Written by Stucco Champions — Southern California’s Authority on Exterior Plastering.
The Stucco Scratch Coat: A Detailed Exploration of the Foundation Layer
In a traditional Three-Coat Stucco system, the strength of the wall comes from the bottom up. While the finish coat gets all the attention for color and texture, it is the Scratch Coat that does the heavy lifting. It embeds the lath, stiffens the wall, and provides the mechanical grip for everything that follows.
If the scratch coat fails, the entire system delaminates. This guide breaks down the physics, the mix, and the critical curing process required by ASTM C926 to ensure your stucco wall stands up to Southern California's seismic activity.
1. What is the Scratch Coat?
The scratch coat is the first layer of cement applied to the wall. Its primary function is to completely encase the galvanized wire lath (mesh) and key into the waterproof paper backing.
Thickness: It should be approximately 3/8 inch thick. This is just enough to cover the wire lath fully without sagging.
2. The "Scratch" Technique (Scarifying)
Why is it called a scratch coat? Because of what we do to it before it dries.
While the cement is still plastic (wet but firm), we use a tool called a Scarifier (a rake-like trowel) to cut horizontal grooves into the surface.
These horizontal ridges create a "mechanical key." When we apply the next layer (the Brown Coat), the new wet cement locks into these grooves, preventing the heavy brown coat from sliding down the wall.
3. The Mix: Strength vs. Workability
A standard scratch coat is a blend of Portland Cement, Lime, and Sand.
The Ratio: 1 part Plastic Cement to 3 to 4 parts Sand.
We use "Plastic Cement" because it contains lime, which makes the mix "sticky" (workable) and allows it to hang on the vertical wire mesh without falling off. A mix that is too rich (too much cement) will crack; a mix that is too lean (too much sand) will crumble.
4. Application: Embedment is Key
Whether applied by hand (trowel) or by machine (pump), the goal is Full Embedment.
The plasterer must push the mud hard enough to force it through the holes in the wire mesh. If the cement just sits on top of the wire ("face skimming"), the wire will rust, and the wall will fail. The cement must surround the steel completely.
5. The Cure: The 48-Hour Rule
The scratch coat must cure before we can add more weight to the wall.
ASTM C926 requires a minimum of 48 hours between the scratch and brown coats.
During this time, the wall must be kept moist. We mist the scratch coat with water (hydration) to slow down the drying process. This prevents shrinkage cracks and ensures the cement reaches its full compressive strength.
In the Inland Empire or during Santa Ana winds, a scratch coat can "flash dry" (lose moisture too fast). If this happens, the cement turns into dust rather than rock. Proper hydration is non-negotiable in hot weather.
Conclusion: The Anchor
The scratch coat is the anchor of the system. It connects the finish to the frame. By using the right mix ratio, achieving full wire embedment, and scarifying the surface for bonding, Stucco Champions ensures that the foundation of your exterior is solid.
Last week, we shared What is a Three Coat Stucco System?. The scratch coat is step one; read on to learn about the Brown and Finish coats.