What kind of mesh or lath should be used under stucco to prevent cracking on a Toronto home?

Galvanized expanded metal lath is the standard reinforcement for traditional three-coat stucco on Toronto homes, and using the correct type and weight is one of the most important factors in preventing cracking over the stucco's lifespan. The lath provides mechanical anchorage for the scratch coat and distributes stress across the wall surface so that normal building movement and thermal cycling don't concentrate into cracks.

For residential stucco in the GTA, self-furring expanded metal lath weighing a minimum of 1.75 pounds per square yard (2.5 lb is preferred) is the standard specification. The "self-furring" feature means the lath has dimples pressed into it that hold it approximately 6 millimetres away from the building paper and sheathing, creating a space for the scratch coat to key behind the lath. This mechanical key is what holds the entire stucco system on the wall — without it, the stucco sits on the surface of the lath rather than locking around it, and delamination becomes a matter of time rather than possibility. The lath must be galvanized (zinc-coated) to resist corrosion. In lakefront GTA neighbourhoods or areas with heavy road salt exposure — the Beaches, Mimico, Port Credit, Lakeshore — some contractors specify stainless steel lath for maximum corrosion resistance, though the cost is significantly higher.

Fastening the lath correctly matters as much as choosing the right product. Lath must be fastened with galvanized roofing nails or galvanized staples driven through the sheathing into the studs, spaced no more than 150 millimetres (6 inches) apart vertically and 400 millimetres (16 inches) horizontally to match stud spacing. Lath sheets must overlap a minimum of 25 millimetres at horizontal joints and 50 millimetres at vertical joints, with the upper sheet overlapping the lower to shed water. Corners require pre-formed galvanized corner bead — never just bent flat lath, which creates weak points that crack within the first few freeze-thaw cycles.

Fibreglass mesh serves a different purpose and is NOT interchangeable with metal lath for traditional stucco. Fibreglass mesh (typically 4mm by 4mm or 5mm by 5mm alkali-resistant mesh) is used in acrylic finish systems and EIFS, where it's embedded in a thin base coat over rigid insulation board or over a cement base coat. It provides crack resistance in thin-coat applications but does not provide the structural reinforcement or mechanical keying that metal lath provides for thick three-coat stucco. Using fibreglass mesh instead of metal lath on a traditional three-coat stucco application is a code violation and a recipe for system failure.

There are situations where additional reinforcement is needed beyond standard lath. Stucco applied over dissimilar substrates — for example, where the wall transitions from concrete block to wood framing — requires a control joint at the transition to accommodate differential movement. High-stress areas around window and door openings should have additional diagonal strips of lath at the corners to resist the stress cracks that commonly radiate from corners. Areas prone to impact (garage door surrounds, lower wall sections accessible to foot traffic or lawn equipment) benefit from heavier-gauge lath or double-layer lath for added durability.

For GTA projects, the Ontario Building Code (OBC Part 9, Section 9.20) and CSA standards govern masonry and stucco applications. Ensure your contractor specifies lath that meets CSA A82.31 and uses fasteners appropriate for the substrate. All work should be done by a contractor with WSIB coverage who can demonstrate experience with proper lath installation — because once the stucco is on, you can't see what's underneath, and the quality of the lath installation determines whether that stucco lasts 5 years or 30.