Roofing paper, often called felt or underlayment, plays a critical role in roof performance, but its waterproofing capacity varies widely by type and installation. Understanding whether roofing paper is waterproof helps homeowners make informed choices about materials, maintenance, and leak prevention.
| Material | Common Name | Water Resistance | Typical Use |
|---|---|---|---|
| Asphalt-Saturated Felt | Traditional Roofing Felt (15/30 lb) | Water-Resistant, Not Fully Waterproof | Underlayment Under Shingles |
| Rubberized Asphalt | Self-Adhered Underlayment | Highly Waterproof | Ice/Water Barrier, Edge Protection |
| Synthetic Fibers | Synthetic Underlayment | Highly Water-Resistant, Fast-Draining | Lightweight Underlayment, Secondary Barrier |
| Peel-And-Stick Membranes | Butyl/Rubber Membrane | Waterproof | Critical Areas: Valleys, Eaves, Penetrations |
What Roofing Paper Is And How It Works
Roofing paper historically refers to asphalt-saturated felt made from organic or fiberglass mats saturated with asphalt; it serves as a secondary barrier beneath shingles. Its primary job is to provide temporary protection from moisture and to improve fire resistance and shingle adhesion, not to act as the main waterproof layer.
Types Of Roofing Paper And Their Water Resistance
There are three main categories: traditional asphalt felt, synthetic underlayment, and rubberized/asphaltic self-adhered membranes. Each type offers different levels of water resistance and durability, so identifying the material is essential when assessing waterproofing.
Asphalt-Saturated Felt (15 lb And 30 lb)
Traditional felt is available in 15-pound and 30-pound weights; 30 lb is thicker and more durable. This material is water-resistant and can handle brief exposure, but it will absorb moisture over time and is not fully waterproof.
Synthetic Underlayment
Synthetic underlayments are made from polypropylene or polyester and are engineered to be lighter, stronger, and more water-resistant than felt. They shed water quickly and resist tears, providing better temporary protection while the roof is open or during severe weather.
Rubberized Asphalt/Peel-And-Stick Membranes
Rubberized asphalt membranes are self-adhering and provide a continuous waterproof barrier when installed correctly. These products are commonly used for ice and water barriers, eaves, and roof penetrations because they are effectively waterproof.
Can Roofing Paper Be Considered Waterproof?
In general, most traditional roofing paper is not fully waterproof. Asphalt-saturated felt is designed to resist moisture but will absorb and hold water if continuously exposed, so it should not be relied upon as the primary waterproofing layer.
Synthetic underlayments and rubberized membranes, however, can be highly water-resistant or fully waterproof depending on product specifications. Manufacturers will often label a product as waterproof or as a water and ice shield when it meets those performance criteria.
Where Waterproof Underlayment Is Needed
Waterproof underlayments are recommended in critical areas such as eaves, valleys, roof penetrations, dormers, and low-slope roofs. Building codes in many climates require ice-and-water barriers at eaves and valleys to prevent damage from ice dams and wind-driven rain.
Performance Factors That Affect Waterproofing
Installation quality, seams, fasteners, and exposure time affect the waterproof performance of underlayments. Even a waterproof membrane can fail if seams are not sealed or if it is improperly fastened or cut during installation.
Seams And Flashing
Seams must be overlapped and sealed per manufacturer instructions, and flashings must be integrated to channel water away. Poorly detailed seams are a common source of leaks, regardless of the underlayment’s rated performance.
Fasteners And Penetrations
Nails and staples used to secure underlayment create potential entry points for water unless products are self-adhering or designed to seal around fasteners. Rubberized underlayments that bond to the deck reduce leakage risk at fasteners.
UV Exposure And Temporary Protection
Some underlayments degrade if left exposed to UV light for extended periods; synthetic ones typically have better UV resistance. Roofing paper is intended for temporary protection during installation and should be covered by shingles within the recommended exposure window.
Comparing Durability, Weight, And Cost
Traditional felt is inexpensive and heavy, while synthetic underlayments cost more but offer better tear resistance and lighter handling. Rubberized membranes are the most expensive but provide the best waterproofing where it’s most needed.
| Property | 15/30 lb Felt | Synthetic Underlayment | Rubberized Membrane |
|---|---|---|---|
| Cost | Low | Medium | High |
| Waterproofing | Limited | High Water-Resistant | Waterproof |
| Tear Resistance | Low-Medium | High | High |
| Weight | Heavier | Lightweight | Variable |
Building Codes And Manufacturer Recommendations
Local building codes and shingle manufacturer warranties often specify the type of underlayment required. In many U.S. jurisdictions, an ice-and-water shield is required along eaves and in valleys to meet code and warranty conditions.
How To Choose The Right Underlayment For Different Roofs
Choice depends on climate, roof slope, material, and budget. For steep-slope asphalt shingle roofs, synthetic underlayment is a strong choice; for vulnerable roof edges and low slopes, rubberized membranes are preferred.
Cold Climates And Ice Dams
In areas with freeze-thaw cycles and ice dams, a self-adhered ice-and-water barrier along eaves and valleys prevents water from backing up under shingles. This strategy significantly reduces the risk of interior water damage during winter months.
Low-Slope Roofs
Low-slope and flat roofs require membranes designed for ponding water and longer-term exposure. Synthetic or rubberized waterproof membranes are recommended over standard felt on these assemblies.
Installation Best Practices For Waterproofing
Proper installation matters as much as product choice. Following manufacturer guidelines for overlap, fastening, flashing, and sealing ensures the underlayment performs as intended.
- Start At The Eave: Lay ice-and-water barrier from eave to the field per code.
- Overlap Correctly: Maintain the manufacturer-specified overlap on seams.
- Seal Penetrations: Use compatible sealants and flashing boots at pipes and vents.
- Minimize Exposure Time: Complete shingle installation within recommended exposure limits.
Common Misconceptions About Roofing Paper
Many homeowners assume roofing paper is a waterproof shield; this is not universally true. Traditional felt provides weather resistance but is not a substitute for proper flashing or a fully waterproof membrane in vulnerable areas.
Signs That Underlayment Is Failing
Symptoms of underlayment failure include interior stains near eaves, wet insulation, and soft or rotted roof decking. Early detection and repair of underlayment defects prevent more extensive structural and interior damage.
Repair And Replacement Considerations
Small underlayment breaches can sometimes be repaired with patching materials, but widespread degradation usually requires replacement. When reroofing, upgrading from felt to a synthetic or rubberized underlayment can improve long-term waterproofing performance.
Cost-Benefit Analysis: Upgrading Underlayment
While premium underlayments raise upfront costs, they can reduce leak risk, extend roof life, and protect interior finishes. Investing in higher-performance underlayment often yields savings by avoiding water damage repairs and warranty issues.
Questions To Ask A Roofing Contractor
Homeowners should ask which underlayment is recommended, where the waterproof membrane will be installed, and how seams and flashings will be handled. Request manufacturer product data sheets and ensure the contractor follows installation instructions to preserve warranties.
Key Takeaways For Homeowners
Not all roofing paper is waterproof; performance depends on material and installation. For critical areas and vulnerable climates, select rubberized or self-adhered membranes and ensure professional installation to achieve true waterproof protection.