What is the Stucco?

‘Stucco’ refers to the branch of the plastering industry that manipulates portland cement based mortars, usually on exterior surfaces. In some localities the term ‘stucco’ is limited, and is only used to refer to a factory prepared finish coat mixture. This artificial limitation is questionable, however, since it severely restricts the understanding of stucco’s vast potential, and the wide array of available stucco finishes.

Stucco is a surface coating that provides superior qualities unmatched by any other available siding. Adaptable to, and applicable over, a wide variety of surfaces, it provides a strong, firm, fire and weather resistant membrane. Stucco will not deteriorate, resists rot and fungus, and has proved to be a durable wall covering in both warm and very cold climates. Possessing properties similar to reinforced concrete, stucco provides long term protection to any surface. It is economical and requires little maintenance. Stucco provides several distinct finishes, and all stucco finishes include a wide variety of texture and color options.

Stucco can be considered as one component of a structure’s wall and ceiling assembly. It is usually either the last, or next to last, element to be installed. Stucco is selected because it possesses characteristics which no other component of the assembly can provide. Among its major functions, stucco serves as a refining element insofar as it can conceal and correct minor deviations in the alignment of the other system components, and provide a base for decoration.

 

The Stucco Finishes

This section is a visual showcase of the varied stucco finishes. It is put here to give the reader initial insight into what is possible.

The Stucco Membrane

Produced in separate successive stages, a conventional stucco application incorporates portland cement based mortar and internal metal reinforcement to produce a stucco membrane. The internal reinforcement also acts to secure the membrane to the supporting structure.

The typical stages required to produce a stucco membrane include the application of lath, the scratch coat, and the brown coat. The membrane may then decorated with one of the stucco finishes.

The terms used to describe the dimensions of the membrane include: surface plane, backing plane, perimeter, profile, and thickness.

As their names imply, the surface plane describes the plane formed by the exposed surface of the membrane, while the backing plane describes the plane formed by the rear of the membrane. Although the surface plane must be uniformly flat, the uniformity of the backing plane may vary depending on the surface offered by the supporting structure.

A perimeter is formed by any interruption in the continuity of a stucco membrane. Several things may produce an interruption, including: stucco stop, projecting or reentrant corners, control joint, flashing, door or window frames, vents, and trim.

The profile and thickness describe similar but different aspects of the membrane. The profile describes the distance the surface plane is from set guide points, and is a constant measure. The thickness describes the distance between the backing plane and the profile of the surface plane. Unlike the profile, the thickness may vary to compensate for any irregularities in the membranes backing plane.

The aim of any conventional stucco application is to form a stucco membrane possessing certain desired characteristics. The membrane must conform to the planes, profiles, and perimeters, required by the specified architectural form; be adequately secured to its supporting structure; have high strength; possess both uniform suction properties and uniform appearance; and if so required, be prepared to receive further applications or treatments such as paint, acrylic, wood trim, or tile.

The Characteristics of a Stucco Membrane

Membrane Strength

The strength of the stucco membrane is determined by: the stucco mortar strength, the bond between the separate laminations of mortar, the extent and effectiveness of internal reinforcement, and the thickness of the membrane.

Mortar strength

If correctly produced and properly hydrated, stucco mortar is an extremely strong and durable substance. For a complete study of the factors that determine the strength of stucco mortar.

Bond between laminations

The bonds between the supporting structure, the lath, and the laminations of stucco mortar are produced by chemical or mechanical processes. A chemical bond is formed between the layers of different stucco mortar applications. A mechanical bond is formed between the supporting structure and the lath, the lath and the stucco mortar, and between the different layers of stucco mortar.

Internal reinforcement

The lath's sources of interlock not only provide a means of securing the stucco mortar to the supporting structure, but it also provides internal reinforcement for the stucco membrane. Additional internal reinforcement is also provided by the larger aggregates used for dry dash and wet dash finishes.

Membrane thickness

A significant factor in the ultimate strength of the stucco membrane, the minimum membrane thickness is determined by the structural integrity of the supporting structure, the elastic properties of the membrane, and to a lesser extent, the chosen stucco finish.

Stucco membranes are usually produced in thicknesses of either 13 mm (½ inch) or 19 mm (¾ inch). Although no dimension should ever fall below the minimum specified in your local building code, the thickness of a conventional stucco membrane should not be less than 13 mm (½ inch). Some stucco finishes, particularly float finishes, benefit as the thickness of the membrane approaches 19 mm (¾ inch).

Fire Protection

Possibly the most vital function of stucco is its ability to confine flame, smoke and superheated gases to the area in which a fire originated, and protect the supporting structure against exposure and collapse. This property places stucco in a class by itself among surface materials.

The performance of a stucco membrane with regard to fire resistance is measured in terms of the period (in hours) between the time of initial exposure to a controlled fire, and the time at which failure occurs as determined by standard criteria. The fire resistance properties exhibited by a stucco membrane depend on the quality of the application. Since building codes recognize only those ratings that are based on actual tests, the quality of an application, with regard to fire resistance, depends upon how precisely the rated application is duplicated on a project.

Resistance to Impact and Abrasion

Stucco provides a durable surface with ample resistance to withstand the impacts and abrasions encountered under normal conditions.

The Structural Functions of Stucco

Stucco performs no function that could be classified as purely structural, and must not be expected to assist the structural frame in supporting its loads.

Weather Resistance

A material's resistance to weather is its ability to resist the penetration of moisture, and to maintain its surface integrity when exposed to extreme cycles of temperature and moisture variation. The required degree of weather resistance depends upon the severity of climate and the degree of exposure. In all climates and conditions, stucco has been proven to provide excellent weather resistance.