Written by Stucco Champions — Southern California’s Authority on Exterior Plastering.
Weather-Resistiant Building Paper: Protecting Your Stucco System from the Ground Up
When installing a stucco system, success starts with the layers you don’t see. One of the most critical components is Weather-Resistant Building Paper (WRBP)—a moisture barrier that protects your structure from water damage and ensures your stucco finish lasts for decades.
Stucco is a reservoir cladding; it absorbs water by design. The only thing preventing that water from rotting your framing is the Grade D paper behind it. This guide explains the physics of the drainage plane and why code compliance is non-negotiable.
1. What Is Weather-Resistant Building Paper?
WRBP is a specially engineered asphalt-saturated kraft paper installed between your home’s exterior sheathing and the stucco. It performs a dual function:
- Water Intrusion Protection: It acts as a drainage plane, redirecting bulk water that penetrates the stucco down to the weep screed.
- Wall Breathability: Unlike plastic vapor barriers, Grade D paper allows trapped water vapor to escape from the inside out, reducing the risk of rot and mold (The "Sweating Wall" effect).
2. Understanding the Two-Layer System
In climates like California’s, building codes (CRC and IBC) mandate a Two-Layer WRBP System over wood sheathing. This isn't redundant; it's structural.
| Layer | Function |
|---|---|
| First Layer (Inner) | The true moisture barrier. It stays separate from the stucco, protecting the wood sheathing. |
| Second Layer (Outer) | The "Sacrificial Layer." It bonds to the wet cement during application, essentially becoming part of the cladding. |
The separation between these two layers creates a Drainage Plane. If you only use one layer, the stucco fuses to it, eliminating the path for water to drain. The two-layer system ensures water always has a way out.
3. Installation Best Practices
Even the best paper fails if installed incorrectly. We follow strict ASTM protocols to ensure the envelope is watertight.
| Technique | Purpose |
|---|---|
| Shingle Overlap Method | Upper sheets must overlap lower sheets to guide water downward and outward (Gravity Flow). |
| Flashing Integration | Paper must tuck under window head flashing and over the sill flashing to prevent leaks. |
| Horizontal Overlaps | Minimum 2 inches (Head Lap). Prevents water from wicking up between sheets. |
| Vertical Overlaps | Minimum 6 inches (End Lap). Ensures sealing at the end of rolls. |
| Weep Screed Lap | The bottom edge of the paper must overlap the metal weep screed flange. |
4. Common WRBP Mistakes to Avoid
When we inspect failed stucco, we often find these critical errors:
Installing a lower sheet over an upper sheet creates a "cup" that catches water and funnels it into the wall. Always work from the bottom up.
- Using "Housewrap" Alone: Standard Tyvek bonds to stucco. If you use housewrap, you must add a layer of Grade D paper over it to act as a bond breaker.
- Skipping the Inspection: Tears happen during lathing. We inspect the paper for rips before applying cement and patch them with moistop sealant or tape.
Conclusion: The Invisible Hero
You will never see the building paper once the job is done, but it is the most important component of your wall. By using a code-compliant two-layer system (like Super Jumbo Tex 60 Minute), Stucco Champions ensures your home remains dry and structurally sound for generations.
Last week, we shared Grade D Building Paper: Code Requirements and Benefits. Dive deeper into the specific ASTM standards for paper selection.