Roof Underlayment Types: Synthetic, Felt & Peel-and-Stick Compared

Your shingles get all the attention. They’re what people see from the street, what you pick colors for, and what takes the first hit from hail and wind. But the layer directly beneath your shingles — the roof underlayment — is what actually determines whether a storm event becomes a living room ceiling stain.

Understanding roof underlayment types is more important than most homeowners realize. The three main categories — synthetic, felt (15# or 30#), and self-adhering peel-and-stick — perform very differently in Alabama and Gulf Coast conditions. Choose the wrong one, or let a contractor default to the cheapest option, and you may compromise your shingle warranty, your FORTIFIED eligibility, and your first line of defense against wind-driven rain.

This guide breaks down every major underlayment type, compares them head-to-head, covers the GAF products Ridgeline installs, and explains how building codes, manufacturer warranties, and the FORTIFIED program all factor into the right choice for your home.

What Is Roof Underlayment and Why Does It Matter?

Roof underlayment is a water-resistant or waterproof sheet material installed directly on top of the roof deck, before shingles are applied. It sits between two worlds: the structural wood deck below and the shingle layer above. When shingles get damaged, shift in high wind, or develop a gap at a flashing point, the underlayment is what stands between your attic and incoming water.

The layer serves four distinct functions that shingles alone cannot provide:

• Secondary water barrier: Shingles aren’t perfectly watertight — they’re designed to shed water, not seal it. Underlayment provides a backup layer that catches wind-driven rain that penetrates or bypasses the shingles.
• Temporary protection during installation: On a multi-day roofing job, underlayment is what keeps your home dry if shingles haven’t been laid on a section yet.
• Deck protection: It insulates the deck from the harsh chemistry of asphalt shingles, reducing thermal cycling stress on the wood.
• Code compliance: Alabama and Florida building codes both mandate underlayment. The type and coverage required varies by roof slope, region, and wind zone.

None of that is visible once the shingles are on. That invisibility is exactly why some contractors cut corners on this layer — and why asking the right questions before installation begins matters more than people realize.

The Three Main Roof Underlayment Types: An Overview

Every residential underlayment falls into one of three categories. Each has a different material composition, performance profile, and appropriate application. Here is a plain-language overview before we go deep on each one.

Cost estimates: industry averages per square foot of material installed, Q1 2026. Labor not included.

Synthetic Underlayment: The Modern Standard for Alabama Roofs

Synthetic underlayment is made from woven or spun polypropylene or polyethylene — engineered polymer materials that don’t absorb water, don’t wrinkle when wet, and hold up under repeated exposure to Alabama’s summer heat without degrading. This is the product Ridgeline installs as standard across all residential replacements in our service areas.

How It’s Made and Why the Material Matters

Traditional felt paper is literally paper — organic fibers saturated with asphalt. Synthetic underlayment replaces all of that with continuous polymer strands, either woven into a fabric or laid in a random-fiber pattern. The result is a product that:

• Repels water rather than absorbing it (felt absorbs moisture and swells)
• Holds its shape in heat — 140°F attic temperatures common in Alabama summers don’t cause wrinkling or buckling
• Tears at 4–6 times the resistance of #30 felt (by ASTM D4533 trapezoid tear test)
• Stays stable on the roof deck for up to 180 days of UV exposure — essential on large projects or after storm damage when re-roofing may be delayed
• Is lighter per roll, making installation faster and easier on crews working steep pitches

Why Synthetic Is Essentially Required in Alabama

Alabama’s climate puts roofing materials through unusual stress. Summer temperatures regularly exceed 95°F with humidity above 70%, creating a combination that accelerates felt paper degradation. The organic fibers in felt absorb moisture during humid seasons, swell, and can cause visible ridging in the shingles above. In North Alabama’s Madison, Limestone, and Morgan counties, and across the Gulf Coast from Spanish Fort to Mobile, that humidity cycle repeats year after year.

Beyond moisture: most GAF shingle warranties require synthetic underlayment to remain valid. The Golden Pledge warranty — an enhanced GAF warranty on qualifying systems, available through Master Elite contractors on qualifying systems — explicitly requires a qualifying GAF synthetic product as part of the roofing system. Felt paper disqualifies the warranty regardless of shingle quality.

Felt Underlayment (15# and 30#): What You Need to Know

Felt underlayment has been used in residential roofing since the early twentieth century. It remains the cheapest option per square foot and is still technically code-compliant in some applications — but it has real drawbacks that make it a poor choice in Alabama and Gulf Coast conditions.

15-Pound vs. 30-Pound Felt

The pound designation originally referred to the weight per square (100 square feet) of material. Modern manufacturing means a “15-pound” product often weighs closer to 7–8 pounds per square, but the naming convention persists.

• 15# felt: Lighter, less expensive, easier to handle. Less moisture resistance and tear strength. Allowed on roofs with slopes above 4:12 under most codes, but many jurisdictions now require 30# in wind exposure zones.
• 30# felt: Heavier, more resistant to moisture and tearing. Required in Florida by the 2020 FBC (7th Edition), which mandates ASTM D226 Type II or equivalent where felt is used. Better performance, but still inferior to synthetic in high-humidity conditions.

The Core Problem with Felt in Alabama

Felt absorbs water. That’s not a flaw exactly — it’s the nature of organic material. In a climate with low humidity, that absorption and drying cycle causes minimal long-term harm. In Alabama, where a hot, humid summer can keep underlayment damp for weeks at a time, that same absorption cycle leads to:

• Wrinkling and buckling, which creates visible rippling in overlying shingles
• Accelerated degradation — felt paper can become brittle and crack within 5–10 years under severe UV and heat exposure
• Weight increase when wet, adding stress to the deck fasteners holding it in place
• Potential for mold on the organic fiber substrate in areas with poor ventilation

Peel-and-Stick Underlayment: The Waterproof Layer for Critical Zones

Peel-and-stick underlayment — also called self-adhering membrane, ice-and-water shield, or leak barrier — is a different product category entirely. Where synthetic and felt underlayments are water-resistant, peel-and-stick is genuinely waterproof. It bonds directly to the roof deck with a self-adhesive rubberized asphalt backing, sealing around fasteners and lapping over seams without the need for additional adhesive or fasteners.

The name “ice-and-water shield” originated in northern climates where ice dams were the primary threat. In Alabama and the Florida Panhandle, ice dams are rare — but wind-driven rain is not, and self-adhering membrane is standard practice at every vulnerable point in a roof assembly regardless of climate.

Where Peel-and-Stick Is Applied

On a properly installed Alabama or Gulf Coast roof, self-adhering membrane appears at:

• Eaves (rafter tails): First 3–6 feet from the roof edge, where wind-driven rain has the best angle of entry
• Valleys: The V-shaped intersections where two roof planes meet — high water concentration, high failure risk
• Around penetrations: Chimneys, skylights, pipe boots, HVAC curbs — anywhere a fastener or flashing penetrates the deck
• Rakes: The sloping edges of the roof, particularly in high-wind zones
• Low-slope transitions: Any area where the roof pitch drops below 4:12 and water drainage slows

On a standard roof, peel-and-stick covers roughly 15–25% of the total deck area. On a FORTIFIED-designated roof, coverage requirements expand significantly.

The FORTIFIED Connection

The IBHS FORTIFIED Roof standard requires a sealed roof deck — defined as a deck where all seams, edges, and penetrations are sealed against wind-driven water intrusion. One of the approved methods for achieving a sealed deck is full or partial coverage with self-adhering underlayment. GAF’s Deck-Armor can satisfy certain FORTIFIED requirements, and self-adhering products like StormGuard and WeatherWatch are frequently specified at critical zones in FORTIFIED installations.

As of November 1, 2025, IBHS requires FORTIFIED roofs to be installed by a credentialed FORTIFIED Wise Roofing Contractor. The underlayment selection, coverage, and fastening documentation must all meet the 2025 standard and be submitted by a Certified FORTIFIED Home Evaluator.

GAF Underlayment Products: What Ridgeline Installs

As a GAF Master Elite and President’s Club contractor, Ridgeline works within the GAF product ecosystem across all underlayment categories. Here is a breakdown of the specific products we use and when each is appropriate.

Deck-Armor vs. Tiger Paw: Which Should Be on Your Roof?

Both products are premium synthetic underlayments with nearly identical tear strength and UV tolerance. The single deciding factor is breathability.

Deck-Armor allows water vapor from inside the home to pass upward through the underlayment and out through roof ventilation. Tiger Paw does not — it acts more as a vapor barrier on the interior side while shedding water from above. For most Alabama homes, especially older construction where soffit and ridge ventilation may not be optimally sized, Deck-Armor’s breathability provides meaningful protection against trapped moisture, mold, and deck rot.

Tiger Paw is the right call on newer homes with confirmed, well-designed attic ventilation — it costs slightly less and still vastly outperforms felt in every measurable metric.

Head-to-Head: Synthetic vs. Felt vs. Peel-and-Stick

Building Code Requirements in Alabama and Florida

Underlayment is not optional — it’s required by code. What is required varies by state, jurisdiction, and the roof’s wind exposure zone.

Alabama

Alabama adopted the 2021 International Residential Code (IRC) as the baseline residential building code, with local amendments. Under IRC Section R905.1.1 and applicable manufacturer specifications:

• For asphalt shingles on slopes 4:12 and above: one layer of underlayment is required across the full deck surface
• For slopes 2:12 to less than 4:12: low-slope applications require modified installation methods and may require a double underlayment layer
• In high-wind design areas (110 mph or greater): some jurisdictions require self-adhering membrane at eaves and rakes, and enhanced fastening of field underlayment
• Local amendments vary — Huntsville, Madison County, and Gulf Coast jurisdictions may have stricter requirements than the baseline IRC

Florida

Florida operates under the Florida Building Code (FBC), which is among the most rigorous in the country. The 2023 FBC 8th Edition (Chapter 15, Section 1518.2) governs underlayment for residential roofing:

• Where felt is used, it must be 30# or equivalent (ASTM D226 Type II or ASTM D4869 Types III or IV) — the 2020 7th Edition eliminated 15# felt as an approved option
• Self-adhering membrane is required at eaves in the South Florida and High-Velocity Hurricane Zone (HVHZ) regions — the Florida Panhandle is not HVHZ but is subject to high-wind code requirements
• Synthetic underlayments must carry Florida Product Approval (NOA or FL number) to be installed in permitted work — all GAF products Ridgeline installs carry this approval
• Our Pensacola and Navarre operations follow the Escambia County and Santa Rosa County amendments, which apply wind-zone requirements consistent with the Gulf Coast exposure

How Underlayment Affects Your Warranty

The single most consequential decision about underlayment type often isn’t the homeowner’s — it’s the contractor’s. But the consequences land on the homeowner when a claim is filed.

GAF’s full Lifetime Roofing System warranty requires three things: a GAF Lifetime Shingle, any three qualifying GAF accessories, and installation by an eligible contractor. Deck-Armor and Tiger Paw count as qualifying GAF accessories. Felt paper does not.

For the Golden Pledge warranty — lifetime limited manufacturing-defect coverage with 50 non-prorated years for qualifying individual-owned detached homes, lifetime limited workmanship coverage for qualifying individual-owned detached homes, including 25 or 30 non-prorated years depending on the eligible shingles, transferable subject to the issued warranty’s conditions and notification requirements — the installation must use a complete GAF system including qualifying synthetic underlayment. At Ridgeline, every Golden Pledge-eligible job goes down with GAF underlayment. There is no workaround.

The WindProven Warranty and Underlayment

GAF’s WindProven limited warranty, available on Timberline UHDZ and certain other products installed with LayerLock Technology, eliminates the maximum wind speed from the warranty. No maximum wind-speed limit does not mean every storm loss is covered; qualifying products, installation, exclusions, and the separate WindProven terms apply. WindProven requires a qualifying full GAF system. That system must include GAF synthetic underlayment. It cannot be achieved with felt.

Common Installation Mistakes That Undermine Underlayment Performance

Choosing the right product is necessary but not sufficient. These are the most common installation errors we encounter when we’re called in to fix someone else’s work.

1. Insufficient Lap at Seams

Underlayment runs in horizontal courses from the eave up. Each course overlaps the one below — by at least 2 inches for standard-slope roofs, more for low-slope applications. When crews rush and cut laps to 1 inch or skip laps on vertical seams, wind-driven rain can push water through the joint.

2. Skipping Self-Adhering Membrane at Valleys

Valleys carry the concentrated runoff from two roof planes. A valley with only synthetic underlayment — no self-adhering membrane — is relying on a water-resistant product to do a waterproof job. Valleys are the most common leak location on Alabama roofs after a major storm event. Self-adhering membrane at valleys is not optional in our installations.

3. Fastening into the Exposure Zone

When the next course of underlayment is rolled out, it should cover all fasteners from the previous course. Fasteners left in the exposed zone — the part that shows before the next course overlaps — are direct water intrusion points. This error is more common with inexperienced crews working fast.

4. Leaving Gaps Around Penetrations

Pipe boots, HVAC curbs, and skylights need to be integrated with self-adhering membrane — cut and formed around the penetration before the field underlayment is applied, then the field material laps over the top. Doing it in the wrong sequence, or not using self-adhering membrane at penetrations, creates the most reliable future leak location on the roof.

5. Using Felt Under a Manufacturer That Requires Synthetic

This happens when a homeowner gets a low-bid quote, the contractor selects the cheapest compliant option, and the homeowner doesn’t know to ask about underlayment. The shingles go on, the project looks fine — until a warranty claim is filed and the manufacturer’s inspector finds non-compliant underlayment below.

Underlayment for FORTIFIED Roofs: What the 2025 Standard Requires

The IBHS FORTIFIED program is the most important residential roofing standard in Alabama and Gulf Coast Florida. Over 20,000 Alabama homes now carry FORTIFIED designation — and the insurance savings in Baldwin County and Mobile County alone have driven strong adoption, with some insurers offering 25–55% premium reductions for FORTIFIED-certified homes.

The 2025 FORTIFIED Home Standard (effective November 1, 2025) updated the installer credentialing requirement: all FORTIFIED Roof installations must be performed by a FORTIFIED Wise Roofing Contractor with an active directory listing. But the underlayment requirements predate that change and remain equally important:

• Sealed roof deck: FORTIFIED requires the deck to be sealed against wind-driven water intrusion. Approved methods include self-adhering underlayment, specific two-layer synthetic applications, and foam adhesive systems.
• FORTIFIED drawing F-SRD-7 (updated November 1, 2024) specifies the two-layer synthetic approach using 40″ and 48″ wide rolls — matching the dimensions of GAF Deck-Armor’s 48″ roll format.
• GAF Deck-Armor is explicitly listed as a product that “can be used to comply with certain FORTIFIED Roof requirements.”
• Self-adhering membrane at eaves, valleys, and penetrations is required under all FORTIFIED designation levels.

If your roof is in Mobile County or Baldwin County — or in the Florida Panhandle counties around Pensacola — and you’re not pursuing FORTIFIED designation, you’re likely leaving thousands of dollars in insurance savings on the table over the life of the roof.

What to Ask Your Contractor Before Installation

Most homeowners receive a quote that says “underlayment included” without any further detail. Here are five questions that will tell you what you’re actually getting:

1. “What brand and product name is the underlayment?” — “Synthetic” is not sufficient. You want the brand and model. If it’s felt, ask why.
2. “Is it compatible with the manufacturer’s full warranty for the shingles you’re proposing?” — A “yes” answer you can’t verify is worth asking the manufacturer directly.
3. “What self-adhering membrane are you using at the eaves and valleys?” — If the answer is “none,” that’s a problem.
4. “Are you pulling a permit?” — Permitted work requires code-compliant underlayment and gets inspected. No permit, no verification.
5. “If I want FORTIFIED designation in the future, will this installation qualify?” — If the contractor doesn’t know what FORTIFIED is, that’s meaningful information.

Frequently Asked Questions About Roof Underlayment Types