One of the most enjoyable natural materials to work with is earth plaster (which is also referred to as mud plaster and clay plaster).The material is as simple as the name implies. Earth plaster is nothing more than sand and clay (with chopped straw, if desired) mixed together in the proper proportions to prevent cracking. When mixed and applied properly, this wall covering will be hard, durable, and beautiful. In many regions of the world, earth plaster can even be used on the exterior walls and may easily last for seven to ten years or more without any maintenance.
One advantage of earth plaster in a natural/hybrid home is that the color of the plaster itself can eliminate the need for any paint. This is good for the environment and can reduce construction costs. Earth plaster walls can be made in almost any color you desire. Even white walls are possible. For color tints, natural pigments can be added to the white plaster, and a range of colors can be created from yellow to red to green.
The wall finish is a major visual attribute to the overall look of the house. The earth plaster walls have a rich patina that is not possible with paint alone. The material is breathable, unlike many paints and stucco. This is ideal for straw construction in that moisture from within the walls can evaporate out. Breath-ability is important in any wall system, and particularly important when building with straw or cob. The material is also easy to apply by beginners. This is especially true for the initial coat. Earth plaster can be applied directly to the walls. The fibers of the straw in the cob or straw/clay walls create a very strong bond with the mud, and stucco netting is neither necessary nor desirable.
Although earth plasters can be very durable, you will not want to use them indoors where direct water contact will be probable. The walls in direct proximity to sinks and showers should be coated with tile or a water-resistant plaster. We will dedicate a follow up article on the waterproofing of earthen plaster in the very near future.
Our straw straw/clay home will use a three-coat system of applying the earth plaster for the straw/clay walls. Two coats will be used for the cob walls. For the straw walls, the first coat is quite thick (around an inch). This coat filled in all of the irregularities of the straw/clay wall. The second coat is used to even out any undesired undulations in the first coat. The final coat is just a skim coat to add the desired color and texture. Earth plasters, set very slowly and can be worked for days. This is quite an advantage for the do-it-yourselfer, and you will be able to work at your own pace to create the desired finish.
And remember, whatever you do, it’s really you.