of being nailed or stapled on, sealing around every nail used to fasten the plywood deck and leaving no open staple holes. That keeps the deck watertight even if a panel is ever damaged, and it also keeps embers from entering those small gaps during a fire. Stick-down underlayment is the most weatherproof and fire-resistant underlayment there is, and it is what we install on every metal roof.

A metal roof needs high-temperature underlayment because metal panels get very hot in the sun, hot enough to melt or break down an ordinary underlayment underneath. A high-temp membrane is rated to withstand that heat without softening, sliding, or losing its seal.

Ordinary felt underlayment is not fire resistant, which is a serious weakness in wildfire country, because once embers get under the panels the felt can ignite and expose the wood deck. The underlayment we install, Polyglass XFR, is Class A fire-rated, so it resists flame and embers and helps the whole roof earn its Class A rating over a wood deck. Fire protection starts under the metal.

No, our fire-resistant self-adhered underlayment is included in every roof we install; it is not an upgrade or an add-on. Cheaper bids skip it and use felt to shave their price, which is one reason a low quote can hide a weaker roof. Even so, working directly with us is typically far lower than competitors, because you cut out the middleman who typically contracts it out (often to us). Standing seam metal roofs generally run about $14 to $18 per square foot.

We are metal roofing specialists, and metal is the only thing we do, so every layer of your roof is built to a higher standard than a general roofer can match. We form each panel on site, lock it down with double clips, seal the deck with fire-resistant self-adhered underlayment, and finish with ember-resistant vents, so the whole system resists water, wind, and fire. You are working directly with the experts other roofing companies hire, at a lower price.