This article will discuss the basic procedure for applying stucco to an exterior wall surface. Stucco is a light-weight, durable wall finish used for both commercial and residential exteriors and interiors that are likely to be subject to moisture. Traditionally made from Portland cement and lime, stucco is similar to masonry mortar and can be troweled on manually or applied pneumatically. Stucco can be installed onto a masonry, concrete or framed (open or sheathed) surface.
Step 1 – Prepare the Substrate
When applied to masonry, the surface or substrate must be cleaned of all loose particles, dust and grease, and if necessary, sandblasted, scored, or hacked so that the stucco gains a firm grip. If the substrate material (be it brick masonry, concrete or a wood frame) is not sufficiently rough, metal reinforcing should be applied. Stucco is made with the same ingredients as concrete and if applied directly to the surface will bond with a concrete substrate. The bond can also be improved by applying a water-based emulsion as a bonding agent.
Step 2 – Install Reinforcment for Unsuitable Surfaces
Stucco is supported and reinforced by an embedded lattice of various types and materials. The lattice can take the form of an expanded metal lath, mesh, netting or wire fabric and is galvanized or coated to resist rust. On a sheathed wall, waterproof building paper is applied first, with the horizontal seams overlapping by at least 3 inches. The metal lath is installed next onto the building paper and suspended at least ¼ inch from the wall. Self-furring lath, furring nails or furring strips can also be used for wood framed walls.
Step 3 – Apply the Base Coats
Stucco is typically applied in 4 coats; a dash coat, the scratch coat, the brown coat and the finish coat.
A dash coat consists of 1 part Type I Portland cement, 2 parts sand and is spattered onto the substrate to strengthen the bond with the scratch coat.
The scratch coat and brown coat are one part Portland cement, ¼ part hydrated lime and 3 to 5 parts sand, with enough water to provide a workable mix. The scratch coat is applied to a thickness of ½ inch and should completely embed the metal lath. Before it hardens, the surface should be roughened or grooved horizontally to provide a good bonding surface for the next coat. Together with the lath, the scratch coat creates a rigid base for the application of the brown coat. After a few hours it should be sufficiently dry so that the brown coat can be applied.
The brown coat is troweled on evenly to a thickness of 3/8 inch to present a level surface for the application of the finish coat. Each coat should be uniformly dampened prior to the application of the subsequent coat.
Step 4 – Apply the Finish
The finish coat is mixed in the same proportions as the base coats, but uses white Portland cement plus mineral oxide pigments to add color. The finish coat is only about 1/16 inch in thickness and can have either a smooth finish or be worked into any desired texture.
Various trim accessories such as corner beads are used to provide a neat edge or a square corner for stucco surfaces. For large surface areas, stucco requires frequent control joints to prevent cracking caused by shrinkage or settling foundations. The ingredients and therefore the curing process for stucco are similar to that of concrete. The finished stucco must be kept moist for at least a few days before it is allowed to dry.