Earthen Floors have existed in the human culture for thousands of years. They are durable, beautiful, and provide thermal mass for any home. However, over time they began to develop a negative connotation, being associated with the developing world. However, earthen floors are making their way into modern society, rightly so. Sukita Crimmel, a pioneer in this movement lead us through two projects during my time at Aprovecho. The first project was located in the pantry of the soon-to-be commercial kitchen. We decided to do a little experimentation and divide the small room into four zones to test out different mixtures. One section was a ratio of 1 part clay, 4 parts sand, and chopped, sifted straw to taste. Another section was the same ratio of clay and sand, but horse manure instead of straw. The next section was just clay and sand and for the last section, we used Sukita’s Claylin mixture. These ratios were based on the materials we were working with in Oregon. Remember to test your mixture first. To figure out the ratio of sand to clay we did three small test patches; 3:1, 4:1, and 5:1. The best results were 4:1 with the least cracking and most smoothness. Like most things in natural building, the sand is the structural integrity, the clay is the binder, and the straw is the tensile strength.
There are two scenarios an earthen floor can be applied: slab-on-grade or slab-on-existing (plywood or concrete). The pantry was on existing, so we made sure there was enough structural support first. A relatively thin layer was poured and leveled using both a laser and a screed. A wooden trowel is good for moving the material around, while a pass with a metal trowel gives it a smooth texture. Keep fans on the floor all night in order to prevent mold. The next day we mist a little water and burnished using a metal trowel. No more than 1/4” of pressure was applied. Trays are good for kneeling and moving around on the floor in order to prevent indentation. After three weeks, the floor was ready for oil and finishing. We applied linseed oil with large brushes. Note: The oil can lift some particles, so when the oil has been on for at least a week, sanding with Brillo pads can smooth it out again. Finally, the floor was ready for waxing.
We worked on a slab-on-grade earthen floor in the Farm Office during the Sustainable Shelter Series. The first layer of the slab-on-grade was tamped and leveled subsoil. The second was a drainage layer (4”) of level and compacted rock. The third layer was a vapor barrier of 6 mil plastic. If the rocks created an unlevel surface, relevel using sand. The fourth layer was insulation (4”). Here, we used pumice rock, but rigid foam can be used as well. The fifth layer was roadbase (4“) and lastly a 1” layer of earthen floor. Sukita’s book, Earthen Floors: A Modern Approach To An Ancient Practice has lots more info and is a great resource. The same steps are taken for the earthen floor as the slab on existing, including oiling and waxing.