The Purpose and Function of Roofing Felt Underlayment
Roofing felt, commonly known as felt paper or tar paper, serves as a critical secondary weatherproofing layer beneath the primary roof covering. When installed over roof sheathing and under shingles or tiles, this material acts as a protective barrier against moisture infiltration. The primary purpose of felt paper is to prevent water from reaching the roof deck in cases where the outer roofing material is compromised, such as from wind-driven rain, ice dams, or damaged shingles. Without this underlayment, even a minor leak in the shingle layer could lead to extensive rot in the roof decking, insulation, and interior finishes.
Beyond moisture protection, felt paper also provides a temporary weather seal during the construction process. Roofers often find that a building can be left exposed to the elements for several weeks after the felt is installed but before the final shingles or tiles are applied. This window of protection is especially valuable in regions with unpredictable precipitation patterns. According to the Asphalt Roofing Manufacturers Association, a properly installed felt underlayment can withstand exposure to rain and snow for up to 30 days before the final roofing material must be applied, giving contractors critical scheduling flexibility on large projects.
The material also improves fire resistance in roof assemblies. Most roofing felts are treated with fire-retardant chemicals or manufactured with mineral-surfaced coatings that achieve a Class II fire rating under ASTM E108 standards. When combined with fire-rated asphalt shingles, the total assembly can achieve significantly better fire performance than the shingles alone. This dual-layer approach to fire safety is required by building codes in many wildfire-prone regions, where roof assemblies must meet specific flame-spread and burning-brand classifications.
Another less obvious function of felt paper is providing a slip-resistant work surface for roofers during installation. The textured surface of organic felt or fiberglass mat gives workers reliable footing on sloped roofs, reducing the risk of falls during the shingling process. This safety benefit, combined with its protective qualities, makes felt paper an indispensable component of professional roofing systems across residential and commercial construction.
Types of Roofing Felt: Grades, Weights, and Material Specifications
Roofing felt is categorized primarily by its base weight, which directly correlates to its durability and protective capacity. The most common types are #15 felt and #30 felt, designations that historically referred to the weight of the material per 100 square feet. However, modern manufacturing processes have changed these specifications. Today, #15 felt typically weighs between 6 and 8 pounds per square (100 square feet), while #30 felt weighs approximately 12 to 15 pounds per square. The heavier weight provides greater tear resistance and longer exposure durability during construction.
| Property | #15 Felt | #30 Felt |
|---|---|---|
| Weight per square | 6-8 lbs | 12-15 lbs |
| Tensile strength (MD) | 28-35 lbf/in | 45-55 lbf/in |
| Perm rating | 5-10 perms | 3-7 perms |
| Max exposure (days) | 14-21 days | 30 days |
| Typical applications | Steep-slope roofs (4:12+) | Low-slope roofs, high-wind areas |
| Roll size | 36 in x 144 ft | 36 in x 72 ft |
The material composition of roofing felt has also evolved significantly over the past three decades. Traditional organic felts are made from recycled paper and cellulose fibers saturated with asphalt. These offer excellent water resistance and cost about 15 to 25 percent less than their fiberglass counterparts. However, organic felts are heavier and more prone to tearing during installation, particularly in cold weather when the asphalt becomes brittle. Fiberglass-based felts, introduced in the 1990s, use a nonwoven glass mat as the reinforcement layer. These felts are lighter, more dimensionally stable, and resist tearing and wrinkling better than organic varieties, though they typically cost more per square foot.
When selecting the appropriate felt type for a project, contractors must consider the roof slope, local climate conditions, and the specific roofing material being installed. For steep-slope roofs with asphalt shingles, #15 felt has been the traditional choice for decades and remains compliant with most building codes. For low-slope applications or roofs in regions prone to high winds and heavy precipitation, #30 felt or a double layer of #15 felt is recommended to provide the additional protection needed. The International Building Code specifies minimum underlayment requirements based on roof slope and climate zone, with heavier grades required for roofs with slopes below 4:12.
Installation Best Practices for Felt Paper Underlayment
Proper installation of felt paper is essential to achieving its intended protective functions. The process begins with preparing the roof deck, which must be clean, dry, and free of protruding nails or debris. The felt is then rolled out horizontally across the roof, starting at the eaves and working upward toward the ridge. Each successive course overlaps the one below it by a minimum of 2 inches horizontally and 4 inches vertically, with greater overlaps required on low-slope roofs or in regions with heavy snowfall. These overlap requirements are specified in ASTM D226, the standard specification for asphalt-saturated organic felt used in roofing.
Fastening the felt securely is critical to preventing wind uplift and water intrusion. Standard practice calls for using galvanized or stainless steel staples or roofing nails with 1-inch-diameter caps, spaced 12 inches apart along the edges of each felt course and 24 inches apart in the center. On roofs with slopes of 6:12 or greater, additional fasteners may be required along the top edge to prevent the felt from sagging or slipping during shingle installation. Contractors working in high-wind zones often apply a continuous bead of approved adhesive along the lap joints to create a more secure seal against wind-driven rain, as recommended in the Florida Building Code for high-velocity hurricane zones.
Special attention must be given to roof penetrations and intersections, which are the most common points of failure in any roofing system. Valleys, hips, ridges, and flashing around chimneys, vents, and skylights all require additional felt layers or specialized detail strips. The typical practice involves installing a 36-inch-wide strip of felt centered over valleys before the main felt courses are applied, then adding a second layer after the valley flashing is in place. These reinforced areas provide critical protection where water volume is highest and where temperature fluctuations create the greatest stress on the weatherproofing membrane. Installers working with flange windows should note that the same felt paper used for the roof can serve as an effective water-resistive barrier around window openings when installed with proper flashing details.
Temperature conditions during installation significantly affect felt paper performance. Asphalt-based felts become stiff and brittle below 40 degrees Fahrenheit, making them prone to cracking when handled or walked on. Conversely, in temperatures above 90 degrees, the asphalt can soften and cause the felt to stretch or wrinkle during installation. The National Roofing Contractors Association recommends storing felt rolls in a temperature-controlled environment for at least 24 hours before installation and scheduling work during moderate weather conditions. When installation in cold weather is unavoidable, felt should be warmed gradually and handled with extra care to prevent cracking at the fold points and edges.
Felt Paper Versus Synthetic Underlayment: Comparing Performance and Value
The roofing industry has seen a significant shift toward synthetic underlayments over the past 15 years, with synthetics now accounting for approximately 35 percent of the residential underlayment market according to industry estimates. Synthetic underlayments, typically made from woven or nonwoven polypropylene or polyethylene, offer several performance advantages over traditional felt paper. They are significantly lighter, weighing about 2 to 3 pounds per square compared to 6 to 15 pounds for felt. Synthetics also exhibit superior tear resistance, with tensile strengths of 60 to 100 lbf/in versus 28 to 55 lbf/in for felt, making them less likely to tear during installation or under wind loads.
Moisture performance also differs between the two material types. Felt paper has a perm rating of 3 to 10 perms, allowing moisture vapor to escape from the roof deck. Synthetic underlayments vary widely in their vapor permeability, ranging from 1 perm or less (Class II vapor retarder) to over 10 perms (Class III), depending on the specific product. The choice between vapor-permeable and vapor-impermeable underlayment depends on the roof assembly design and climate zone. In cold climates where ice damming is a concern, a vapor-permeable underlayment helps the roof deck dry to the exterior, reducing the risk of moisture accumulation and rot. Many synthetic products now include non-slip surface treatments that provide equal or better traction than felt paper, addressing an earlier drawback of early synthetics.
The cost comparison between felt and synthetic underlayment is more nuanced than simple material pricing. #15 felt typically costs $0.08 to $0.15 per square foot, while synthetic underlayments range from $0.12 to $0.35 per square foot. However, the installed cost difference narrows when factoring in labor. Synthetic rolls are wider (48 to 60 inches versus 36 inches) and longer, covering more area per roll and reducing the number of horizontal laps required. A study by Building Science Corporation found that synthetic underlayments can reduce installation time by 30 to 40 percent compared to felt, offsetting much of the material cost premium. For a typical 2,500-square-foot roof, the total installed cost difference between #15 felt and a mid-range synthetic is $150 to $300, or 5 to 10 percent of the total low-slope roofing project cost.
Long-term durability is where the choice becomes most consequential. Felt paper has a proven service life of 15 to 25 years in typical roof assemblies when not directly exposed to sunlight. However, synthetic underlayments can last 30 to 50 years or more, matching or exceeding the lifespan of the primary roofing material. This extended durability is particularly valuable for roofs with steep slopes or complex geometries where replacement access is difficult and costly. The International Code Council now recognizes many synthetic underlayments as equivalent or superior to traditional felts, and an increasing number of building jurisdictions accept synthetics as compliant with code requirements. For homeowners and builders focused on long-term performance, the higher upfront cost of synthetic underlayment often delivers better value over the full life cycle of the roof.