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The ideal weather conditions for applying stucco involve an overcast day with low wind, and temperatures of 50 to 60F (1016C). Delay the job if the temperature is expected to drop below 40F (4C) or rise above 90F (32C) in the next week. If the weather is sunny, store your stucco and sand under a tarp between uses. If they feel warm to the touch, don't attempt to use either the stucco or the sand. You may stucco over any rigid material attached to your support studs. The most common surfaces for stucco are plywood, oriented strand board (OSB), cement board, and exterior grade gypsum sheathing. Make sure to follow local building codes while installing the sheathing material. It is possible to stucco over an open frame, but this results in a less even and less structurally secure wall. If you plan to go this route, drive nails halfway into the studs, spaced out vertically in 5-6 inch (13-15 cm) intervals. String line wire horizontally along the protruding nails. You may also use a draining house wrap such as Tyvek drainwrap. Most building codes require at least 2 layers of "Grade D" building paper or an equivalent water-resistant barrier. You may also use 15 lb per 100 square foot roofing felt (6.8 kg per 9.3 square meters) or some types of housewrap, but do not use a plastic housewrap not intended for stucco. Overlap the paper by at least 4 inches (10 cm) and fasten with roofing nails. Start at the bottom and overlap the paper or felt as you work your way up. While not required by most codes, an air gap between the 2 layers is highly recommended to prevent wall rot. A 3D plastic drainage mat between the 2 barriers is one way to accomplish this. Install casing bead as a plaster stop at the corners of doors and windows. Install weep screed at the base of the wall for better drainage. For this project, these 2 materials are not interchangeable. Selecting the correct lath and installing it properly is one of the more difficult parts of this project. Consulting a local contractor is recommended. In most cases, you should nail or staple the lath to the studs (not the sheathing) at no less than 7-inch (18 cm) intervals. Overlap the lath by at least inch (1.25 cm) along the long side and 1 inch (2.5 cm) at the end, but not much more. Be sure to use only galvanized roofing nails or staples designed for use with stucco. Otherwise, your nails or staples will rust and pull loose. In all exterior stucco applications, you must use G-60 hot-dipped galvanized lath. Choose lath with at least inch (6mm) furring, or make up for non-furred lath using furring strips or furring nails. Without this furring, the stucco will not adhere properly to the lath. To reduce cracking, divide the wall into rectangular panels using control joints, spacing them no more than 18 ft (5.5 m) apart. Also install control joints wherever 2 dissimilar walls meet. If the lath is expanded metal (rather than stucco mesh), cut it behind each control joint to fully separate this rigid material into panels. Make the panels as close to square as possible, and no larger than 144 ft2 (13 m2). . Mix the scratch coat from 1 part cement material and 2 to 4 parts plaster sand. If you are using Type I Portland cement instead of plastic cement, you'll need to add your own hydrated lime; count the final cement and lime mixture as the "1 part cement." Mix with just enough potable water that you can trowel the stucco; any more, and it is likely to sag. Make sure to use cool water when mixing your stucco, especially if your hose has been sitting in the sun. If the water is hot or warm, it will cause your stucco to dry too quickly. The aggregate in the cement should be clean and well graded. Apply the scratch coat with a square trowel at a 45 angle, firmly pushing it into the lath. This layer should be inch (9.5 mm) thick. The first scratch coat should be thick enough to cover your wire grid. You may find it easier to use a hawk for some portions of the application. The first coat is called the "scratch coat" because of the shallow, horizontal lines scored into it by a notched trowel. This will ensure a good bond with the next coat. When stuccoing over stud construction, the thick scratch coat must be allowed to cure for 48 hours. During this time, it is critical to protect the stucco from drying out. Mist or fog the stucco twice daily unless the relative humidity is above 70%. Protect the wall with a windscreen or sunshade if necessary. You can also use a rotating lawn sprinkler to keep the stucco damp. Keep the sprinkler turned down to a low setting and pull it away from the house enough so that it keeps the surface damp without soaking it. Mix another batch using 1 part cement and 3 to 5 parts sand. Apply another inch (9.5 mm) layer of stucco and screed to an even thickness, for a total thickness so far of inch (19 mm). Once the brown coat loses its sheen, float it smooth. Wet cure as you did the scratch coat, but this time allow at least 7 days. The first 48 hours are the most important, but you should continue to mist or fog it for this whole period, whenever it looks like it's about to dry. This final inch (3 mm) layer determines the texture of your stucco wall. Apply and float it as you did the brown coat, but this time use 1 part cement to 1 to 3 parts sand. You may purchase a finish which already contains pigment, or trowel on a plain finish and paint over it once it has had at least a week to cure. Colored stucco finishes work best in pastel shades. If the finish is colored, wetting the surface during floating may cause mottling. A fog coat may make the color more even. Many decorative textures are possible when applying this final layer. Judge the appearance from at least 30 feet (9m) back before settling on a look.
Check the weather forecast. Put up sheathing materials. Cover the plywood with building paper and come back over that with a dimple screen or other rain screen. Install weep screeds and casing bead. Attach metal lath. Install control joints. Mix the scratch coat Trowel the scratch coat into the lath. Score the scratch coat lightly. Wet cure the scratch coat. Mix and apply the brown coat. Wet cure for at least 7 days. Cover with the finish coat.