Image

Masonry cement and plastic cement are used to make stucco for residential buildings.

Credit: PCA

The Real Thing

Traditional portland cement plaster is a time-tested exterior finish. It consists of portland cement-based materials and sand mixed with water to form a workable plaster that hardens to provide a strong, durable, and tough surface. Portland cement plaster is applied either by hand or machine to exterior and interior wall surfaces in two or three coats.

It may be applied directly to a solid base such as masonry or concrete walls, or it can be applied to metal lath attached to frame construction, masonry, or concrete. Applied directly to concrete masonry, portland cement plaster provides a tough ½-inch-thick facing that is integrally bonded with the masonry substrate. When applied to metal lath, three coats of plaster form a 7/8-inch-thick facing. When applied to water-sensitive sheathing or framing, the plaster is separated with a vapor-permeable, water-resistant building paper. Portland cement plaster has high-impact resistance and sheds water, but it also breathes, allowing water vapor to escape. It's a proven system that works in all climates.

Strength And Beauty

Traditional portland cement stucco provides a tough exterior that resists attack—from woodpeckers to weed whackers to grocery carts. That inherent toughness is beautifully complemented by the variety of colors and textures available. Whether it's your business or your home, appearance is an important consideration in selecting a wall cladding. You can select a color and texture consistent with the message and image you wish to convey.

Color is determined by selecting cement and aggregate color, often modified by adding mineral oxide pigments to the plaster mix. In many areas, factory-prepared finish coat products are available. Prepigmented packaged cements also can be used to achieve the desired stucco color, or preweighed mineral oxide pigments can be added to the finish coat stucco during mixing.

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. To confirm the suitability of a desired color and texture, be sure to ask your plasterer to provide a sample panel for evaluation prior to starting work.

Durability And Versatility

Portland cement plaster has a well-documented history of proven performance in extreme climates from the desert southwest of Arizona to the severe winters of Minnesota to the hot-humid Florida summers. It performs in all climates. That durability is complemented by its versatility. Portland cement plaster provides an ideal finish or cladding for any building construction system and it can be applied to any flat or curved surface either inside or outside of any structure or building. More exotic uses include simulated mountains, trees, and rocks in amusement parks and zoos.

Complement To Concrete Concepts

The durability and versatility of traditional portland cement stucco provide a great finish for other concrete building systems including concrete panels, insulating concrete forms (ICFs), and autoclaved aerated concrete. 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 directly bonded to the concrete, a dash bond coat or surface-applied bonding agent typically is needed to enhance the bond between the plaster and the concrete. 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, ICFs, or autoclaved aerated concrete walls to provide a base for a durable, traditional three-coat portland cement plaster exterior.

For the right planning and preparation in the construction of portland cement plaster, the first step is to follow the requirements of ASTM C 926, Standard Specification for Application of Portland Cement Plaster, and the guidelines given in the Portland Cement Plaster (Stucco) Manual available from the Portland Cement Association. The result will be an attractive stucco finish that has the toughness and durability of concrete.

For further information, contact the Portland Cement Association at 5420 Old Orchard Road, Skokie, Ill., 60077. Call 847-966-6200 or visit www.cement.org.