Expanded metal lath stucco (C) Daniel Friedman Stucco Wall Recipes & Stucco Application Methods

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Recipes for mixing stucco wall coatings:

This article discusses recipes or mixes to prepare traditional stucco and includes details of how stucco is applied to a building surface.

This article series discusses best practices construction details for building exteriors, including water and air barriers, building flashing products & installation, wood siding material choices & installation, vinyl siding, stucco exteriors, building trim, exterior caulks and sealants, exterior building adhesives, and choices and application of exterior finishes on buildings: paints, stains.

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Stucco Recipes & Stucco Mixtures

Expanded metal lath stucco (C) Daniel Friedman

This article series includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.

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Our page top photo shows exposed metal lath in the stucco exterior of a poorly-finished home in New York. It looks as if the top coat of stucco may have not been applied at all.

Stucco is a mixture of Portland cement, sand, and water, with a little lime or a plasticizer added for workability. A proper mixture has good tensile strength and weather resistance and the ability to bond well to the mesh or substrate. It is also easy to trowel on and resists sagging. In cold climates, it must also have freeze-thaw durability, usually obtained by using air-entrained plaster.

The cement base can be masonry cement, plastic cement, or Portland cement, which may have air-entraining additives. Do not add lime or a plasticizer to masonry cement or plastic cement since these already contain plasticizers. While approximate proportions are well established, the right mix for a job depends on the weather exposure of the wall and weather conditions during application (see Table 1-9).

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Table 1-9: Stucco Base Coat Mixes (C) Wiley and Sons, S Bliss

Other than the right proportions, the keys to a good stucco mix are clean, good quality sand and clean potable water. Since sand makes up about 97% of the stucco mixture by volume, it is critical to use good sand.

The sand used to prepare stucco should be free of vegetable matter, loam, clay, silt, and soluble salts and should conform to ASTM C897, which designates the distribution of particle sizes (gradation). Impurities in the sand or water can affect the strength of the mix, and poor grading of the sand will hurt its workability. Salts can cause staining on the finished surface from efflorescence.

Stucco Wall Application Procedure

Stucco can either be hand troweled or blown with a machine. Some stucco contractors use a pump for the base coats but apply the finish coat by hand. Although the mixes are slightly different for the two approaches, both can produce a high-quality finish. In threecoat stucco, the first and second coats are 3/8-inch thick, and the finish coat is 1/8-inch for a total thickness of 7/8 to 1 inch.

Applying the Stucco First or Scratch Coat

Expanded metal lath stucco (C) Daniel Friedman

The first, or “scratch,” coat, which forms the base for the next two coats, should completely encase the reinforcement.

While still wet, the plaster is scored horizontally with a special metal rake or trowel to create a good mechanical bond with the second coat (vertical scratching promotes cracking at studs).

For proper curing, the scratch coat needs to be kept moist by misting or fogging with water for 48 hours.

Except in very moist weather, misting should start as soon as the freshly applied stucco lightens in color and be repeated at the start and end of each day until the second coat goes on.

Applying the Stucco Wall Brown Coat

Plaster layer (C) Daniel Friedman

The second, or “brown,” coat should go on as soon as the first coat is hard enough to accept the second coat without cracking, but at least 48 hours later, according to the IRC.

The second coat fills any cracks in the scratch coat, and the additional sand in the brown coat helps prevent new shrinkage cracks. Whether it is hand troweled or machine-applied, it must be leveled with a straightedge (“rodded”) and floated to produce an even surface for the final coat.

A short delay between the first and second coat helps to create a good bond between the two and strengthens the scratch coat by rewetting it for a more complete cure.

Any cracks larger than 1 1/16-inch in the brown coat should be patched before the top coat goes on. In the Southwest, where adobe is popular, the brown coat is often steel troweled for an adobe look and serves as the final coat.

Applying the Stucco Wall Finish Coat

After the second coat is allowed to cure for a minimum of 7 days (14 will allow a more complete cure), the top coat is applied to provide the finish color and texture. Many contractors now use premixed color coats, some with acrylic additives to increase water resistance and flexibility.

Creating a uniform color and texture requires a skilled applicator, uniform mixing, favorable weather (avoid direct sun), and a uniform substrate without variations in texture or water absorption. Problems in the substrate will tend to show through the thin finish coat. It is best to do an entire side of the building in one batch with no cold joints. A modest amount of color variation is considered part of the character of traditional stucco, but too much is a sign of substandard work.

A certain amount of shrinkage cracking is also inevitable in stucco exteriors. Application over wood-frame construction results in more cracking than over concrete block or other more stable substrates.

Coarse textures in the finish will tend to hide the cracks better than smooth finishes. Even under the best of conditions, small shrinkage cracks of less than 1/16-inch will occur in the finished stucco and are to be expected. Generally these do not leak or indicate substandard work.

Expanded Metal Lath Supporting Exterior Stucco: Details

At below left we show a cut-cross-section of a steel-lath supported exterior wall on a Minneapolis, MN. home. Our photo at below right shows the marks left by metal lath on felt paper that had been used as a building sheathing wrap on the same home. These details were exposed during construction work for an addition.

Plaster lath board (C) Daniel Friedman

Details about interior stucco or plaster and additional photos and information about the use of expanded metal lath are

Plaster lath board (C) Daniel Friedman

Also see

-- Adapted with permission from Best Practices Guide to Residential Construction.

Stucco Finish System Articles


Continue reading at STUCCO THIN COAT APPLICATION or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see SIDING EIFS WALL LEAK POINTS for a photo guide to common leak points in synthetic stucco applications.

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