What Is Metal Roof Underlayment, and Why Does It Matter in Florida?
A closer look at the layer nobody sees once the panel goes on — and why MRM built one specifically for it.
Last updated October 22, 2026
Most conversations about a metal roof start and end with the panel — the color, the profile, the wind rating. Underneath that panel is a layer that gets a lot less attention but does a surprising amount of work: the underlayment. This guide walks through what it is, what it's supposed to do, and how MRM's Tornado Underlayment line fits into that picture.
What Is Metal Roof Underlayment, Really?
Underlayment is the material installed directly on the plywood roof deck, before the metal panel goes on top of it. It's the roof's first line of defense against water that gets past the panel, and in a well-built system, it's doing several jobs at once rather than just lying there as a moisture barrier.
How the Layers Stack Up
Water that gets past the panel, at a fastener, a seam or a flashing detail, lands on the underlayment. Because each course of underlayment overlaps the one below it, like shingles, that water runs downslope toward the eave instead of into the deck.
The Three Main Types of Underlayment
Most metal roofs in Florida go down over one of three kinds of underlayment. They differ mainly in how they attach to the deck and how they handle heat.
| Type | How it attaches | Around fasteners | Under hot metal |
|---|---|---|---|
| Asphalt-saturated felt ASTM D226 / D4869 | Nailed down in overlapping rows | Relies on laps; nail holes are not sealed | Standard felt can dry out and turn brittle under high heat |
| Synthetic | Fastened with cap nails | Plastic caps help shed water at fastener holes but don't bond to the deck | Varies by product; check its rated temperature |
| Self-adhered membrane ASTM D1970 | Bonds directly to the deck | Designed to seal around nails and screws, and tested for it | High-temperature versions are made for use under metal |
None of these is automatically right for every roof. Code requirements, roof slope, wind zone, budget and the panel manufacturer's instructions all play a part. In Florida, high-temperature self-adhered membranes are a common choice under metal because of the heat and wind-driven rain.
What the Underlayment Does
MRM Tornado Underlayment is built around five confirmed jobs. It is rated for Class A fire resistance, the highest class under ASTM E108 / UL 790, when installed as part of a tested roof assembly. It acts as a radiant energy barrier, reflecting heat rather than absorbing it. It works as a sound barrier, taking the edge off rain and hail noise on a metal roof. It functions as a secondary water barrier, a self-adhered layer that helps keep water off the deck if the panel above is damaged. And it acts as a self-adhered leak barrier (the kind often sold as “ice and water shield”; in Florida its real job is wind-driven rain), helping seal around fasteners, with edges, seams and penetrations detailed during install.
Where Does It Actually Sit?
MRM Tornado Underlayment installs on the plywood roof deck, underneath the MRM Tornado steel or aluminum panel. It's a paired system: the panel and the underlayment are approved to work together, rather than two products from unrelated suppliers that were never tested together.
The Tornado Lineup, in Plain Terms
There are four products in the current lineup. Two — the SWR 5000 FR and SWR 2000 FR — carry a reflective aluminum face and are marketed as the Radiant Barrier options, with the 5000 FR as the heavier-duty of the pair. A third, the Duo Barrier 1500 HT, is a self-adhered duo-layer product without the aluminum face. A fourth, the White SBS Cap Sheet 4000 FR, is a mineral-coated cap sheet chosen where surface durability matters most. Thickness and roll length vary by SKU, so it's worth confirming specs for the specific product on a given quote rather than assuming one number applies across the line.
What Underlayment Can't Do
- It doesn't hold the roof down. Wind uplift resistance comes from the panels, their clips or fasteners and how the deck is attached.
- It isn't a permanent roof. Every underlayment has a limit on how long it can sit exposed to sun. The panels need to go on, and damaged panels need prompt repair.
- It doesn't replace flashing. Walls, chimneys, valleys, skylights and pipe penetrations still need proper flashing.
- It depends on installation. Laps running the wrong way, wrinkles or a dirty deck can let water through even with a good product.
Florida Code Basics
The Florida Building Code requires underlayment under metal roof panels, and the rules vary with roof slope and wind zone. In the High-Velocity Hurricane Zone (Miami-Dade and Broward counties), roofing products need a Miami-Dade Notice of Acceptance (NOA). Elsewhere in Florida, products show compliance through a Florida Product Approval number or local approval. Your permit documents and local building department have the final word on what your roof needs.
Certifications Worth Checking For
When comparing underlayment products, look for: Miami-Dade County approval, a Class A fire resistance rating, compliance with ASTM D1970 (the standard for self-adhered polymer-modified bitumen membranes), a stated warranty term, confirmation the product is manufactured in the USA, and — where the panel and underlayment come from the same manufacturer — a paired-system approval rather than two products that simply happen to be sold together. MRM Tornado Underlayment carries a standard 20-year limited warranty and a paired-system approval with MRM's own Tornado steel and aluminum panels.
Where This Fits If You're Planning a Roof
If you're already comparing metal panels for a Florida re-roof, it's worth asking what underlayment is included and what it's rated for. MRM Tornado Underlayment is manufactured to pair with MRM's own Tornado panel line, and the certifications above are the ones worth asking about on any quote you're comparing it to.
Frequently Asked Questions
- Is underlayment required under a metal roof in Florida?
- Yes. The Florida Building Code requires underlayment under metal roof panels. The type and installation method depend on roof slope, wind zone and the approvals of the products being used.
- Is synthetic or self-adhered underlayment better under metal?
- It depends on the roof. Self-adhered membranes bond to the deck and seal around fasteners; synthetics are lighter and usually cost less. Under metal in Florida, a high-temperature self-adhered membrane is a common choice because of the heat and wind-driven rain.
- Will underlayment stop leaks if a panel is damaged?
- It can limit how much water reaches the deck for a while, but it isn't designed to replace the panel. Damaged panels should be inspected and repaired promptly.
- What does ASTM D1970 mean on a product label?
- ASTM D1970 is the standard specification for self-adhering polymer modified bituminous underlayment. Products that meet it have passed tests that include adhesion and sealability around nails.
- What does a Class A fire rating mean?
- Under ASTM E108 and UL 790, roof coverings are rated Class A, B or C for resistance to fire from outside the building, with Class A the highest. The rating applies to the roof assembly as tested, not to one layer on its own.
Sources and Further Reading
- ASTM D1970/D1970M: specification for self-adhering polymer modified bituminous underlayment
- ASTM D4869/D4869M: specification for asphalt-saturated organic felt underlayment
- ASTM E108: fire test methods for roof coverings (Class A, B and C)
- Florida Building Code (ICC Digital Codes)
- Florida Product Approval search (Florida Building Commission)
- Miami-Dade County Building (Notice of Acceptance program)