ASTM C1063 and C926: The Two Stucco Standards You Shouldn’t Ignore
Written by Stucco Champions — your go-to experts for code-compliant stucco systems in Southern California.
When stucco is installed properly, it can last for decades — but cutting corners or skipping steps can lead to major problems down the road. That’s why two national standards exist to guide every professional stucco system: ASTM C1063 and ASTM C926.
Understanding these codes helps homeowners make better decisions and keeps contractors aligned with required best practices.
What Are ASTM C1063 and C926?
ASTM C1063 covers how to install lath, fasteners, control joints, and trim accessories — essentially everything that supports the stucco.
ASTM C926 explains how to properly apply each stucco coat, how thick each layer should be, and how long to let it cure.
These two standards are the foundation for every three-coat stucco project done right.
What ASTM C1063 Covers
This standard details:
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Water-resistive barriers (WRBs): Typically 2 layers of Grade D paper
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Lath installation: Galvanized steel mesh, attached to framing with proper spacing
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Control joints: Installed to reduce cracking on large walls
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Fasteners: Must penetrate framing deep enough to hold lath securely
At Stucco Champions, every job starts with ASTM C1063 compliance. We follow each detail to avoid structural issues or water intrusion.
What ASTM C926 Covers
This standard focuses on applying stucco properly:
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Three required coats: Scratch, brown, and finish
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Minimum thickness: ⅜ inch for both scratch and brown, ⅛ inch for finish
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Curing time: Each coat must fully cure before applying the next
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Final thickness: At least ⅞ inch over lath
These guidelines ensure long-term durability, better bonding, and fewer issues with cracking or moisture.
Why It Matters
For Homeowners:
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Your stucco will last longer and resist cracking or water damage
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You avoid future repair costs due to poor installation
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You pass inspections and meet insurance or warranty requirements
For Contractors:
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Avoid liability or failed inspections
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Ensure long-term client satisfaction
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Build a reputation for quality and compliance
Common Questions
Is compliance optional?
No. Most cities in California require ASTM standards for permitted stucco work.
How do I know if my contractor follows them?
Ask directly, or request that compliance with ASTM C1063 and C926 is written into your estimate or contract.
What if a contractor doesn’t follow the standards?
You may run into structural issues, water intrusion, or failed city inspections — all expensive to fix later.
Final Thoughts
If you’re investing in a stucco system, make sure it’s built on the right foundation. ASTM C1063 and C926 aren’t just technical guidelines — they’re industry standards that protect your home, your budget, and your peace of mind.
At Stucco Champions, we follow these codes on every job, with zero exceptions.
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