Stucco and the Test of Time
Stucco, or portland cement plaster, is a versatile facing material that can be applied to flat or curved surfaces either inside or outside any building or structure. Stucco has been used for more than a century as a surface finish because of its utility, low first cost, and minimum need for maintenance.
Traditional stucco consists of portland cement-based materials and sand, mixed with water to form a workable plaster. Portland cement plaster is applied either by hand or machine to exterior and interior wall surfaces in two or three coats.
Stucco is used for facing in a wide variety of commercial and residential structures. It may be applied directly to a solid base such as masonry or concrete walls, or it can be applied to a metal lath attached to frame construction, solid masonry, or concrete construction.
Durability and breathablity are the major sustainability benefits of cement stucco. Stucco is a water-shedding, hard, impact-resistant, fire-resistant, and color-retentive, exterior finish material. Because plaster is breathable, or able to transmit moisture vapor, water that gets behind it does not become trapped. This means stucco is resistant to rot and fungus, helping to protect indoor air quality. Stucco has proved to be a durable wall cover in all climates, whether wet, dry, hot, or cold.
The final appearance of the finish coat can be varied by changing the size and shape of the aggregate, using colored cement, adding pigments, changing the consistency of the finish mix, or changing the method or equipment used for plastering. By using integral color, the need to paint is eliminated.
Assembly: Applied directly to concrete masonry, stucco provides a tough 1/2-inch (13-mm) thick facing that is integrally bonded with the masonry substrate. When applied to metal lath, three coats of plaster form a 7/8-inch (22-mm) total thickness. A vapor-permeable, water-resistant building paper separates the plaster and lath from moisture sensitive sheathing or framing.
Concrete Construction. Depending on the condition of the concrete surface, portland cement plaster may be directly bonded to the concrete or applied to metal lath attached to the wall. When concrete surfaces are extremely smooth or contaminated with excessive form oil, metal lath is used as a plaster base. Metal lath can be mechanically attached to concrete, ICF, or to autoclaved aerated concrete walls to provide a base for durable, traditional three-coat portland cement plaster exterior. The use of open-textured concrete masonry units promotes mechanical keying between the plaster and base masonry wall. Because plaster and concrete masonry undergo similar volume changes, they are inherently compatible.
Stucco has been used over frame construction in the United States for more than a century, long enough to develop proven procedures for installation. When stucco is selected as the exterior surface for frame construction, metal lath is attached to framing members. Vapor permeable, water-resistant paper is applied over sheathing before attachment of the lath. The paper protects the sheathing and interior of the wall from outside moisture intrusion without trapping moisture vapor in the wall.
Color is determined by selecting cement and aggregate color, and quite often modified by adding mineral oxide pigments to the plaster mix. In many areas, factory-prepared finish-coat products are available. Pre-pigmented packaged cements can be used to achieve the desired stucco color or pre-weighed mineral oxide pigments can be added to the finish coat stucco during mixing.
Texture. Texture is achieved by selecting aggregate size, controlling finish mix consistency, and using special treatment techniques during and after application of the finish coat plaster. Texture gives substance and character to the plaster surface. It can be used to provide highlights, depth, continuity, segmentation, and even achieve the look of a completely different construction material such as wood timbers, brick, or stone masonry construction. A sample panel should be requested prior to construction to confirm the suitability of a desired color and texture.
What is the Difference between Stucco and EIFS?
Portland cement plaster (stucco) should not be confused with the exterior insulation finish systems (EIFS) or synthetic stucco systems that have become popular but may have performance problems, including moisture damage and low impact-resistance.
Synthetic stucco is generally a fraction of the thickness of portland cement stucco, offering less impact resistance. Due to its composition, it does not allow the inside of a wall to dry when moisture gets trapped inside. Trapped moisture eventually rots insulation, sheathing, and wood framing. It also corrodes metal framing and metal attachments. There have been fewer problems with EIFS used over solid bases such as concrete or masonry because these substrates are very stable and are not subject to rot or corrosion.