Roof Underlayment: Synthetic vs. Felt, and Where Ice-and-Water Goes

What Underlayment Does

Shingles shed most of the water. Underlayment catches what gets past them: rain driven under the shingles by wind, water at valleys and walls, and leaks around penetrations. It also protects the deck during installation. On a sloped roof in Albuquerque's monsoon season, with intense rain and gusty thunderstorm winds, it's the layer that keeps a lifted shingle from becoming a ceiling stain.

Synthetic vs. Felt

The details, from manufacturer data (manufacturer claims, not independent testing):

• Strength and weight. GAF says its Deck-Armor has at least 600 percent greater tear strength than #30 felt. Owens Corning says ProArmor is up to 12 times stronger and up to 4 times lighter than felt.
• Wrinkling. GAF notes felt can absorb water and wrinkle or warp, and CertainTeed says wet felt can wrinkle enough to show through the shingles.
• UV exposure. Synthetics are rated for limited sun exposure before the shingles go on: 90 days for GAF FeltBuster and Owens Corning ProArmor, 180 days for GAF Deck-Armor. GAF notes these ratings are about UV degradation only, not water, snow or wind.

Felt still meets code. The choice is about performance during and after installation, and what the shingle manufacturer's warranty program calls for.

What New Mexico's Code Requires

New Mexico adopts the 2021 IRC through 14.7.3 NMAC, without amending the underlayment or ice barrier sections.

• Underlayment is required under asphalt shingles, meeting ASTM D226, D4869 or D6757 (R905.1.1). Synthetics typically qualify by meeting those standards' physical requirements or through an evaluation report.
• Low slopes need two layers. On slopes from 2:12 up to 4:12, underlayment goes on in two layers, with a 19-inch starter strip and 36-inch sheets overlapped 19 inches (Table R905.1.1(2)). Asphalt shingles can't go on slopes under 2:12 at all (R905.2.2).
• 4:12 and steeper take one layer, lapped 2 inches, with 4-inch end laps.

Does Albuquerque Require Ice-and-Water Shield?

The IRC requires an ice barrier only in areas the local jurisdiction designates as having a history of ice damming (R905.1.2, Table R301.2). The City of Albuquerque's climatic design table, published in its Uniform Administrative Code and its homeowner permit guide, lists "Ice Shield Under-Layment Required: N/A." So the code doesn't require an eave ice barrier in the city. Confirm with your building department, especially at higher elevations outside city limits.

Self-adhered membrane (often called ice-and-water shield) is still standard in places that see concentrated water, and it's part of manufacturer warranty programs:

• Valleys and penetrations. GAF's System Plus warranty requires leak barrier in valleys and around all penetrations when that category is used. Its Golden Pledge warranty calls for it at valleys, walls, chimneys, vents and skylights.
• Under metal roofs. Metal gets very hot. Owens Corning notes underlayments under metal roofs must meet standards for high-temperature use. GAF describes its StormGuard, with a 250°F compound, as suitable for most metal roofing systems, and Grace's Ice & Water Shield HT is rated to 260°F. Warranty coverage still depends on the specific product and roof assembly.

Code Minimum, Manufacturer Requirements and What We Recommend

Three different things decide what goes under your shingles:

• The code minimum. Underlayment under the shingles, two layers on 2:12 to 4:12 slopes, and no ice barrier required in Albuquerque's climatic design table.
• The manufacturer's requirements. Warranty programs can require specific products and placements, such as leak barrier in valleys and around penetrations, or the manufacturer's own deck protection. These depend on the product and the warranty you choose.
• Recommended extra protection. A quality installation often goes beyond the minimum, such as self-adhered membrane at valleys, walls and penetrations even where code doesn't require it. Your estimate should say which items are which.

What to Look for on an Estimate

1. Which underlayment, by name. "Synthetic underlayment" or a product name, not just "underlayment."
2. Where leak barrier goes. Valleys, around chimneys, skylights and penetrations, and along walls.
3. Two layers on low slopes. If any part of your roof is between 2:12 and 4:12.
4. High-temperature membrane under metal.
5. Whether the warranty program requires specific components. GAF's enhanced warranties require GAF accessories, including roof deck protection.

Our guide to how to read a roofing estimate covers the rest. Timberwolf installs GAF roofing systems; see roof replacement and asphalt shingle roofing.