By: Jim B. Smith, Hampton Construction of Southwest Florida, Inc
* Article as appears in February 2004 edition of CCCIA Builders Showcase

Portland cement-based plaster, often referred to as stucco, is a two or three coat system, usually 1/2 inch to 7/8 inch thick, an inflexible membrane which commonly provides the exterior skin for structures in South Florida and other parts of the country.

Structural penetrations, such as windows, doors, plumbing, electric outlets, light fixtures, vents, etc., pierce the thin plaster membrane. Such penetrations function as focus points or lines for stress inherent in the supportive skeleton of the building.

Cracking in plaster occurs when the forces or stresses acting on it exceed the tensile strength of the plaster itself. Since Portland cement-based plaster gains strength gradually, it is most susceptible to cracking during its early, weaker stage.

There are only two factors that produce all cracking in plaster. One is the normal change in volume of Portland cement intrinsic to the hydration of the moisture required for hydration. This process causes internal stresses.

The other factor is stress transferred to the plaster membrane from external sources. Classes as external stress, examples are sonic resonance, seismic vibration, deflections of supporting members, thermal expansion and contraction, wind loads, settlement and/or subsidence.

Like concrete, Portland cement plaster must be mixed with sufficient water to render it plastic and workable. Part of the water chemically combines with the cement for hydration. A significant amount of water is lost either by evaporation or absorption. Accompanying the setting of the plaster is a natural shrinkage loss of mass or itâs build. If the loss of moisture is not uniform throughout the thickness of the plaster, shrinkage will create internal stresses within the plaster membrane. Normally contraction occurs toward the center of the plastered area or panel.

Plaster will gain its largest percentage of the ultimate strength during the first seven days following its application. This is the same period which most of the shrinkage of the plaster takes place. The two natural but counteractive processes develops during a period of time when plaster should be expected to achieve its gain in strength, while at the same time, the greatest distorinal stress from loss of volume occurs, resulting, at times, in shrinkage cracks.

In stud wall systems, a significant degree of transferred stress often is generated in thermally induced expansion and contraction of the wall framing elements. This is unavoidable. Unfortunately, thermo-induced expansion and contraction are further differentiated by dissimilar materials, thus inducing a high maintenance area at these conditions.

Cracking is more apparent in certain stucco textures. A float finish will highlight cracks more than a skip-trowel or other coarse textures patterns, which tend to absorb or conceal cracks.

Because of the nature of the material, some hairline cracking in Portland cement-based plaster is very normal and should be expected. Design consideration should be given to ASTM C-926.

Cement-based plaster under any conditions is not a flashing. Water induced into the wall system by any means (e.g. Water intrusion, condensation, or cracking) will compromise the integrity and industry standards. The bonding characteristics, mechanically and chemically, will be rendered insufficient. De-lamentation will result and often times mildew will occur in the interior of wall systems if the problem is not corrected in a short amount of time.

Knowledge of industry standards and education are key to successful construction projects. Prevention of problems, their resulting liabilities and system solutions, are available from quality subcontractors in our community.

Hampton Construction Company of Southwest Florida, Inc. would like to provide quality work and service on your projects in metal framing, exterior finish systems, lath and plaster and gypsum drywall. Please call 832-437-4785 or 239-994-4220 for more information.