DIYing Portland cement plaster for exterior of house
Some home projects we do for the joy of the process. Some we do for a just perfect outcome. Others, we do because we can’t find a professional who wants the job.
Stuccoing the outside of my parents’ home falls in the latter category. Between the scaffolding and the multi-layered process, it was not a job we really relished. Wanting traditional stucco, rather than acrylic, we struck out to hire someone a few years ago.
In case you aren’t well versed on cement terms, stucco is the common name for Portland cement plaster on exterior surfaces. In our case, the exterior surface is the upper half of a large house.
The process has several steps. First, we secured waterproof paper membrane onto the outer layer of plywood covering the house. The next layer consists of lath, a metal mesh paneling, which allows the stucco to go through the holes and form a strong bond, so it can’t just chip off the wall.
After that, we applied layers of cement stucco. The first layer, the scratch coat, pushes through the lath, and then we used a toothed trowel to scratch ridges into it, so the second (brown coat) could bond to it.
Finally, we applied a third skim stucco coat, about 1/8 inch thick, for a total of less than an inch of stucco and associated layers. We used a white top stucco skim coat with added colorant to achieve the final shade we wanted.
Each coat is made of a pre-mix of cement and sand. Portland cement is a mix of limestone, iron and clay or shale. For the scratch and brown coats, we just added water and a pinch of fiberglass for strength to type S mortar, and for the finish coat, we added colorant, latex and water to the pre-mixed white stucco.
Slathering the cement, sand and water mix, I sat two and sometimes three stories up on scaffolding. When my arms got tired, I took a break and gazed out over our hollow and admired the panorama of trees and flowers gone a little wild.
My sister and I both enjoyed this view, while competing over who spread the plaster faster.
Wondering where we fell in the history of this trade, I did a little research after finishing one side of the house with all three layers.
Turns out, stucco has been used worldwide for centuries. The Greeks used it on the walls of temples as early as 1400 BCE. The Aztecs used it in Mexico. The Romans used it on huge monuments as early as 120-130 CE, and Islamic architecture in North Africa and Spain also included stucco.
Traditional stucco uses breathable materials, so if moisture gets in, it can dry out. It also insulates and helps block outside noise.
It can be manipulated to look many different ways by the use of different grits of sand or addition of small stones. Or it can be painted, or used to create low-relief modeling.
Modern synthetic stuccos, made from a mix of acrylic resins and applied to foam board, lack breathability. Water potentially can leak in around windows or doors and, thus trapped, can cause mold or other damage to the structure.
Technology is improving and manufacturers may improve the breathability of synthetic stucco. When painted, traditional and synthetic stucco are hard to tell apart.
We opted for traditional and therefore, it turns out, for DIY. Through the process, I’m gaining a skill, arm muscles and appreciation of this trade and the folks who do it professionally.
