Felt paper (also called roofing felt, tar paper, or building paper) is one of the most important but often misunderstood materials in building construction. It serves as a secondary weather-resistant barrier beneath roofing, siding, and other exterior finishes. Understanding what felt paper does, how it works, and when to use it is essential for any builder or homeowner.
The Primary Functions of Felt Paper
Felt paper serves several critical functions in a building envelope:
| Function | Description | Importance |
|---|---|---|
| Secondary weather barrier | Catches water that bypasses the primary roof or wall covering (shingles, siding) | Critical – prevents water from reaching the structure |
| Moisture drainage plane | Provides a path for water to drain down and out of the wall or roof assembly | Essential for moisture management |
| Air infiltration barrier | Reduces air movement through the building envelope | Improves energy efficiency |
| Temporary weather protection | Protects the structure during construction before the final siding or roofing is installed | Important during construction phase |
| Separation layer | Prevents incompatible materials from contacting (e.g., asphalt shingles against wood decking) | Prevents chemical reactions and staining |
| Vapor retarder (some types) | Slows moisture vapor transmission through the assembly | Varies by climate and assembly design |
Types of Felt Paper
| Type | Weight (per 100 sq ft) | Material | Water Resistance | Permeability (perms) | Common Use |
|---|---|---|---|---|---|
| #15 asphalt-saturated felt | 15 lbs | Organic felt + asphalt | Moderate | 5-10 (Class II vapor retarder) | Roofing underlayment |
| #30 asphalt-saturated felt | 30 lbs | Organic felt + asphalt | Good | 3-5 (Class II vapor retarder) | Steep-slope roofing, under slate |
| Synthetic/polypropylene underlayment | Varies (typically 8-12 lbs) | Woven polypropylene | Excellent | 2-10 (varies) | Modern roofing underlayment |
| Grade D building paper (asphalt-saturated) | Varies | Kraft paper + asphalt | Good | 5-10 (Class II vapor retarder) | Wall sheathing wrap |
| Grade B building paper (asphalt-saturated) | Varies | Kraft paper + more asphalt | Better | 3-7 (Class II vapor retarder) | High-performance wall wrap |
| Non-perforated felt | 15-30 lbs | Asphalt felt (no perforations) | Very Good | 1-3 (Class I or II) | Under concrete, ice dam protection |
Felt Paper in Roofing Applications
In roofing, felt paper is installed between the roof deck and the shingles. Here is how it performs in various conditions:
Water Testing Results (ASTM D226)
| Felt Type | Water Absorption (24 hr) | Pliability (low temp) | Tensile Strength (MD, lbs/in) | Service Life (UV exposed) |
|---|---|---|---|---|
| #15 organic felt | 15-25% | Moderate (freezes at 25°F) | 25-35 | 30-60 days |
| #30 organic felt | 10-20% | Moderate (freezes at 25°F) | 35-50 | 60-90 days |
| Synthetic underlayment | 1-3% | Excellent (to -20°F) | 40-70 | 6-12 months |
Installation Best Practices
Roofing Application
- Start at the eave (bottom) and work up, overlapping each course
- Horizontal overlaps: minimum 2 inches for #15, 4 inches for #30
- Vertical overlaps (end joints): minimum 6 inches, stagger between courses
- Fasten with cap nails or staples at 12-inch spacing on edges, 24-inch spacing in field
- Install a self-adhering ice and water shield at eaves (3 ft minimum in cold climates)
- Avoid walking on wet felt (can tear or stretch)
Wall Application
- Install horizontally, starting at the bottom of the wall
- Overlap each course minimum 2 inches
- Wrap around corners (minimum 12 inches)
- Install flashing before siding at windows, doors, and penetrations
- Staple or nail at 12-inch spacing
Felt Paper vs. Synthetic Underlayment
| Characteristic | Felt Paper (#15 or #30) | Synthetic Underlayment |
|---|---|---|
| Weight per roll | 60-80 lbs | 25-40 lbs |
| Tensile strength | 25-50 lbs/in | 40-70 lbs/in |
| UV resistance | 30-90 days | 6-12 months |
| Cost per sq ft | $0.04-0.08 | $0.08-0.15 |
| Slip resistance (walking) | Moderate (slippery when wet) | Good (textured surfaces available) |
| Moisture absorption | 10-25% | 1-3% |
| Permeability | 5-10 perms | 2-10 perms (varies) |
| Environmental impact | Some asphalt content | Petroleum-based plastic |
Common Myths About Felt Paper
Myth: Felt paper is a waterproof barrier.
Fact: Felt paper is water-resistant, not waterproof. It is designed to shed water while allowing trapped moisture to escape through vapor transmission.
Myth: More layers of felt provide better protection.
Fact: A single properly installed layer is adequate. Double layering can trap moisture between layers and slow drying.
Myth: Felt paper is a vapor barrier.
Fact: Most felt papers are vapor-permeable (Class II vapor retarder, 5-10 perms). Only specially laminated or heavy-asphalt-coated felts approach vapor barrier performance.
Conclusion
Felt paper is an essential component of a properly constructed building envelope. It provides a critical secondary weather barrier, drainage plane, and temporary protection during construction. While synthetic underlayments have gained popularity for their superior strength and UV resistance, traditional asphalt-saturated felt remains a reliable, cost-effective choice. The key to performance is proper installation with correct overlaps, fastening patterns, and integration with flashings and other building components. For more on building envelope design, see our guide on building insulation principles.