Synthetic vs. Felt Underlayment: What Florida Homes Need

Florida roofs work harder than almost any other roofs in the country. Between the summer heat that pushes attic temperatures past 150°F, the near-daily afternoon thunderstorms, and the ever-present threat of a named storm rolling through, the materials under your shingles or tiles matter just as much as the ones on top. One of the most important — and most overlooked — layers is the roofing underlayment, the moisture barrier that sits between your roof deck and your primary roofing material.

If you're scheduling a roof replacement or repairing significant damage, your contractor will ask you to choose between traditional felt underlayment and synthetic underlayment. That choice has real consequences for how well your roof handles Florida's punishing climate. Here's what you need to know before you decide.

What Is Roofing Underlayment and Why Does It Matter?

Underlayment is a secondary water-resistant layer installed directly on top of the roof deck (usually plywood or OSB) before shingles, tiles, or metal panels go on. Its job is straightforward: if water ever gets past your primary roofing material — through a cracked tile, wind-driven rain, or a loose shingle — underlayment is your last line of defense before that water reaches the decking, rafters, and eventually your interior.

In most of the country, this layer goes largely untested for years at a time. In Florida, it earns its keep constantly.

Traditional #30 Felt: The Longtime Standard

For decades, #30 felt — sometimes called tar paper — was the default underlayment across the country and throughout Florida. It's made from a cellulose or fiberglass mat saturated with asphalt, and it comes in at a lower material cost than synthetic alternatives.

Where felt falls short in Florida:

• Moisture absorption. Felt is water-resistant, not waterproof. When it gets wet — whether from condensation, a delay between installation and shingle application, or a leak — it absorbs moisture and swells. Repeated wet-dry cycles cause it to wrinkle, buckle, and ultimately degrade faster than it should. In a state where the average relative humidity sits above 70% for much of the year, that's a real problem.
• Heat damage. Felt becomes brittle under sustained high heat. Florida roof decks regularly reach extreme temperatures in summer, and over time that thermal stress breaks down felt's flexibility, leaving it prone to cracking and tearing — exactly when you need it to be intact.
• Tear resistance during installation. Felt tears easily in wind, which creates a real risk during the window between deck prep and final shingle installation. If a pop-up storm catches an open roof with felt laid down, you can end up with a compromised underlayment before the job is even finished.
• Short exposure window. Most felt manufacturers recommend covering it with primary roofing material within a day or two of installation. In Florida's unpredictable weather, scheduling that tightly isn't always realistic.

Felt still meets minimum code requirements in many applications, and it remains widely used. But "meets minimum code" is not the same as "best choice for this climate."

Synthetic Underlayment: Built for the Conditions Florida Creates

Synthetic underlayment is made from woven or spun polypropylene or polyethylene — essentially a high-tech plastic fabric engineered specifically for roofing performance. The premium you pay over felt is real, but so are the differences in how each product performs in heat and humidity.

Key advantages for Florida homeowners:

• Superior moisture resistance. Synthetic underlayment doesn't absorb water. It sheds it. That makes it far more forgiving when installation spans multiple days or when a sudden storm arrives before the roof is closed in. It also resists the rot and mold growth that trapped moisture can trigger in felt-covered decking.
• Heat tolerance. Quality synthetic underlayments are rated to handle temperatures well above what even a Florida summer can produce, without becoming brittle or losing their integrity. Some products carry UV-exposure ratings of several months, meaning they can be left uncovered far longer than felt without degrading.
• Tear strength and wrinkle resistance. Synthetic underlayments are significantly stronger pound-for-pound than felt. They resist tearing in high winds, and they stay flat and smooth rather than wrinkling — which matters because wrinkles can telegraph through finished shingles and create low spots where water pools.
• Lighter weight and easier handling. A roll of synthetic underlayment covers more square footage and weighs less than the equivalent felt, which makes installation faster and reduces crew fatigue on steep-pitched roofs.

A note on Florida Building Code and insurance:

Florida's building code, updated regularly in response to hurricane experience, sets strict requirements for wind-driven rain resistance and underlayment performance — especially in the High-Velocity Hurricane Zone (HVHZ) that covers Miami-Dade and Broward counties. Many synthetic underlayments carry Florida Product Approval numbers (FL-PA), making them explicitly compliant with these requirements. Some insurance carriers in Florida's difficult market also look favorably on premium underlayments when evaluating policies or setting rates. Your contractor should be able to confirm that any product they specify carries the appropriate Florida Product Approval for your zone.

What Does the Upgrade Actually Cost?

The honest answer is that pricing varies by product tier, roof size, pitch complexity, and regional labor rates. In general, synthetic underlayment adds a moderate premium over felt on material cost alone, but that gap narrows when you factor in the larger coverage area per roll and faster installation time. Over the life of the roof — typically 20 to 30 years for a quality Florida installation — the durability difference is where synthetic truly justifies its price. A compromised underlayment that allows deck rot can turn a routine roof repair into a much larger and more expensive project.

If you're dealing with storm damage and your insurer is covering replacement, this is an ideal moment to discuss the underlayment upgrade with your contractor — the cost difference relative to the total claim is usually modest.

Which Option Is Right for Your Altamonte Springs Home?

For most homeowners in Florida, synthetic underlayment is the stronger long-term choice. The combination of humidity resistance, heat tolerance, wind resistance, and longer safe-exposure windows addresses nearly every chronic roofing challenge this climate creates. Felt remains a workable option in limited applications, but if you're investing in a full replacement or a significant repair, the incremental cost of going synthetic is difficult to argue against.

Understanding the difference is one thing — getting the right product installed correctly by a licensed professional is another. Rune Roofing can connect you with a vetted, licensed local roofer in Altamonte Springs who can walk you through your options and provide a free inspection to assess your current underlayment and overall roof condition. Call us today and we'll match you with a licensed contractor who knows exactly what Florida's climate demands — so your next roof is built to last.