Building Paper — 15# Felt Weather-Resistive Barrier

A building paper is an asphalt-saturated kraft paper used as a weather-resistive barrier behind exterior wall cladding to block liquid water intrusion while allowing water vapor to escape from wall cavities.

Building Paper diagram — labeled parts and installation context

What It Is

Building paper — commonly called 15-pound felt or tar paper — is the original water-resistive barrier in residential construction. Despite the "15-pound" designation (a legacy reference to the weight per 100 square feet before modern manufacturing changes), it remains a widely used and code-accepted water-resistive barrier for wall applications.

The material functions through a combination of hydrophobicity and vapor permeance. Liquid water beads on the surface and drains down and out; water vapor in a wicking state can pass through the paper over time. This vapor permeability is important because it allows wall assemblies to dry to the exterior — trapping vapor inside walls leads to condensation and decay.

Building paper is installed shingle-fashion, with each horizontal course lapping over the top of the course below by a minimum of 2 inches. Vertical seams lap a minimum of 6 inches. Cap nails with large plastic or tin washers fasten the paper to the sheathing at the required interval.

Modern synthetic housewrap products (Tyvek, Typar, etc.) have largely replaced building paper in new construction because they install faster and provide better resistance to tearing during installation. Building paper remains common in stucco assemblies because it is required under metal-lath by code in many jurisdictions, and in re-roofing applications.

Types

15-pound building paper (ASTM D226 Type I) is the standard wall product. It is a kraft paper core saturated with asphalt.

30-pound building paper (ASTM D226 Type II) is heavier, less vapor-permeable, and more tear-resistant. It is used under roofing as felt underlayment and in applications requiring greater water resistance.

Grade D building paper (ASTM C1032) is the code-specified type for use behind stucco in California and other western states. It has specific permeance and water resistance requirements distinct from standard 15-pound felt.

Where It Is Used

Building paper is installed over wall-sheathing before any exterior cladding is applied. It runs from the base of the wall to the top, installed bottom to top so upper courses lap over lower courses (reverse lapping allows water to run behind the barrier). In stucco assemblies, two layers of Grade D building paper are commonly required under the lath. In roofing, 15-pound or 30-pound felt is installed on the roof deck as roofing-underlayment beneath shingles.

Building paper is also used as a vapor-barrier in some crawl space applications, though polyethylene sheeting is more common there.

How to Identify One

Building paper visible at wall edges, penetration flashings, or during renovation appears as black or dark brown kraft paper. It has a distinctive asphalt odor when freshly cut. It tears relatively easily compared to synthetic housewrap. The rolls are typically 36 inches wide and printed with horizontal chalk lines to assist in alignment.

Replacement

Building paper behind existing cladding is not routinely replaced unless the cladding is being removed. When siding, stucco, or other cladding is removed for repair, the exposed building paper should be inspected for tears, gaps, and lap failures. Damaged sections are repaired with new paper and cap nails before new cladding is installed.

Building Paper — FAQ

What is the difference between building paper and housewrap?

Both are weather-resistive barriers installed over sheathing before cladding. Building paper (15# felt) is an asphalt-saturated kraft paper. [Housewrap](/wiki/housewrap/) (Tyvek, Typar, etc.) is a spun-bonded polyolefin plastic. Housewrap installs faster and is more tear-resistant. Building paper is required under stucco lath in many codes and remains common in remodeling work where matching existing material is preferred.

Does building paper expire or degrade over time?

Building paper behind intact cladding can last for decades. Its main enemies are prolonged exposure to sunlight (UV degradation before cladding is applied) and repeated wetting and drying cycles when laps are inadequate. Once installed and covered by cladding, building paper typically outlasts the cladding above it.

Can I use roofing felt as wall building paper?

30-pound roofing felt can be used on walls but is lower in vapor permeance than 15-pound felt, which may not be desirable in certain climate zones. For stucco, the code specifically requires Grade D building paper (ASTM C1032) in many western jurisdictions — standard roofing felt does not meet that requirement. Use the material specified by the code for the application.

Is building paper required by code?

Yes. The International Residential Code (IRC) requires a water-resistive barrier behind exterior wall cladding. Building paper (15# felt or equivalent) satisfies this requirement. The specific product and lap requirements vary by cladding type — stucco, for example, requires two layers of Grade D paper in many jurisdictions.

How is building paper fastened?

Cap nails — roofing nails with a large plastic or tin washer — are the standard fastener. The large cap prevents tearing at the fastener point. Staples are used in some applications but are less code-accepted because they puncture the paper and can allow moisture infiltration at the fastener point. Fastener spacing is typically 12 to 18 inches along the field and at edges.