As previously mentioned in order to begin building an earthen home you must look around and use what materials are local. Most buildings use some form of a mud mixture whether for plaster, mortar, or a mud mixture. So, the first step is to test the type of soil on the site. Soil contains clay, sand, and silt in various proportions. Natural buildings require clay as a binder, sand as structure/stability, and fiber for tensile strength. When conducting a soil test do not use top soil; dig 50 cm deep into the subsoil for a sample. Next, find a glass jar, add a tablespoon of subsoil, and fill with water. To speed up the separation process add a drop of detergent or salt and let settle for a day.
Obtaining soil from one´s property is the most affordable and sustainable way to build. Because large amounts of soil are used the location of dirt excavation is important. For instance, a house on a slope can have multiple levels or terraces if the excavation is done in place. At Mama Roja the dirt is obtained from the front of the property and will be turned into an artifical pond after building.
If you don´t want to use soil from the site or it is not the right type, sand and dirt can be brought in to make the right mixture. Often times construction sites discard soil so it is not difficult to obtain. After soil testing, Kim and Marcelo determined the correct amount of soil to sand ratio. Since the soil has a lot of clay, more sand needed to be added. The mortar mix which binds the adobe bricks is a 20:8 or 10:4 bucket ration of sifted soil to sand. The soil is sifted to remove all rocks form the pit. After we fill the wheel barrel with the right amount of sifted soil we take it to the site and dump it on a large tarp for mixing. Sand and pine needles are added with water, in increments. A few people stomp on it and mix it around with their feet to get an even consistency. Also, tossing the tarp back and forth reveals areas that need more stomping. Once it is ready, we fill buckets and begin building.
The walls are made from adobe bricks which are essentially mud bricks that are sun-dried instead of cooked in an oven. The foundation is built with cooked bricks and cement. Kim and Marcelo experimented with different types of foundations such as stone, but found that this bricks and cement were best. Again, everything is location dependent. Some places are very humid and require a layer of plastic between the foundation and the start of the wall to prevent moisture from the ground to travel up. In general, although concrete and metal are used in some natural buildings, they are used in the smallest of quantities because of the high amount of energy needed to produce them.
The bricks and mortar are laid by hand but guided by a simple string level system in which a string is wrapped around two bricks and are placed on opposite sides of the wall. A plumb bob makes sure the line is in the correct level placement. The roof is built made of tongue and groove slats painted with borax solution to protect it against fungus and insects. Next, slip straw is spread evenly across the roof. This is a mixture of mud, straw, and alfalfa. Basically, it is a dry mix that is bond together with a bit of mud.