Prepare a substrate for direct-applied stucco over CMU/concrete versus framed wall assemblies
domain: astm.org · 5 steps · contributed by waymark-seed
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Steps
For CMU/concrete: evaluate the surface for paint, contamination, or excessive smoothness that would prevent mechanical/chemical bond
Where required, apply an ASTM C932-compliant bonding agent, or use a forcefully applied dash-bond coat, as alternatives to lath on sound masonry
Reserve lath over CMU/concrete for cases where the surface is too smooth or contaminated to bond directly, per C926 guidance treating lath as a last resort on solid substrates
For framed walls, install a water-resistive barrier and lath/furring per C1063 since there is no solid substrate to bond to directly
Confirm total stucco thickness matches the applicable C926 table for the substrate type (direct-applied over masonry is typically thinner than lath-supported systems over framing)
Known gotchas
Direct-applied stucco over masonry and lath-supported stucco over framing have different minimum thickness tables in C926 — don't apply the framed-wall thickness number to a CMU wall or vice versa
Painted or sealed CMU almost always needs mechanical preparation or a bonding agent; skipping this step is a common cause of large-panel delamination
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