Tar paper, also known as building felt or roofing felt, has been used as an underlayment beneath wood flooring for generations. While modern synthetic underlayments have gained popularity, tar paper remains a cost-effective and proven material for reducing moisture transmission, cushioning the floor, and minimizing squeaks. Understanding when and how to use tar paper under wood flooring is essential for achieving a long-lasting, stable installation.
The Purpose of Underlayment Beneath Wood Floors
Underlayment materials serve several critical functions in wood floor installations. They provide a moisture barrier between the subfloor and the wood flooring, cushion minor subfloor imperfections, reduce friction that causes squeaks, and can provide a small amount of thermal insulation. Tar paper has been the traditional material of choice for these purposes, particularly in nail-down installations over plywood or OSB subfloors.
| Function | How Tar Paper Performs | Importance |
|---|---|---|
| Moisture barrier | Asphalt-saturated felt blocks vapor migration | Critical — prevents cupping and buckling |
| Friction reduction | Smooth surface allows wood to move during expansion/contraction | High — prevents squeaks |
| Cushioning | Minor compression bridges small subfloor voids | Moderate |
| Thermal break | Thin air layer provides minor R-value | Low — negligible insulation |
Types of Tar Paper and Their Ratings
building felt is classified by its weight per square (100 square feet). The most common grades for flooring applications are 15-pound and 30-pound felt. The weight designation is a historical reference to the approximate weight per square; modern materials may differ slightly.
| Grade | Weight per Square | Thickness | Best Use |
|---|---|---|---|
| #15 felt | ~15 lb | ~0.030 in (30 mil) | Standard wood floor underlayment |
| #30 felt | ~30 lb | ~0.060 in (60 mil) | High-moisture areas, concrete subfloors |
| ASTM D226 Type I | Varies | ~0.025 in | General purpose, meets code |
| ASTM D226 Type II | Varies | ~0.050 in | Heavy-duty, steeper slopes |
Installation Over Plywood Subfloors
For nail-down solid hardwood flooring over plywood or OSB subflooring, the installation sequence is straightforward. After the subfloor is clean, dry, and flat (within 1/4 inch over 10 feet), roll out the tar paper parallel to the direction of the flooring. Overlap adjacent rows by 2 to 4 inches. Use a stapler or cap nails to fasten the paper every 12 to 18 inches along edges and at the center of each sheet. Avoid excessive fastening that might create bumps under the flooring.
The paper should extend to within 1/2 inch of walls and obstructions. It should not be wrapped up the walls, as this would interfere with baseboard installation and might trap moisture against the wall framing. For glue-down installations, tar paper is not used because the adhesive must bond directly to the subfloor.
Installation Over Concrete Slabs
When installing wood flooring over a concrete slab, the moisture protection requirements are more stringent. A 6-mil polyethylene vapor barrier is generally required between the slab and the wood subfloor or sleeper system. Tar paper alone is insufficient as a primary vapor barrier over concrete because moisture vapor can pass through it. However, 30-pound felt can be used over a polyethylene vapor barrier as an additional cushioning layer. Concrete slabs must be at least 60 days old and must test below 3 pounds of moisture per 1,000 square feet per 24 hours using a calcium chloride test before any wood flooring is installed.
Moisture Performance Data
Comparative testing by flooring manufacturers has shown that 15-pound asphalt-saturated felt transmits approximately 5 to 8 perms of moisture vapor (a perm is a measure of water vapor transmission). By contrast, 6-mil polyethylene transmits less than 1 perm. This means that tar paper provides only moderate vapor retarder performance and should not be relied upon as the sole moisture barrier in high-humidity environments or over concrete slabs.
However, for typical wood-frame subfloors in climate-controlled buildings, the perm rating of tar paper is adequate to prevent moisture-related issues. The key is to maintain the building’s relative humidity between 30 and 50 percent, which keeps the wood flooring within its stable moisture content range of 6 to 9 percent.
Alternatives to Tar Paper
Several modern alternatives to tar paper offer improved performance in specific areas. High-density foam underlayments provide better sound insulation. Rubberized membranes offer superior moisture protection. Self-adhering vapor-retarder sheets combine the functions of the vapor barrier and the cushioning layer in a single product. However, these alternatives cost significantly more than tar paper, which remains a cost-effective choice for standard nail-down installations over wood subfloors.
| Material | Cost per sq ft | Moisture Rating | Acoustic Rating (STC) | Best For |
|---|---|---|---|---|
| #15 tar paper | $0.05 – $0.10 | Moderate (5-8 perms) | 45 | Nail-down over plywood |
| Foam underlayment | $0.15 – $0.30 | Low (10+ perms) | 55-60 | Floating floors, condos |
| Rubber membrane | $0.50 – $1.00 | High (1-2 perms) | 60-65 | Concrete subfloors |
| Polyethylene film | $0.03 – $0.08 | Very high (0.5-1 perm) | 40 | Direct vapor barrier over slab |
Common Mistakes to Avoid
One of the most common mistakes is using tar paper that is too thick or too thin for the application. Using 30-pound felt when 15-pound would suffice can create a surface that is too soft, allowing the flooring nails to lose grip over time. Conversely, using lightweight felt (less than 15 pounds) may not provide enough moisture protection. Another common error is failing to overlap the seams adequately or leaving gaps that allow moisture vapor to bypass the barrier.
Installing tar paper over a damp subfloor is a critical mistake. The paper traps the moisture against the wood flooring, leading to cupping, warping, and mold growth within months. Always verify that the subfloor moisture content is below 12 percent using a pin-type moisture meter before laying the underlayment.
Tar paper remains a reliable and time-tested component in wood flooring installations. When selected correctly for the specific subfloor type and environmental conditions, and installed with proper overlap and fastening, it provides the moisture protection and slip-reduction necessary for a durable, squeak-free wood floor.