Written by Stucco Champions — Southern California’s Authority on Exterior Plastering.
Weather-Resistant Building Paper: Protecting Your Stucco System from the Ground Up
Stucco is an incredible material. It resists fire, withstands impact, and can last for 50 years. But it has one vulnerability: It is not waterproof. Stucco is a porous, cementitious cladding that absorbs moisture by design.
The real hero of your home's exterior isn't the textured finish you see; it's the invisible layer underneath known as Weather-Resistant Building Paper (WRB). If this layer is installed incorrectly, the water absorbed by the stucco has nowhere to go but into your wood framing, leading to dry rot and mold. This guide explains the physics of the drainage plane.
1. What is Grade D Paper?
In the stucco trade, we use Grade D Building Paper. This is an asphalt-saturated Kraft paper engineered to breathe. Unlike plastic vapor barriers, Grade D paper allows water vapor from inside the home to escape while repelling bulk water from the outside.
2. The Two-Layer Code Requirement
In Southern California, applying stucco over wood sheathing requires a Two-Layer System. This isn't just for extra thickness; it serves a specific mechanical function known as a "Bond Breaker."
- Layer 1 (The Sacrificial Layer): When wet stucco is troweled onto the wall, it bonds to this outer layer of paper. They essentially fuse together.
- Layer 2 (The Drainage Plane): Because the outer layer is stuck to the cement, the inner layer remains separate. This creates a microscopic air gap between the two sheets. Water that penetrates the stucco hits this gap and drains down to the weep screed by gravity.
3. Material Selection: 10-Minute vs. 60-Minute
The "minute" rating refers to ASTM D779 (The Boat Test)—a measure of how long the paper can hold back standing water.
- 10-Minute Paper: The bare minimum allowed by code. It is thin and tears easily during lath installation. We rarely recommend it.
- 60-Minute "Super Jumbo Tex": The professional standard. It is significantly thicker, resists tearing, and offers 600% more water resistance than the minimum standard. For custom homes and coastal properties, this is non-negotiable.
4. Installation Best Practices
The best paper in the world will fail if installed backward. We follow the "Shingle Principle."
- Bottom-Up: Installation starts at the foundation and works up.
- Overlapping: Each upper sheet must overlap the lower sheet by a minimum of 2 inches. Vertical seams must overlap by 6 inches.
- Weep Screed Integration: The paper must lap over the metal flange of the weep screed. This ensures water exits the wall. If the paper is tucked behind the screed, water runs into the foundation plate.
Can you use Tyvek (Housewrap)? Yes, but not alone. Stucco bonds to Tyvek, eliminating the drainage plane. If you use Tyvek for air sealing, you must install a layer of Grade D paper over it to act as a bond breaker. Furthermore, the surfactants (soaps) in wet cement can degrade standard housewraps.
Conclusion: The Cheapest Insurance
Once the stucco is applied, you cannot change the paper without demolishing the wall. Investing in a high-quality, two-layer 60-minute paper system adds pennies per square foot to the cost but adds decades to the life of the building envelope.
Last week, we shared Grade D Building Paper for Stucco: Code Requirements. Dive deeper into the ASTM standards.