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What is stucco?

Traditional stucco is a cement mixture used for siding, usually on Mission or other Spanish style homes. The cement is combined with water and inert materials such as sand and lime. Usually, wooden walls are covered with tar paper and chicken wire or galvanized metal screening. This framework is then covered with the stucco mixture. Sometimes, the cement mix is applied directly to specially prepared masonry surfaces.

Stucco when and where?

Although stucco-sided homes became popular in twentieth century America, the concept of using cement mixtures in architecture goes back to ancient times. Wall frescoes by ancient Greeks and Romans were painted on fine-grained hard plaster surfaces made of gypsum, marble dust and glue. Stucco techniques were elaborated by the Italians during the Renaissance and spread through Europe. This marble dust compound could be molded into decorative shapes, polished to a sheen or painted. Many homes built after the 1950s use a variety of synthetic materials which resemble stucco. Mock stucco siding is often composed of foam insulation board or cement panels secured to the walls. Although synthetic stucco may look authentic, real stucco tends to be heavier. Walls made of genuine stucco sound solid when tapped and will be less likely to suffer damage from a hard blow. Also, genuine stucco holds up well in wet conditions. Although it is porous and will absorb moisture, it will dry easily, without damage to the structure.

Solid base for Stucco?

People often ask us if stucco, or portland cement plaster, will adhere to concrete or concrete masonry. This question probably arises because plaster is often attached to wall surfaces that have metal lath affixed to them. Metal lath is regularly used over stud wall construction with or without sheathing materials. With a substrate of concrete or concrete masonry, is it necessary to use lath? Stucco is made from the same materials as concrete and concrete masonry. As such, they have a great affinity for each other. Portland cement adheres well to lots of materials, especially to other materials made from the same type of ingredients. ASTM Standard C 926, Specification for Application of Portland Cement-Based Plaster, permits direct application of stucco to solid surfaces like concrete and masonry as long as bond is sufficient. Concrete masonry surfaces are both absorbent and textured, two things necessary for bond. If contamination is present on the substrate surface, good bond is inhibited. This is generally not a concern with new walls. Older walls may have bond-inhibiting characteristics, in the form of paint, sealer, or some other coating or dirt on the surface. The entire surface must have uniform bonding potential or you can run into problems. Partial bond will create undesirable stresses and can lead to delamination and cracking of the stucco layer. Potential for bond can be quickly checked by a simple test: sprinkle the wall with some water to see how it absorbs. If it is readily absorbed, then the surface is expected to bond well with stucco. Immediately prior to plastering, the wall should be prewetted. This prepares the unit surface to absorb paste from plaster. It dampens the unit, reducing its water demand and the potential of premature dryout. Unless it's very hot, dry and/or windy, moisture on the wall together with that in the plaster is usually sufficient for curing. Generally, stucco that is applied directly to solid surfaces is placed in two coats that together total 5/8 in. of thickness. If water sprinkled on the wall is not readily absorbed, then the surface must be handled differently. Depending on the contamination, you might only need to wash with water. With worse conditions, you need more aggressive techniques. These include sandblasting or acid etching. Alternately, you might apply a dash-bond coat or a bonding agent to allow for direct application of stucco. If bond can't be attained by cleaning or bonding agents, you will have to use paper-backed lath to assure mechanical anchoring to the wall. This completely isolates the stucco from the wall surface but supports its weight so it doesn't fall off. Then the application of plaster over concrete or concrete masonry becomes very similar to application over open stud or sheathed frame construction.

Performance

Exterior portland cement plaster is subject to all types of weather conditions, from hot to cold and wet to dry. The weather-resistant finish is durable in all climates.