Stucco

Stucco is a material made of an aggregate, a binder, and water. Stucco is applied wet, and hardens when it dries. It is used as a coating for walls and ceilings and for decoration. In Europe the term render is more commonly used. Stucco may be used to cover less visually appealing construction materials such as concrete, cinder block, or clay brick and adobe.

Stucco from the House of Borujerdi-ha, 1850s, Kashan, Iran.

Composition

The difference in nomenclature between stucco, plaster, and mortar is based more on use than composition. Until the later part of the nineteenth century, it was common to have plaster, which was used inside the building, and stucco, which was used outside the building, consist of the same primary materials: lime and sand (lime and sand are also used in mortar). Animal or plant fibers were often added for additional tensile strength. In the later part of the nineteenth century, Portland cement was added with increasing frequency in an attempt to improve its durability. At the same time, traditional lime plasters were being replaced by gypsum plaster.

Traditional stucco is made of lime, sand, and water. Modern stucco is made of Portland cement and water. Lime is often added to decrease the permeability and increase the workability of modern stucco. Sometimes additives such as acrylics and glass fibers are added to improve the structural properties of the plaster as well as its workability. This is usually done with what is considered a "one coat" stucco system—as opposed to the traditional 3 coat method.

Lime stucco is a relatively hard material that can be broken or chipped by hand without too much difficulty. The lime itself is usually white; color comes from the aggregate or any added pigments. Lime stucco has the property of being self-healing to a limited degree due to the slight solubility of lime (lime in solution can be deposited in cracks where it later solidifies). Portland cement stucco is very hard and brittle and can easily crack if the base on which it is applied is not stable. Typically its color was gray due to the innate color of most Portland cement (white Portland cement is also used). Today's stucco manufacturers offer a very wide range of colors that can be mixed integrally in the finish coat.

Traditional stucco

A stucco face from the ancient Greek city of Ai Khanoum, Afghanistan, 3rd-2nd century BCE.

Traditionally Stucco is directly applied to a masonry surface such as brick or stone. In wood-framed buildings, stucco was applied over a wood lath, similar to the lath and plaster technique used on the interiors of buildings. Modern stucco is usually applied over an expanded metal lath that is fastened thru the the wall sheathing and into the studs with staples or screws. The material is generally applied in three coats, the scratch coat, the brown coat and the finish coat. The finish is usually an integral color and is where you also achieve your texture.

Stucco can be hand applied or sprayed. The finish material can be trowled smooth, hand textured, floated to a sand finish or sprayed.

In some parts of the country (California, Arizona, New Mexico and Florida), stucco is the predominant exterior in use on both residential and commercial construction.

Stucco has also been used as a sculptural and artistic material. Baroque and Rococo architecture makes heavy use of stucco. Examples can be found in churches and palaces, where stucco is mostly used to provide a smooth, decorative transition from walls to ceiling, decorating and giving measure to ceiling surfaces. Stucco is an integral part of the art of belcomposto, the baroque concept that smoothly integrates the three classic arts, architecture, sculpture, and painting.

Since stucco can be used for decorative purposes as well as for figurative representation, it provides an ideal transitive link from architectural details to wall paintings such as the typically baroque trompe l'oeil ceilings, as in the work of the Wessobrunner School. Here, the real architecture of the church is visually extended into a heavenly architecture with a depiction of Christ, the Virgin Mary or the Last Judgment at the center. Stucco is being used to form a semi-plastic extension of the real architecture that merges into the painted architecture.

Islamic art makes use of stucco as a decorative means in mosques and palaces. Indian architecture knows stucco as a material for sculpture in an architectural context. It is rare in the country area.

Due to its "aristocratic" look, baroque looking stucco decoration was used frequently in upper-class apartments of the 19th and early 20th century.

Beginning in the 1920's stucco, esp. in its Neo-Renaissance and Neo-Baroque materialization became increasingly unpopular with modern architects in some countries, resulting in a widespread movement to remove the stucco from tenements.

Stucco was still employed in the 1950's in pre-molded forms for decorating the joins between walls and ceilings inside houses. It was generally painted the same color as the ceiling and used in designs where a picture rail or rat rail was in use.

Stucco used as an exterior coating on a residential building.

Modern stucco

Modern Stucco usually consists of 1 layer of wire lath and 2 layers of Portland cement-based plaster. This is due to its rapid strength development and durability. However, cement's crystalline structure cannot accommodate significant movements in the building structure (as lime does) and is thus prone to cracking. This is why an additional acrylic finish on top is often applied. It adds flexibility for surface movements and is waterproof.

First a wire mesh is attached to vapor permeable, water-resistant "tar" paper if stucco is being applied to a wood framed structure or light-gauge steel frame structure. The paper protects the sheathing and interior of the wall from outside moisture intrusion without trapping moisture vapor in the wall. A wide variety of accessories such as weep screeds, control and expansion joints, corner-aids and architectural reveals are sometimes also incorporated into the lath. Wire lath is used to give the plaster something to attach to and may be expanded-metal lath, woven-wire lath, or welded-wire lath. In Europe, wire lathing with fired clay an integral "brick-mesh" is used in many applications instead of expanded metal lathing.

The first layer of plaster is called a "scratch coat", consisting of plastic cement and sand. A brush is used to scratch the surface horizontally or in a criss cross pattern to provide a key for the second layer.

The next layer is called the "brown coat" or leveling coat. It also consists of sand, cement and lime. It is leveled with tools called "Darbys", "Rods" and "Federege" scraped smooth, and floated to provide a nice even surface onto which to apply the finish coat. It is then allowed to dry (cure) for 7-10 days minimum to allow "checking" (shrinkage)and cracking to take place. Sometimes the dry surface is sprayed with water for one or more days to speed up the curing process.

The exterior stucco layer is the final layer and is referred to as the "finish coat". There are 2 different recommended types of finish coats:

1. Color Coat is a colored sand, cement and lime mixed finish and is typically 3 mm (1/8") thick. A base coat of stucco is applied over the 2nd coat of cement (brown coat). This then can be floated with water for a sandy finish or later textured over with a trowel to create various styles of troweled finishes. Pre-mixed stucco is what's mostly used and comes in coarse graded sand and finer graded sand for creating a very smooth troweled finish. It is made in a variety of colors.

2. Acrylic Finish is an acrylic based finish from 1,0 mm to 4,0 mm thick. It can be applied in many ways, and is the most recommend finish for long lasting quality. It also can be ordered in any color.

3. Hard Coating is another method of adding a finish to the stucco wall, although no longer recommend. In the 60's and 70's people added a variety of materials like glass chunks, stones or marble (calcium carbonate finished with crushed natural marble (Marmalrino)) into the wet stucco wall. This kind of finish coat is very heavy and inflexible, and also hard to repair.

See also

• Exterior Insulation Finishing System
• Plaster
• Tadelakt
• Polished plaster

External links

• Anne Grimmer, The Preservation and Repair of Historic Stucco, published by the Technical Preservation Services, Heritage Preservation Services Division, National Park Service [1], and duplicated all over the web
• About.com article on stucco
• Modern Wall Systems

• How to Apply Stucco Veneziano
• Graymont - major producer of stucco materials