Stucco is one of the oldest building materials in continuous use. We can find examples of stucco wall covering that 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 as well, especially in Italy and Germany.
Traditionally, stucco was applied over masonry walls, as a finish. Today’s equivalent to that is cement block construction. But more often than not, stucco is applied today to homes which are made of standard wood construction, covered with plywood sheathing. Either softwood construction-grade exterior plywood can be used or OSB. 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 to be an acceptable replacement for softwood construction plywood, even to the point that the building code allows it, the two are not the same. 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 that the substrate can receive moisture from.
When stucco is applied over any engineered wood sheathing, the wood sheathing must cover the entire surface which will receive the stucco. It doesn’t work to only use plywood 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, regardless of whether it is CDX construction plywood or OSB, depends on surface preparation more than anything else. Stucco requires a rough surface that it can bond to, like the aforementioned masonry. 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 from prolonged exposure to heat. Adding this space allows room for the plywood to expand, rather than buckling.
Building code then requires two layers of waterproof building paper, as a vapor barrier. This can be roofing felt or another type of vapor barrier. The same requirement exists whether the home’s sheathing is plywood, OSB, exterior gypsum board or concrete board. This should be nail in place, with nails which are six inches apart along the edges, overlapping vertical seams by six inches and horizontal seams by four inches. Be sure to install it so that upper layers overlap Lower ones, like shingles. The paper should also overlap corner by at least 16 inches.
With the two layers of paper in place, weep screed and other accessories that are required by code should be installed. This weep screed is installed at the bottom of any are to be stuccoes and serves as a stop to the stucco, putting a small gap behind it for moisture to drip down and exit the wall.
In addition to this 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 installation of stucco over plywood are different than those used for installation 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 required 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 that there is one inch of overlap on horizontal seams and two inches of overlap on vertical ones. As with the waterproof building paper, the lath should extend a 16” around corners. Use galvanized nails or long staples to install it, placing them every six inches, both vertically and horizontally. They must penetrate into the studs a minimum of one inch.
Corner trim should be installed at all outside corners, much like corner bead is installed over drywall, before finish it and for much the same purpose. Impact to 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 the stucco is to install control or expansion joints over the lath, breaking the wall into sections that are no larger than 144 sq. ft. Stucco will shrink as it dries and this joint helps to prevent cracking as it dries. If there are expansion joints in the existing wall, then control joints must be placed over them, as well as on inside corner and anyplace where there is a change in substrate.
Applying the Stucco
Building code requires three coats of stucco to be applied over plywood substrates, with each coat being approximately ¼” thick. The easiest 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 wet 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, so as to press the stucco into the lath, working from the bottom to the top. This first coat should be troweled into place at a uniform thickness of 3/8”. To even it out, once applied, go over the stucco with a straight edge or darby, screeding the stucco to a final thickness of 3/8”.
In order 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 thumb print. 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 applied over the base coat. As with the base coat, this coat should also be screeded 3/8” thick. If necessary, fill any low spots or voids with additional stucco base coat and screed again.
At this point, there should be a total of ¾” of stucco base coat on the wall. As soon as the sheen is gone from the second coat, 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 basecoat and brown coat (second coat) cured, it is time for the finish coat, which is done with finish coat stucco, a slightly different material. This material comes in different colors, as well as there being pigments which 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 coat is a decorative coat, applied to give texture and color to the underlying material. It is applied 1/8” thick. There are a number of different ways of applying the finish coat, providing various different patterns and textures, from a light to a very heavy texture. 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 application is finished, keep the wall damp for several days, with a fine mist, while the finish coat cures.