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OBC DIVISION B · PART 9 · SECTION 9.28 — STUCCO

What are the lath and mix requirements for exterior stucco in Ontario?

Exterior stucco must be applied over corrosion-resistant metal lath or wire reinforcement, fastened at prescribed spacings with furred attachment, and mixed to code proportions of Portland cement, lime and clean sand, applied in coats to the required total thickness.

Code reference
Stucco reinforcement must be galvanized or otherwise corrosion-protected metal lath or wire mesh, lapped at joints and fastened to supports at limited spacings so the mesh is held clear of the backing for full embedment. Stucco mixes must follow prescribed proportions of cement, lime and aggregate with clean water and sand, applied over a moisture barrier in successive coats.
OBC Division B, Subsections 9.28.4. and 9.28.5.

Paraphrased for clarity — open this provision in the BUILDAiD Library for the exact wording.

What this means in practice

Traditional three-coat stucco is unforgiving of shortcuts. The lath must be self-furred or installed on furring so mortar keys fully around the wire; mesh stapled flat against the paper leads to delamination and cracking. Laps at mesh joints and corner reinforcement matter for crack control. Mix discipline matters too: excessive cement makes brittle, shrinkage-prone stucco, while weak, sandy mixes dust and erode. Curing each coat properly, especially in hot or windy weather, is essential. Note that proprietary acrylic EIFS systems are a different assembly evaluated under their own standards, not this section.

Plain language

Real cement stucco needs a wire mesh backbone properly nailed to the wall, and a cement-lime-sand recipe the code spells out. Done right it lasts decades; done with the wrong mix or loose mesh it cracks and falls off in sheets.

Related questions

How much clearance does stucco need above the ground in Ontario?

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This page is provided for general information. Building code requirements depend on your specific project, occupancy and municipality, and code editions change. Always verify requirements against the current Ontario Building Code and confirm with your local building department.