Cracking In Plaster

Technical Bulletin: Cracking in Plaster

March 24, 2015

Rapid Set® EISENWALL® provides the highest strength, greatest durability, and most crack resistant base coat of nearly all wall-coating systems. However, while EISENWALL can greatly reduce the tendency for cracks to appear in the exterior plaster system, using EISENWALL does not guarantee that the structure will be crack-free. To achieve a successful application, the plastering contractor, builder, and architect must understand the limitations of any exterior plaster system. Exterior plaster, commonly referred to as stucco, is subjected to many forces, both internal and external, that can cause cracking.

The primary internal force affecting exterior plaster is drying shrinkage. As the cement in the plaster hydrates (chemically combines with water to form a rigid structure), excess water can leave the plaster due to evaporation or absorption. The result is drying shrinkage, or an overall reduction in bulk volume of the plaster. In normal portland cement based plaster products, only about half of the water required for mixing is actually used in the hydration process. This leaves a tremendous amount of unused water for the evaporation and shrinkage process to occur, and results in drying shrinkage cracks in the plaster. Unlike Portland cement based plaster products, EISENWALL uses virtually all the mixing water in the plaster for this hydration process. The result is far less drying shrinkage (less than one third), and virtually no cracking due to drying shrinkage.

NOTE: Cracking due to drying shrinkage should not be confused with plastic shrinkage cracking. Plastic shrinkage occurs when excessive water is lost from the plaster while it is still in the plastic state (before hydration, when the plaster is still workable). Common causes include dry, cold, or windy weather, and lack of sufficient water curing.

External forces that affect the integrity of the exterior plaster are far more numerous. External forces acting on the plaster can include the following:

In wood frame construction, new, or “green” lumber contains water in the form of free moisture. As the building is framed and the lumber loses this free moisture, the wood changes dimensionally in the form of shrinkage, warping, bowing, bending, twisting, etc. As these changes occur, the entire structure is subjected to stresses that act on all the structural elements, including the plaster. Even kiln dried lumber can contain a significant amount of water. Open framing left to the elements during rain or wet weather only exaggerates this condition.

When wood sheathing (plywood, particle board, or OSB sheeting) is exposed to moisture, it warps and swells. If a minimum 1/8″ gap is not maintained between adjacent sheets, this swelling can cause buckling of the wood sheathing and subsequent cracking in the plaster. It is extremely important that adequate weather barriers be carefully installed before lathing and plastering takes place. Applicable building codes and ICC Evaluation Reports should be carefully followed regarding adequate moisture barriers, flashing, sealing and caulking. Any water intrusion that occurs can trap moisture in the interior of the wall exacerbating the swelling and warping of wood framing and sheathing.

In metal frame construction, thermal expansion and contraction of steel studs and structural beams can result in forces acting on the exterior plaster. The coefficient of thermal expansion for steel (the amount of expansion or contraction per degree Fahrenheit [roughly 5.5 millionths/F]) can mean greater than a quarter inch of expansion in a 50-foot length. That accounts for almost ten 1/32″ width cracks, or one every five feet. This effect is more severe when the wall system is welded and includes no relief mechanism to deal with the stresses.

Lack of proper moist curing of the plaster can also cause cracking. Any hydraulic cement, by its very nature, relies on the presence of water to hydrate and obtain strength. Moisture must be present during the hydration or curing process, so that the plaster can gain sufficient tensile strength in order to resist cracking. Portland cement based products require a minimum of 7 days curing time, which is nearly impossible to keep the plaster continuously moist and rarely done in practice. Rapid Set EISENWALL achieves significant tensile strength in the first hour of hydration, and only requires moist curing for 90 minutes. EISENWALL can be easily moist cured for the majority of the strength gaining hydration process, providing an extremely high strength, fully cured plaster—something that is rarely achieved with Portland cement based plaster products.

Additional causes of external stress and cracking include: foundation settlement, concrete creep and sag, mechanical vibrations, building movement, intersecting walls, ceilings, pilasters, and corners, structural joints, structural subsidence, wind loads, and service loading of floors. While the significantly higher strength of EISENWALL resists this cracking more than conventional Portland cement based systems, these stresses can sometimes cause EISENWALL to crack.

Cracking will usually occur at the corners of openings and penetrations in the plaster. These right angles are the weakest points in a plaster wall and act as concentration areas for stress. An analogy is somewhat like cutting a piece of glass; the glass is scored and in effect a tiny “corner” is created; as soon as the glass is subjected to stress, the pane breaks at the weakest point, the score mark, or “corner”.

CrackingAndPlasteringC_1To minimize cracking at corners, expansion joints (control joints) should be located at all corners of doors and openings, and at other locations to create free floating panels with maximum area of 100 square feet. Additionally, the longer dimension of the panel should not exceed 2.5 times that of the other. Control joints should also be located over the line of abutment of differing types of construction (i.e. where wood-frame meets masonry, etc.). Generally the designer has the responsibility to provide proper control joints to minimize cracking. Where control joints are not possible or not allowed for in the design of the structure, additional reinforcement in the form of 3.4 expanded metal lath should be located at all corners, and has been found to be quite effective in minimizing cracking. Fiberglass mesh embedded in a suitable material can also be used on top of the base coat to keep cracks from coming through to the finish.

Exterior plaster is a beautiful finish that is amazingly flexible in design possibilities. With the proper design, product selection, and construction, excellent results can be achieved. Choosing Rapid Set EISENWALL is a key step in achieving those results.