ASTM C1063 sets the minimum installation requirements for lathing and furring that will receive portland cement-based plaster, covering both exterior stucco and interior plaster systems.1ICC Digital Codes. ASTM C1063-21 The International Residential Code references the standard directly for lath materials and lath attachment, making it the baseline inspectors use to approve or reject a plaster assembly before any mud goes on the wall.2ICC Digital Codes. 2021 International Residential Code (IRC) – R703.7.1 Lath The current edition is C1063-25a, though many jurisdictions still enforce the 2022 version depending on which code cycle they have adopted.
Framing Spacing and Alignment
Everything downstream depends on the framing. For exterior wood framing, the standard calls for a maximum of 16 inches on center. Spacing out to 24 inches is allowed only when heavier lath is used, with a minimum weight of 3.4 pounds per square yard for horizontal surfaces and 2.5 pounds per square yard for vertical.
Framing members also need to be aligned within tight tolerances. Surfaces must be straight and true so the lath sits flat and the plaster coat stays uniform in thickness. Where studs bow or twist out of plane, plaster builds up unevenly in the low spots and cracks form in the thick areas.
Ceilings and Soffits
Overhead installations carry extra requirements. For ceilings and soffits, the standard requires rib lath or self-furring lath with a minimum weight of 3.4 pounds per square yard. Standard diamond mesh is not acceptable overhead. Rib lath can span joists set at 24 inches on center for these horizontal applications, with the nose of the ribs touching the framing members to maintain proper contact and support.
Weather-Resistive Barriers
Behind every exterior lath installation sits the weather-resistive barrier. The IRC’s stucco provisions distinguish between climate zones, and the requirements differ depending on whether the project is in a dry, moist, or marine climate.
In dry climates, the barrier must consist of either two independent layers of Grade D paper, each providing a separate continuous plane, or a single layer of 60-minute Grade D paper separated from the stucco by foam sheathing or a designed drainage space. In moist or marine climates, those same configurations apply with an additional drainage layer on the exterior side of the barrier, at least 3/16 inch deep, or a drainage system tested to at least 90 percent efficiency.3UpCodes. R703.7 Exterior Plaster (Stucco) The two-layer approach with flashing directed between the layers is the most common field installation in residential work.
Lath Orientation and Self-Furring
The long dimension of every lath sheet must run perpendicular to the framing members. For expanded metal (diamond mesh) lath, the cups formed during manufacturing must face upward on walls and away from the framing on ceilings. When the cups point the wrong direction, the plaster has nothing to key into and can delaminate from the lath over time.
On solid substrates such as plywood or OSB sheathing, the lath must be furred away from the surface by at least 1/4 inch. Self-furring lath with built-in dimples or V-grooves satisfies this requirement without additional furring strips. On open framing where the bearing surface of the stud is 1-5/8 inches or less, self-furring is not required, because the lath naturally sits proud of any solid backing. Using non-furred lath on a solid substrate traps the plaster against the sheathing and prevents the scratch coat from wrapping around the mesh to form a mechanical bond.
Fasteners
All fasteners in contact with the lath or the plaster system must be corrosion-resistant: galvanized nails, zinc-coated staples, or corrosion-resistant screws. Fasteners that corrode behind the plaster create hollow spots that stay invisible until the stucco starts popping off in sheets.
Penetration depth varies by framing type and lath product:
- Driven fasteners into wood need a minimum 3/4-inch penetration into the structural member, including when lath is applied over sheathing.
- Screws into horizontal wood framing need a minimum 5/8-inch penetration and must engage at least three strands of lath.
- Screws into metal framing with standard lath must be long enough to extend through the steel with at least three exposed threads on the back side.
- Screws into metal framing with rib lath need a minimum 3/8-inch penetration and must pass through without deforming the rib.
Along framing members, fasteners are spaced no more than 7 inches on center for metal plaster bases. For rib lath, every rib that crosses a framing member gets a fastener to maintain uniform tension across the surface.
Lapping Sheets and Corners
Where two sheets of lath meet, the overlap has to keep the plaster surface continuous. Side laps, where sheets meet along their long edges, require a minimum 1/2-inch overlap, and those laps must land on a framing member. Between supports, side laps are tied with wire at intervals of no more than 9 inches. End laps, where sheets meet along their short edges, need at least 1 inch of overlap. If an end lap falls between supports rather than on one, it gets laced with wire to hold it together.
Installers should stagger end laps so that no single horizontal line across the wall has laps from adjacent sheets lining up. When multiple seams stack along the same line, the plaster develops a weak plane that invites cracking. At corners, the lath wraps around the edge to cover at least one stud spacing on the adjacent wall. Corner reinforcement strips, known in the trade as cornerite, go on at both inside and outside corners before the scratch coat.
Control and Expansion Joints
Plaster is rigid, and rigid materials crack when they cannot move. Control joints give the system predetermined lines where movement can occur without damaging the finish. The standard limits wall or ceiling plaster panels to a maximum of 144 square feet without a control joint. No single panel dimension can exceed 18 feet, and the length-to-width ratio of any panel should not exceed 2.5 to 1.
Control joints are also mandatory wherever the substrate material changes, such as where plywood sheathing meets concrete block. The lath must be cut and discontinued at the joint so the plaster on each side can move independently. If the lath runs through the joint, it bridges the very movement the joint was designed to accommodate. All accessories, including control joints, expansion joints, and casing beads, must be installed and secured before any plaster is applied.
Weep Screeds
At the base of every exterior plaster wall, a weep screed provides a clean termination point and a path for trapped moisture to escape. The bottom edge of the weep screed must sit at least 4 inches above raw earth or 2 inches above paved surfaces.3UpCodes. R703.7 Exterior Plaster (Stucco) These clearances keep the plaster from wicking ground moisture up into the wall assembly through capillary action.
When site conditions make those clearances impossible, such as when a driveway or patio slab sits too close to the foundation, the project needs an alternative design approved by the local building department. Burying the weep screed below grade or omitting it entirely is one of the most common code violations inspectors flag on stucco jobs.
What the Lath Inspection Covers
Most jurisdictions require a lath inspection before any plaster can be applied, and the inspector verifies the full assembly in one visit. A typical inspection covers the weather-resistive barrier installation, the fastener schedule and penetration depth, the placement of all accessories (control joints, casing beads, corner reinforcement, and weep screeds), lath overlap and orientation, and framing spacing.
Failing the lath inspection means the scratch coat cannot go on until the deficiencies are corrected and the work passes re-inspection. Photographs of the completed lath installation, taken before the inspector arrives, give the contractor a record that can resolve later disputes about what sits behind the finished plaster.