Stucco Over Plywood

Stucco is one of the oldest building materials in continuous use. Examples of stucco wall coverings have survived over 1500 years. Nor is stucco limited to one geographic area of the world. Here in the United States, we recognize stucco construction as coming north from Mexico, yet it was used extensively in Europe, especially in Italy and Germany.

Traditionally, stucco was applied over masonry walls as a finish. Today’s equivalent is cement block construction. But more often than not, stucco is applied to homes made of standard wood construction, covered with plywood sheathing. Either softwood construction-grade exterior plywood or OSB can be used. Building codes vary from place to place but typically call for a minimum of 3/8” of sheathing underneath stucco.

While OSB is often considered an acceptable replacement for softwood construction plywood, the two are different, even if the building code allows it. OSB is more susceptible to moisture than plywood is. Not only does it absorb more water, but it expands more when that moisture is absorbed. While a moisture barrier is required when installing stucco over wood products, the stucco isn’t the only place where the substrate can receive moisture.

When stucco is applied over any engineered wood sheathing, the wood sheathing must cover the entire surface that will receive the stucco. It doesn’t work to use plywood only in the corners, with Styrofoam sheathing in between. The Styrofoam isn’t strong enough to support the stucco.

Success Comes from Preparation

Installing stucco over plywood, whether CDX construction plywood or OSB, depends more on surface preparation than anything else. Stucco requires a rough surface that it can bond to, like the masonry mentioned above. Plywood products don’t offer this, so lath needs to be applied, providing something for the stucco to bond to.

But there are other parts of the preparation to do before the lath. It starts when the plywood is hung on the home’s framing. At that time, most installers recommend leaving a 1/8” gap between sheets to account for expansion. Plywood products can expand due to absorbing moisture or prolonged heat exposure. Adding this space allows room for the plywood to grow rather than buckle.

Building code requires two layers of waterproof building paper as a vapor barrier. This can be roofing felt or another type of vapor barrier. The exact requirement exists whether the home’s sheathing is plywood, OSB, exterior gypsum board, or concrete board. This should be the nail in place, with nails six inches apart along the edges, overlapping vertical seams by six inches and horizontal seams by four inches. Be sure to install it so the upper layers overlap the Lower ones, like shingles. The paper should also overlap the corner by at least 16 inches.

With the two layers of paper in place, weep screed and other accessories required by code should be installed. This weep screed is installed at the bottom of any stucco to be installed and serves as a stop to the stucco, leaving a small gap behind it for moisture to drip down and exit the wall.

In addition, casing beads should be installed to neatly terminate the stucco at the end of the wall and around doors and windows. The casing beads for installing stucco over plywood are different from those used for installing it over cement. Only two coats of stucco are required over concrete, so a ½” casing bead is used. In contrast, three layers of stucco are necessary for application over plywood, so a ¾” casing bead needs to be installed.

Once the edging accessories are installed, it’s time to install the galvanized expanded metal lath or one-inch woven wire stucco netting for the stucco to stick to. This must be installed so there is a one-inch overlap on horizontal seams and two inches on vertical ones. As with the waterproof building paper, the lath should extend 16” around corners. Use galvanized nails or long staples to install it, placing them every six inches, both vertically and horizontally. They must penetrate the studs a minimum of one inch.

Corner trim should be installed at all outside corners before finishing them, much like corner beads are installed over drywall, for the same purpose. Impact on the corner trim may bend it, but this is better than chipping a piece of the stucco off the corner.

The final part of preparing the wall for stucco is installing control or expansion joints over the lath, breaking the wall into sections no larger than 144 sq. ft. Stucco shrinks as it dries, and this joint helps to prevent cracking. If there are expansion joints in the existing wall, control joints must be placed over them, on the inside corner, and anywhere else there is a change in substrate.

Applying the Stucco

Building code requires three coats of stucco to be applied over plywood substrates, each approximately ¼” thick. The most accessible material for this is to use a manufactured stucco mixture. Add water to the mixture, wetting it thoroughly. When properly mixed, the stucco should stay on a trowel when the trowel is held perpendicular to the ground.

Stucco that is too watery will sag on the wall, providing an unattractive, uneven appearance and possible failure, falling off the wall. On the other hand, if the stucco isn’t wet enough, it won’t adhere to the metal lath and will fall off. The ideal mix is as moist as possible without the stucco falling off the trowel.

One 80-pound bag of stucco base coat mix will cover about 27 square feet of wall, 3/8” thick.

Apply the prepared stucco base coat mix to the wall with a square trowel, using firm pressure to press the stucco into the lath, working from the bottom to the top. This first coat should be troweled at a uniform thickness of 3/8”. Once applied, go over the stucco with a straight edge or darby, screeding the stucco to a final thickness of 3/8”.

To ensure that the second layer of stucco can bond to the first, scratch 1/8” deep horizontal lines into the stucco using a raking tool. This should not be done until the stucco is hard enough not to accept a thumbprint. Then, allow the base coat to cure for 24 to 48 hours, keeping the surface damp with a fine water mist to help prevent cracking.

Once the base coat is cured, a second 3/8” thick coating of stucco base coat, mixed and applied the same way, can be used over it. As with the base coat, this coat should also be screeded 3/8” thick. If necessary, fill any low spots or voids with an additional stucco base coat and screed again.

At this point, there should be a total of ¾” of stucco base coat on the wall. When the sheen from the second coat is gone, float the surface, smoothing it out and giving it a uniform appearance. As with the first coat, allow this coat to cure for 24 to 48 hours, spraying it from time to time with a fine water mist.

With the base coat and brown coat (second coat) cured, it is time for the finish coat, which is done with the finish coat stucco, a slightly different material. This material comes in various colors, and pigments can be added to the finish coat, allowing the color to be adapted to the specific application. When mixing these colors, be sure to mix enough at one time to cover the entire wall.

This decorative coat is 1/8 “thick and applied to give texture and color to the underlying material. There are several different ways of using the finish coat, providing various patterns and textures, from light to very heavy. Before applying any of them, it’s a good idea to practice on a hidden area of the wall or some scrap material before applying stucco to the entire home.

Regardless of the type of texture desired, entire walls or sides of houses should be done at one time to avoid the possibility of inconsistencies in color. Once the application is finished, keep the wall damp for several days with a fine mist while the finish coat cures.