NOTE: it is important to keep the surface damp for by applying a fine water mist over several days. Building a Stucco Wall For centuries, Stucco has proven to be one of the most enduring, versatile and weather resistant exterior wall finishes available - with its variety of colors and textures, stucco continues to be one of the most popular wall systems.
Project Instructions Step 1 Attach two layers of Grade D, waterproof building paper using galvanized nails or staples in a shingled fashion over the wall sheathing extending 16 inches around all corners. Step 2 Install trim accessories. Step 3 Install casing beads. They are directly attached to support studs then covered with building paper or other weather resistant barrier WRB. Metal lath attached over the sheathing and into the supports carries the plaster.
The weather resistant barrier is intended to resist water penetration, so the sheathing is protected from moisture. That means that many materials are suitable for this application, but the common ones remain plywood, oriented strand board OSB , cement board, and exterior grade gypsum sheathing.
In the early s, basic research on stucco systems looked at the then-current practice of using board lumber not panels as sheathing. These were commonly 6- to 8-inch wide boards attached to support studs at 45 degree angles.
During that period, diagonal placement of the boards transitioned to horizontal placement, and was followed by a move to 4-byfoot panels. Both wood and metal lath were common substrates for plaster at least through the teen years of the s. It should also be noted that it is possible to place stucco over open frame construction. This is accomplished by stretching line wires between studs and adjusting the wire to support paper as the backup to plaster.
The paper is supported by the wire creating the backstop for plaster as it is applied to create the wall face. Because this is not as rigid as a board, the open frame method can allow for more variation in the thickness of the plaster.
In addition it is important to understand that the open frame must be adequately rigid to resist deformation due to wind or other forces—that often means bracing the frame.
The stucco layer may not be considered as part of the stiffening system. Common sheathing materials today come in 4-byfoot boards. These are readily available building materials of consistent quality and are easy to install over wood or steel frames. The boards assure a more uniform thickness of the plaster layer and add structural rigidity to the building frame. For a variety of reasons, then, the currently preferred practice in many regions is to use sheathing boards for frame construction.
As noted above, sheets of plywood, oriented strand board OSB , cement board, and exterior gypsum sheathing are the most common materials for independent veneer plaster applications. There are many types of hydraulic cements that can be used as a binder for stucco, or portland cement plaster. Hydrated lime may be used with blended cement or hydraulic cement, too, but is not used with masonry cement, mortar cement, plastic, or stucco cement.
Those materials already contain workability agents and the addition of lime is neither necessary nor allowed. If used with those materials, lime poses potential problems such as reduced strength or durability.
White cements are also available for use in stucco. They are generally specified under C, C91, or C Specifying white cement - sometimes white cements are blended with pigment in the factory or on the job to create colored mixes. Where white or colored plaster mixes are used, they often are applied only as the finish coat.
Standards for all of these materials are available from ASTM. Sand blast or water blast to remove the paint in its entirety, then direct apply a two coat system. It is essential to have a surface that is uniformly absorptive to accept the plaster coating. In addition, it may be beneficial to use a bonding agent or dash bond coat with this approach.
Attach paper backed lath or install appropriate building paper between wall and attached metal lath to provide a moisture barrier and to serve as a bond breaker. Apply traditional three coat stucco to metal lath and accessories. In this approach, the idea is to treat the plaster like a sheathed system, using metal lath to support the plaster on the substrate, while completely isolating the plaster layer from the backup with building paper.
This prevents a partial bonding situation, which could set up undesirable stresses in the plaster and lead to cracking. Wet and Dry The scratch coat, which is the first base coat, is named for horizontal scratches that promote water retention for curing and a mechanical keying with the brown, or second, base coat.
Colored finishes are usually cured by wetting the brown coat to provide curing moisture from behind. In more extreme conditions, they may be covered to prevent drying. Hot and Cold Proper curing also requires that plaster be in a medium temperature range.
Joint spacing in stucco is more of an issue over frame construction. No matter if the joint is called a control joint, a contraction joint, or an expansion joint, it is there to relieve stresses. The rules for the maximum spacing between joints and the maximum size of panel are well established and discussed in a separate FAQ on joint spacing.
Stucco is a like a thin layer of concrete. When direct applied to a solid substrate, the jointing rules are simply to follow what is present in the backup. The building itself should contain joints to limit random cracking. Complex jointing patterns using multiple types of jointing accessories in framed construction can be confusing, because people wonder if one joint is somehow different from the others.
They are different, but they have one important similarity: a joint relieves stresses and provides a separation between various sections. Again, because plaster is so thin, it must be sectioned into panels to control stresses due to volume change. If there is a potential for out of plane movements, then the joint must be able to move in different directions, both in plane and out of plane. If a joint is a contraction or expansion joint, it simply has to move back and forth.
For these reasons, it is important to choose the right accessory for the joint. The one piece accessories usually have a pleat or accordion shape. They can move in plane but do not handle out of plane movements well. For those locations control joints , such as the interface between different types of substrates, borders, penetrations doors, windows, etc. These may be prefabricated or may be field-constructed by placing casing beads or other accessories back to back.
Note that at a window or door frame, a casing bead adjacent to the frame serves the same purpose. Joint accessories: one-piece and two-piece. A distinction should be made between framed construction and solid backing such as masonry or concrete. Framed construction requires the installation of moisture-resistant paper behind the lath.
There is no need to have plaster bond to the paper, so bonding agents are not used with framed construction, only solid surface substrates. When plastering over concrete masonry, walls should be free from contamination that might impair bond.
Bonding agents have different chemical formulations, so they have different performance characteristics. Bonding agents do not guarantee performance. Research the material to find out which is best suited to particular conditions. Where prepared surfaces seem questionable, and lathing is not an option, a bonding agent may be beneficial. Plaster finish on a concrete masonry wall. Dash Bond Coat Rather than using bonding agents, another option for low-absorption surfaces is to apply a dash bond coat.
This question comes up in both new and repair construction. Designers need to know how much weight the stucco adds to the wall so that they can be sure the structural system provides adequate support. The joint spacing should meet the following criteria: No length should be greater than 18 feet in either direction. No panel should exceed square feet for vertical applications.
No panel should exceed square feet for horizontal, curved, or angular sections. Its hard surface resists abrasion and can take a lot of physical abuse. It stands up to all sorts of climates, from cold to hot and wet to dry. Many homes built in the early s have had very little maintenance and remain in good shape today. Contact information Association of the Wall and Ceiling Industry. This layer, the brown coat, functions as a strengthening layer and must harden for at least 36 hours before you resume the application process.
During this time period, the surface should be misted with water periodically to prevent rapid drying, which can cause stucco to become brittle. This layer is always hand-troweled and may be texturized or applied as a smooth finish. Pigment may be added to the stucco mixture prior to application to add color to the layer. Let the finish coat harden for at least 36 hours. Mist to promote strength in the finished product.
Once all the stucco material is sufficiently dry and hard — which may take several weeks — it may be painted. Apply an asphalt-saturated or plastic sheet called a stucco wrap over the lath to minimize cracking.
Once this base coat is partially dry, use a plasterer's rake to scratch a horizontal or crisscross pattern in it. The scratch coat should dry for a minimum of 36 hours before resuming application.
During this time, it should be misted periodically to keep it from drying too quickly. Once the scratch coat is dry, apply the next layer and scrape it smooth. This is the leveling coat. Repeat the drying process. After 36 hours, apply the finish coat using a flat finishing trowel. This layer forms the visible exterior and should be allowed to cure for several days with periodic misting. You may also use 15 lb per square foot roofing felt 6. 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 weep screeds and casing bead. Install casing bead as a plaster stop at the corners of doors and windows. 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. Otherwise, your nails or staples will rust and pull loose. In all exterior stucco applications, you must use G hot-dipped galvanized lath. Install control joints. To reduce cracking, divide the wall into rectangular panels using control joints, spacing them no more than 18 ft 5. Also install control joints wherever 2 dissimilar walls meet.
Make the panels as close to square as possible, and no larger than ft 2 13 m 2. Mix the scratch coat. 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. 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. Trowel the scratch coat into the lath. You may find it easier to use a hawk for some portions of the application.
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. 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. 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 and apply the brown coat. Mix another batch using 1 part cement and 3 to 5 parts sand. Wet cure for at least 7 days. 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. Cover with the finish coat. 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.
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