Roofing Underlayment Types and Why They Matter

What Underlayment Does

The primary roofing material (shingles, tiles, metal panels) is the first line of defense against rain, but it is not perfectly watertight under all conditions. Wind-driven rain can force water sideways under shingle tabs and tile overlaps. Snow melt from ice dams backs up under courses from the bottom edge. Condensation forms on the underside of metal panels during temperature swings. In all of these scenarios, water reaches the underlayment layer, which must prevent it from reaching the plywood or OSB roof deck below.

Underlayment also protects the roof deck during the construction period between deck installation and roofing material installation. If rain hits the exposed deck before shingles are installed, the underlayment prevents water damage to the deck surface and the interior below. On large or multi-day roofing projects, this temporary protection is essential.

Building codes require underlayment on virtually all residential roofing applications. The specific type required varies by location, climate zone, and roofing material. The International Residential Code (IRC) mandates underlayment for asphalt shingle roofs and specifies additional requirements for ice dam protection in cold climate zones.

Asphalt-Saturated Felt (Tar Paper)

Asphalt felt, commonly called tar paper, is the traditional underlayment that has been used for over a century. It consists of a base mat of organic fiber (cellulose) or fiberglass saturated with asphalt to provide water resistance. Felt comes in two weights: 15-pound (single layer) and 30-pound (heavier, more durable).

Fifteen-pound felt costs $0.15 to $0.20 per square foot and is the minimum code-compliant underlayment for standard asphalt shingle roofs in most jurisdictions. It provides basic water resistance but tears easily during installation, can wrinkle when exposed to moisture before the roofing material is applied, and degrades relatively quickly when exposed to UV radiation. If the primary roofing material is ever compromised and UV light reaches the felt, it deteriorates within weeks to months.

Thirty-pound felt costs $0.20 to $0.30 per square foot and provides roughly double the tear resistance and water shedding capability of 15-pound. It is commonly specified under heavier roofing materials like wood shake and tile because it provides a more robust secondary barrier under materials that allow more water penetration than tightly overlapping asphalt shingles.

The main drawbacks of felt underlayment are its susceptibility to tearing during installation (especially in windy conditions), wrinkling when it gets wet before the roofing material is applied, and lower overall durability compared to synthetic alternatives. Felt also becomes slippery when wet, creating a safety hazard for roofing crews working on steep pitches.

Synthetic Underlayment

Synthetic underlayment is made from woven or spun-bonded polypropylene or polyethylene, creating a lightweight, strong, and highly water-resistant sheet. Synthetic products have largely replaced felt as the preferred underlayment for new construction and quality reroof projects because they outperform felt in virtually every measurable category.

Synthetic underlayment costs $0.25 to $0.75 per square foot, with standard products at the lower end and premium woven products at the upper end. The cost premium over felt is $0.10 to $0.50 per square foot, which adds $200 to $1,000 to a typical 2,000 square foot roof. Given the performance advantages, this premium is one of the best value upgrades in any roofing project.

The advantages of synthetic over felt are substantial. Synthetic is 3 to 5 times more tear resistant, so it survives the installation process and wind exposure without damage. It lies flat and does not wrinkle when exposed to moisture. It provides much better traction for workers, reducing fall risk on steep roofs. It weighs 60 to 70 percent less than felt, making it easier to handle and less burdensome on the roof structure. And it resists UV degradation for 3 to 6 months of direct exposure, providing a much longer window of temporary protection if roofing material installation is delayed.

Synthetic underlayment is not self-sealing around nail and screw penetrations, which is an important distinction from self-adhered products. Water can wick through the holes where fasteners pass through the synthetic sheet. This is acceptable under standard conditions where the primary roofing material provides the main water barrier, but in areas prone to ice dams or wind-driven rain, self-adhered underlayment is the better choice for vulnerable sections.

Self-Adhered (Peel-and-Stick) Underlayment

Self-adhered underlayment, commonly called ice and water shield or peel-and-stick, is a rubberized asphalt membrane with an adhesive backing that bonds directly to the roof deck. When a nail or screw penetrates the membrane, the rubberized asphalt self-seals around the fastener shaft, creating a watertight seal at every penetration point. This self-sealing property is what makes peel-and-stick uniquely effective at preventing leaks from ice dam backup, wind-driven rain, and condensation.

Self-adhered underlayment costs $0.75 to $3.00 per square foot, making it significantly more expensive than felt or synthetic. The wide price range reflects differences in thickness (20 mil to 65 mil), self-sealing performance at various temperatures, and brand reputation. Premium products like Grace Ice and Water Shield and CertainTeed WinterGuard occupy the upper end.

Building codes in cold climate zones (IECC Climate Zones 5 through 8) require self-adhered underlayment at all eaves, extending at least 24 inches past the interior wall line. This requirement targets ice dam protection, where backed-up meltwater must be prevented from reaching the roof deck at the eave overhang. Best practice in severe cold climates is to extend ice and water shield 36 inches past the interior wall line or to the first 3 feet of roof surface, whichever is greater.

Beyond code requirements, self-adhered underlayment is recommended at all valleys (where two roof planes intersect and concentrate water flow), around all penetrations (plumbing vents, exhaust fans, skylights), at sidewall transitions (where a roof meets a vertical wall), and anywhere the roof slope changes. These are the areas where leaks are most likely to occur, and self-sealing underlayment provides the strongest secondary defense.

Which Type to Use Where

The most effective and cost-efficient approach is to use self-adhered underlayment in the highest-risk areas and synthetic underlayment across the remaining roof field. This hybrid approach costs approximately $0.50 to $1.25 per square foot averaged across the entire roof, compared to $0.15 to $0.30 for felt-only or $0.75 to $3.00 for full self-adhered coverage.

Apply self-adhered membrane at eaves (minimum 24 inches past interior wall, 36 inches in severe climates), all valleys, around every penetration with an 18-inch margin, at sidewall and headwall transitions, and around skylights and chimneys. Apply synthetic underlayment over the remaining roof deck from the top edge of the ice and water shield to the ridge.

Full-roof self-adhered coverage is justified in extreme environments: coastal hurricane zones where wind-driven rain can breach any primary material, high-altitude mountain locations with severe ice dam conditions, or on very low-slope roofs (2:12 to 4:12) where water drains slowly and has more opportunity to penetrate the primary material.

Underlayment and Roof Lifespan

The underlayment does not directly determine how long your shingles or metal panels last, but it determines how much damage occurs when the primary material fails to keep all water out. A roof with quality synthetic and self-adhered underlayment may experience a missing shingle during a windstorm and suffer zero interior damage because the underlayment catches the water. The same storm on a roof with deteriorated felt underlayment may result in water-stained ceilings, damaged insulation, and mold growth in the attic.

Think of underlayment as insurance within the roof system. The primary material handles 99 percent of water management, but the underlayment handles the 1 percent of events where the primary material is compromised. That 1 percent includes the events that cause the most expensive interior damage.