Roofing felt, often referred to as tar paper or felt paper, is a long-standing component in traditional roofing assemblies. It is an integral layer installed directly onto the roof deck (plywood or oriented strand board, OSB) before the final roofing materials (shingles, tiles, metal panels) are applied. Understanding its precise function is key to realizing its limitations regarding moisture protection.
Short Answer: Roofing Felt Is Water-Resistant, Not Waterproof
While roofing felt provides significant protection against moisture penetration, it does not meet the technical definition of “waterproof,” which implies total imperviousness to water, even under hydrostatic pressure.
Purpose of Roofing Felt in Roofing Systems
The primary role of roofing felt is to act as a secondary weather barrier and a vapor retarder. It is designed to shed water that manages to bypass the primary roofing material (e.g., wind-driven rain getting under shingles or broken tiles) and protect the underlying roof deck from moisture absorption. It also keeps chemicals from the primary roofing material from reacting directly with the decking.
Why It’s Considered Secondary Protection
The primary protection against water is always the outer layer—the shingles, metal panels, or tiles. Roofing felt is the backup. Its inherent composition, typically asphalt-saturated organic or fiberglass mat, makes it highly resistant to water penetration, but a tear or prolonged submersion (like from severe ponding) can allow water to eventually penetrate the felt and reach the deck.
How Roofing Felt Protects Your Roof
Roofing felt performs several essential functions that contribute to the longevity and weather-tightness of a roof assembly.
Temporary Water Shedding
During heavy rainfall or melting snow, the felt works by allowing water that breaches the primary roof surface to run down its smooth surface and off the roof edge. It acts as a slip sheet, guiding moisture harmlessly away from the structure. This is critical for preventing wood rot in the decking.
Protection During Installation
A significant benefit of roofing felt is protecting the bare roof deck during the construction phase. If a rainstorm occurs after the old roof is removed but before the new shingles are installed, the felt ensures that the exposed decking remains dry, preventing delays and costly repairs due to swelling or warping of the wood.
Barrier Against Wind-Driven Rain
In high winds, rain can be forced sideways or even slightly upward, pushing water past the overlaps of shingles. Roofing felt acts as a continuous sheet barrier, preventing this high-pressure water from penetrating directly into the attic space or wall cavities.
Roofing Felt vs. Synthetic Underlayment
In modern construction, synthetic underlayments have become a common alternative to traditional felt, offering improved performance in several areas.
| Feature | Traditional Roofing Felt (e.g., 15 lb or 30 lb) | Synthetic Underlayment |
| Durability | Prone to tearing, cracking, and wrinkling when wet. | Highly resistant to tearing (often 20x stronger than felt) and foot traffic. |
| Moisture Resistance | Water-resistant; can absorb some moisture, which causes wrinkles and deck movement. | Completely water-resistant; many varieties are truly waterproof. |
| Performance in Extreme Weather | Susceptible to cracking in very cold temperatures and softening/shriveling in extreme heat. | Maintains integrity across a wider temperature range; does not absorb moisture or crack. |
| Weight | Very heavy (e.g., 30 lb felt is cumbersome to carry and install). | Very lightweight and easy to handle; a single roll covers significantly more area. |
Durability Comparison
Synthetic underlayment is typically made of polypropylene or polyethylene, offering superior tensile strength. This makes it far more durable against foot traffic and high winds during installation compared to felt, which can easily tear, especially around fasteners.
Moisture Resistance Differences
Felt paper is oil-saturated and can absorb a minimal amount of moisture over time, which can lead to buckling and telegraphing (showing wrinkles) through the finished roof, especially with lighter-weight shingles. Synthetic underlayment is 100% synthetic and will not absorb water, ensuring it remains flat and stable.
Performance in Extreme Weather
Synthetic products are UV-resistant, often capable of being exposed for up to six months without degradation. They are also less prone to becoming stiff and brittle in the cold or sticky and soft in the heat, providing a more consistent and reliable barrier in diverse climate zones.
When Roofing Felt Fails
The lifespan and effectiveness of roofing felt are limited by both its inherent material properties and external factors.
Tears and Punctures
The most common cause of failure is mechanical damage. Dropped tools, heavy foot traffic, or improperly placed nails/staples can puncture the felt. Once the continuous surface is compromised, water can enter the hole and spread beneath the felt, saturating the deck below.
Prolonged Exposure to Water
While felt is water-resistant, leaving it exposed to the elements for weeks or months can lead to UV degradation, thermal cracking, and eventual saturation. If the primary roof fails and allows a continuous stream of water to hit one area of the felt, the material will eventually break down and fail its water-shedding function.
UV Degradation
Asphalt-saturated felt is not designed for long-term UV exposure. Direct sunlight causes the asphalt to break down, drying out the felt, making it brittle, and reducing its effectiveness as a moisture barrier. This is why roofing felt should be covered by the finished roofing material as quickly as possible.
FAQs
Can roofing felt be left exposed?
No, roofing felt should only be exposed temporarily (typically no more than a few days or weeks) during the construction process. Prolonged exposure will lead to rapid UV degradation and a high risk of failure, requiring the felt to be replaced before the final roofing material can be installed.
How long does roofing felt last?
When properly covered by a primary roofing material like shingles, 15 lb or 30 lb roofing felt is designed to last the full lifespan of the shingles (typically 15 to 30 years). Its lifespan is protected by the primary roofing layer above it.
Is synthetic underlayment better?
In most professional opinions, yes. Synthetic underlayment offers superior durability, is easier to handle, does not wrinkle or absorb moisture, and provides better tear resistance and UV protection, making it a more robust and reliable secondary barrier than traditional felt paper.
Planning your next roofing project?
Ensure you choose the correct underlayment for your home’s needs and climate. Contact a qualified roofing professional today to discuss your options.