Stucco a House

Traditional stucco is simply a variety of concrete, applied in several layers to create a strong bond to the wall. Stucco is popular for many reasons, including its low cost, earthquake resistance, and breathability in humid climates. This article covers exterior stucco applications over a wooden or steel framework, or over a solid wall. This project is fairly advanced, but within the range of an experienced home handyman.

Steps

Applying Stucco Over a Stud Wall

  1. Check the weather forecast. The ideal weather conditions for applying stucco involve an overcast day with low wind, and temperatures of 50 to 60ºF (10–16ºC).[1] Delay the job if the temperature is expected to drop below 40ºF (4ºC) or rise above 90ºF (32ºC) in the next week.[2]
  2. Put up sheathing materials. 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.[2] 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.[3]
  3. Cover the plywood with two layers of building paper. Most building codes require at least two layers of "Grade D" building paper or an equivalent water-resistant barrier.[4] You may use asphalt felt or some types of housewrap, but do not use a plastic housewrap not intended for stucco.[5] Overlap the paper by at least 4 inches (10 cm) and fasten with roofing nails.[3]
    • While not required by most codes, an air gap between the two layers is highly recommended to prevent wall rot. A 3D plastic drainage mat between the two barriers is one way to accomplish this.[6]
  4. Install weep screeds and casing bead. Install casing bead as a plaster stop at the corners of doors and windows.[7] Install weep screed at the base of the wall for better drainage.[8]
    • For this project, these two materials are not interchangeable.[9]
  5. Attach metal lath. 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 ½" (1.25 cm) along the long side and 1 inch (2.5 cm) at the end, but not much more.[10]
    • In all exterior stucco applications, you must use G-60 hot-dipped galvanized lath.[10]
    • Choose lath with at least ¼" (6mm) furring, or make up for non-furred lath using furring strips or furring nails.[9] Without this furring, the stucco will not adhere properly to the lath.
  6. Install control joints. 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 two dissimilar walls meet.[11] If the lath is expanded metal (rather than stucco mesh), cut it behind each control joint to fully separate this rigid material into panels.[12]
    • Make the panels as close to square as possible, and no larger than 144 ft2 (13 m2).[13]
  7. Mix-Cement. Mix the scratch coat from 1 part cement material and 2¼ 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."[3][2] Mix with just enough potable water that you can trowel the stucco; any more, and it is likely to sage.
    • The aggregate in the cement should be clean and well graded.
  8. Trowel the scratch coat into the lath. Apply the scratch coat with a square trowel at a 45º angle, firmly pushing it into the lath. This layer should be ⅜" (9.5 mm) thick.[13]
    • You may find it easier to use a hawk for some portions of the application.
  9. Score the scratch coat lightly. 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.
  10. Wet cure the scratch 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.
  11. Mix and apply the brown coat. Mix another batch using 1 part cement and 3 to 5 parts sand. Apply another ⅜" (9.5 mm) layer of stucco and screed to an even thickness, for a total thickness so far of ¾" (19 mm).[3] Once the brown coat loses its sheen, float it smooth.
  12. Wet cure for at least seven days. Wet cure as you did the scratch coat, but this time allow at least seven 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.
  13. Cover with the finish coat. This final ⅛" (3 mm) layer determines the texture of your stucco wall.[3] Apply and float it as you did the brown coat, but this time use 1 part cement to 1½ 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.[14]
    • 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.[1]

Applying Stucco Over Concrete or Masonry

  1. Prepare the wall's surface. You may apply stucco directly over these rigid, solid surfaces, but only if the surface is roughly textured and absorbent. If the wall does not absorb a sprinkle of water, or if there's obvious surface contamination, wash the surface thoroughly. If the wall is coated in paint or sealer, or if it's too smooth to support stucco, try one of the following treatments:[15]
    • Acid-Etch-Concrete
    • Sandblasting
    • Bush hammer or roughing machine (for unpainted, smooth surfaces)[3]
    • Applying a bonding agent, referring to specific product instructions. Do not use over water-soluble paint.[16]
    • If there is any doubt that the wall can support the stucco, follow the instructions in the stud wall section above, attaching metal lath and plastering over it.
  2. Wet the surface. Wet the wall right before you apply the first coat of plaster, preferably with a fog spray. This improves the suction bond and reduces the amount of water the wall absorbs from the plaster, preventing premature drying. The surface should be damp, but not soaked.[3]
    • Delay the job if the next week of weather involves freezing temperature, hot weather (above 90ºF / 32ºC), or high wind. These conditions will interfere with the curing process.
  3. Mix-Cement. This coat should be 1 part cement material (including lime) and 2¼ to 4 parts sand.[2] Plastic cement, which has been pre-mixed with lime, is usually the easiest to mix and work with. Combine this with plaster sand from a dry material yard.[17]
    • Only add enough water to allow you to trowel the plaster, or it may sag or fail to stick to the wall.
  4. Apply and score the scratch coat. Trowel on a layer ¼ inch (6.4 mm) thick.[18] Score this with shallow, horizontal lines using a notched trowel, keeping the tool perpendicular to the wall.[19] These grooves will help the next coat bond onto the surface.
    • Acid-etched surfaces (among others) may not be quite rough enough for a strong bond with this method. Instead, dash on the scratch coat using a cement gun, or by whipping it on with a fiber brush or whisk broom. This forces out air to create a stronger bond.[3]
    • Some builders combine the scratch coat and brown coat into one base coat. If you choose to do this, plan on a total stucco thickness of about ⅜" (9.5 mm) for cast concrete, and ½" (12.7 mm) for unit masonry.[2] Allow about ¼" (6.4 mm) of this thickness for the finish coat.[3]
  5. Apply the brown coat after a few hours. With modern cement on a rigid surface, there is no need to wait for the scratch coat to cure completely. For a stronger bond, trowel on the second, "brown" coat as soon as the scratch coat is rigid enough to resist cracking, usually after four or five hours.[3][20] Rod and float this surface until it is level and ¼" (6.4 mm) thick.[18]
    • The brown coat mix should contain 1 part cement materials and 3 to 5 parts sand.[2]
    • It may help to compact this layer with a shingle float.
  6. Keep the brown coat moist while it cures. Over the next 48 hours, it is critical to keep the stucco moist. If the Calculate-Humidity of the air is below 70%, you will need to mist or fog the surface once or twice a day.[2] Wait at least seven days for the brown coat to cure, wetting periodically if it begins to dry prematurely. Some organizations recommend 10 or even up to 21 days of drying for even more crack resistance.[21]
    • In extremely hot or windy conditions, put up a windbreak and sun shade. You may even need to cover the moistened surface with polyethylene.
  7. Put on the finish coat. The decorative finish coat contains 1 part cement materials and 1½ to 3 parts sand.[2] Optionally, it may include pigment as well to add color. Trowel and float this on in a thin layer, about ⅛" (3 mm) thick.[18] Allow it to cure completely before painting (if desired), following the same moist curing instructions above if the weather is hot.



Tips

  • As you stucco a house, work on the back and sides before the front. This gives you a chance to improve your technique before working on the walls that are most visible from the street.

Things You'll Need

  • Plywood
  • Galvanized nails
  • Nail gun
  • Building paper
  • Mesh lath
  • Furring nails
  • Hammer
  • Bonding agent (optional)
  • Large bucket
  • Masonry cement
  • Portland cement
  • Medium-grain sand
  • Pigment (optional)
  • Trowel
  • Stucco sprayer (optional but a real time saver)

Related Articles

Sources and Citations

  1. ↑ 1.0 1.1 http://www.askthebuilder.com/applying-stucco/
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 http://www.cement.org/for-concrete-books-learning/materials-applications/stucco/stucco-frequently-asked-questions
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 http://www.free-ed.net/free-ed/Resources/Trades/carpentry/Building01/default.asp?iNum=1404
  4. ↑ http://www.tsib.org/pdf/technical/60-216_Water-Resistive-Barrier-Physical-Properties.pdf
  5. ↑ http://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-water-resistive-barriers
  6. ↑ http://www.greenbuildingadvisor.com/blogs/dept/musings/install-stucco-right-include-air-gap
  7. ↑ http://amico-lath.com/lath/casing_bead.htm
  8. ↑ http://www.hometips.com/buying-guides/walls-siding-stucco.html
  9. ↑ 9.0 9.1 http://www.titanamerica.com/uploads/Common%20Errors%20in%20Lath%20and%20Accessories%202013-09-25.pdf
  10. ↑ 10.0 10.1 http://www.awci.org/cd/pdfs/9612_c.pdf
  11. ↑ http://www.homedepot.com/catalog/pdfImages/0a/0afff321-4174-47da-96bc-768e2f52d66d.pdf
  12. ↑ http://www.stuccoguru.com/resources/article.cfm?articleID=B-04010&resourceID=4
  13. ↑ 13.0 13.1 https://www.quikrete.com/AtHome/Video-Building-a-Stucco-Wall.asp
  14. ↑ http://www.tsib.org/pdf/plaster-assemblies-chapter-03-three-coat-plaster-stucco.pdf
  15. ↑ http://www.cement.org/for-concrete-books-learning/materials-applications/stucco/stucco-installation-standards
  16. ↑ http://www.awci.org/cd/pdfs/9310_a.pdf
  17. ↑ http://www.stuccoguru.com/resources/article.cfm?articleID=A-01050&resourceID=3
  18. ↑ 18.0 18.1 18.2 http://www.awci.org/cd/pdfs/9010_e.pdf
  19. ↑ http://www.stuccoguru.com/resources/article.cfm?articleID=A-01300&resourceID=3
  20. ↑ http://www.titanamerica.com/uploads/Double-back%20method%202013-09-20.pdf
  21. ↑ http://www.stuccoguru.com/resources/article.cfm?articleID=B-03055&resourceID=1

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