Exterior Finishes: Stucco 3-Coat

Wood sheathing helps support the wall system. If it’s installed only at corners or above and below windows, it creates an uneven surface for applying the stucco. It’s difficult to apply stucco at a uniform thickness across an uneven surface. For best results, install sheathing continuously, positioning the boards horizontally. Leave a 1/8” space between the edges of the boards to allow for moisture expansion. Make sure wood sheathing is the proper moisture content and grade in order to prevent unnecessary cracking.

For open-frame construction, line wires are installed instead of sheathing to support the wall system. Make sure line wires are spaced 6" apart and stretched tight. However, keep in mind that sheathed frame construction ensures a more uniform stucco application.

A drainage plane is a layer of water-resistant material that completely covers the home's exterior. When water penetrates the exterior finish, it moves down the drainage plane and back to the exterior. The drainage plane protects sheathing and framing from getting wet and prevents moisture buildup in exterior wall cavities. In production homebuilding, housewrap is the material of choice for creating the drainage plane on exterior walls. After installing sheathing, wrap the home shingle style with housewrap, starting from the foundation and using only manufacturer recommended fasteners. Extend the housewrap past the wall-to-foundation intersection a minimum of 1". Overlap all horizontal and vertical seams a minimum of 6". Tape all vertical seams with construction tape, using the manufacturer's recommended tape. Ensure that there are no gaps or tears in the housewrap.

Flashing is an essential part of water management on a wall. It's a thin sheet or strip of water-resistant material that's installed to guide any water that gets behind the stucco downward and away from the home. Make sure flashing is installed at vulnerable areas, such as windows, doors, intersections, and penetrations, and that it's properly integrated with the drainage plane.

Weep screeds are installed at the bottom of all stucco walls. They act as a stop for stucco and allow water to weep from behind the stucco. At wall-to-foundation and roof-to-wall intersections, install a weep screed so water can exit the wall system and drain away from the home. The weep screed should be installed over the housewrap, and building paper should be layered shingle style over the weep screed flange.

Because stucco can chemically react with the drainage plane and deteriorate it, a layer of building paper should be installed over the housewrap. The building paper acts as a bond break, preserving the housewrap beneath it to effectively shed water from the home. Install the building paper shingle style, layering it over the weep screed flange.

Self-furring metal lath is installed on a wall to act as reinforcement and anchorage for the scratch coat. The lath has to be installed correctly in order for the scratch coat to mechanically bond to it. If the lath isn’t double-sided, the side with the stand-offs must be installed against the wall surface. The stand-offs hold the lath about 1/8" to ¼” off the wall, allowing the stucco to fully encase it. Make sure the fasteners securing the lath to the wall are attached to the framing and aren't overdriven. Ensure that the lath is continuous across the entire wall. The lath should also be applied to all architectural features. The alternative to using self-furring lath is to use lathing stand-offs.

Install corner beads at corners, casing beads at windows and doors, and stucco stops at soffits. Windows with an integral stucco stop don't need to have a casing bead installed. Make sure they're square and level; use a long level to check for level.

Control and expansion joints are designed to relieve stresses on stucco walls to prevent extensive cracking from occurring where the material expands and contracts or where the home experiences framing movement. Install control joints at least every 144 square feet. When possible, try to align control joints with window or door openings to provide protection against cracks and to create a look that's more aesthetically pleasing to the homeowner. Install expansion joints between floors and between changes in mass, such as the house-to-garage intersection.

The scratch coat is the first coat of stucco applied to the wall. Its name comes from the fact that it's scratched horizontally to promote both a mechanical and chemical bond between it and the brown coat, which is applied next. Prepare the scratch coat according to the manufacturer’s instructions. Ensure that the proper proportions of cement, stucco-grade sand, and water are used in the mix. It’s particularly important to have the correct amount of sand, as too much sand can weaken the system and too little can increase the likelihood that it will crack. Let the mix sit for 20 minutes before remixing it for application.

Apply the scratch coat to the wall at a uniform thickness of 3/8” to a maximum of 1/2”. Make sure the scratch coat completely encases the metal lath. After it's applied, scratch the coat horizontally. Scratching promotes a good mechanical bond between the scratch and brown coats and helps the scratch coat retain water during hydration.

Wait for the scratch coat to set up to 10 to 12 hours, depending on the air temperature and humidity, before hydrating the scratch coat. Hotter, drier climates will require that the hydration process begin earlier. To hydrate the scratch coat, flood the wall surface with a gentle stream of water, working from the bottom to the top until the water runs off in sheets. Applying water from top to bottom will cause the water to run down in rivulets, potentially eroding the stucco. Once the water has been absorbed by the surface of the scratch coat, it’s ready for another cycle of hydration. Hydrate each exterior wall at least once, but up to as many as three times, if the wall continues to soak up water. The stucco should retain moisture for the first 48 to 72 hours after it’s applied in order to gain the strength needed to fully support the brown coat.

The brown coat is the second coat of stucco applied to the wall. The brown coat creates the level surface that's desirable for the finish coat. Mix the brown coat, following the manufacturer’s instructions. Ensure that the proper proportions of cement, sand, and water are used in the mix. Generally, the brown coat requires slightly more sand than the scratch coat in order to make it weaker. Let the mix sit for 20 minutes before remixing it for application. Make sure to dampen the scratch coat before applying the brown coat.

Apply the brown coat to the wall at a uniform thickness of ¼” to 3/8”, except at architectural details, where it may be thicker. The brown coat provides architectural character, such as slopes and edges. After it's applied, screed the brown coat, and create a smooth, consistent surface with an 8' level or straight edge. Screeding causes stucco to drop off the wall onto horizontal surfaces, such as manufactured stone or brick, that may already be in place. Clean excess stucco from horizontal surfaces immediately after screeding. Hydrate the brown coat in the same manner as the scratch coat.

The finish coat is the final coat of stucco applied to the wall. Out of the three coats, it's the thinnest and is meant to provide a decorative finish. Apply the finish coat to the wall at a thickness of 1/16” to 1/8”. If the finish coat is cementitious, apply it the same day as the brown coat, so it can adhere properly to the brown coat through suction. If the finish coat is acrylic, it can be applied up to several days after the brown coat. Hydrate the finish coat in the same manner as the scratch and brown coats. If the finish coat is acrylic, it doesn’t need to be hydrated beyond the needs of the brown coat.