What is Stucco?

Stucco is an exterior cement plaster wall covering. It is usually a mix of sand, Portland cement, lime and water, but may also consist of a proprietary mix of additives including fibers and synthetic acrylics that add strength and flexibility. Modern synthetic stucco can be applied as one base layer and a finish layer, which is thinner and faster to apply, compared to the traditional application of three-coat stucco.

Applying stucco.As with any cement-based material, stucco must be reinforced to resist movement cracking. Plastic or wire mesh lath, attached with nails or screws to the structural framing, is embedded into the base coat to provided stiffening for the stucco. One method often used to help conceal the smaller surface cracks that may appear is the application of one of a variety of pre-mixed acrylic finishes. Flexible acrylic finishes have the ability to stretch and bridge over cracks, improving appearance and limiting the passage of moisture behind the stucco.

Where stucco is to be applied to a structure of wood-framing or light-gauge steel framing, the framing is protected from moisture damage by applying a vapor-permeable, water-resistant weather barrier; typically an asphalt-saturated paper or one of a variety of manufactured plastic-based sheets, known as "building wraps" or "stucco wraps". The properties of the weather barrier must not only protect the framing from rain and moisture, but at the same time allow the free passage of any water vapor generated inside the building to escape through the wall.

A wall finished with a stucco overlay. A wide variety of stucco 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 to add strength. Types include expanded-metal lath, woven-wire lath, and welded-wire lath.

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 crisscross pattern to provide a key for the second layer. The first coat is allowed to dry (cure) before the second layer is applied.

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 "darbies," "rods," and "federeges," scraped smooth, and floated to provide a smooth, even surface onto which the finish coat is applied. It is then allowed to dry (cure) for 7–10 days minimum to allow "checking" (shrinkage) and cracking to take place.

If applied during very dry weather, the layers of stucco are sprayed with water for one or more days to keep a level of moisture within the stucco while it cures, a process known as "moist curing." If the stucco dries too soon, the chemical hardening ("hydration") will be incomplete, resulting in a weaker and brittler stucco.

The final, exterior layer is the "finish coat," of which there are two recommended types:

Color Coat is a colored sand, cement, and lime mixed finish and is typically 3 mm (0.12 in) thick. It is applied over the second coat (brown coat) and can be floated with water for a sandy finish or textured over with a trowel to create various styles of finishes. Premixed, bagged stucco is gaining in use and is available in coarse graded sand and finer graded sand for creating a variety of troweled finishes; it is available in a variety of colors.

Acrylic Finish is an acrylic-based finish from 1 to 4 mm (0.039 to 0.16 in) thick. It can be applied in many ways; it is the most recommended finish for long-lasting quality. It also can be ordered in any color.

Hard Coating is another method of adding a finish to the stucco wall, although no longer recommended. In the 1960s and 1970s people added a variety of materials like glass chunks, stones or marble into the wet stucco wall. This kind of finish coat is very heavy and inflexible and is hard to repair.

Source: Wikipedia, the free encyclopedia