Stucco

Rock dash stucco used as an exterior coating on a house on Canada's west coast. The chips of quartz, stone, and colored glass measure approx. 3-6 mm (1/8" - 1/4").

Then with the introduction and development of heavy timber and light wood-framed construction methods, stucco was adapted for this new use by adding a reinforcement lattice, or lath, attached to and spanning between the structural supports and by increasing the thickness and number of layers of the total system. The lath added support for the wet plaster and tensile strength to the brittle, cured stucco; while the increased thickness and number of layers helped control cracking.

The traditional application of stucco and lath occurs in three coats—the scratch coat, the brown coat and the finish coat. The two base coats of plaster are either hand-applied or machine sprayed. The finish coat can be troweled smooth, hand-textured, floated to a sand finish or sprayed.

Originally, the lath material was strips of wood installed horizontally on the wall, with spaces between, that would support the wet plaster until it cured. This lath and plaster technique became widely used.

Because of its "aristocratic" appearance, Baroque-looking stucco decoration was used frequently in upper-class apartments of the 19th and early 20th century.

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 trowel is used to scratch the surface horizontally or in a crisscross pattern to provide a key for the second layer. A brush is not used because it will cause delamination. 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 "feathereges", 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:

Damaged stucco that has begun to delaminate from its masonry substrate.

The repair of historic stucco should begin by identifying the cause of the damage to the stucco finish. Historically, the application of stucco was quite similar to the process of applying lime plaster. Repairs should be carried out as soon as problems become visible, as the damage will only become worse over time. Cracks may form in the stucco due to building settling or direct damage to the exterior coating. Once water is able to breach the coating, whether through an opening in the stucco itself or from beneath its surface, fragile stucco can begin to buckle and crumble. Wood is a common structural material that is often used as substrate beneath stucco. It can absorb moisture from at or below ground level and draw it away from the original source of the problem. Stucco can also be applied to masonry such as brick or stone, which can also be damaged by moisture infiltration.

Diagram showing the use of wire mesh as a substrate for an exterior application of Portland cement.

A professional plasterer or contractor familiar with the process of making repairs to historic stucco should carry out necessary repairs. Typically, a homeowner should not attempt to repair stucco finishes on their own.