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The first part of building your cob home will be the foundation. This is the first detail to consider for building any structure, and the information presented in this video is relevant to just about any natural building you might construct. The foundation is a unified, stable base which holds the weight of your walls, upper floors, roof, and any live loads. It also distributes the weight of the house over a large area and prevents it from sinking into the ground, which can be catastrophic to a building’s structure. You must build on solid, dry ground.
Foundations can be built using a variety of materials, such as: stone, concrete, and gravel. A good foundation is vital to the life of your building and it should be given the consideration it deserves as a critical piece of the structure.
Foundations for cob buildings can include the rubble trench below ground and the above-ground stem wall that holds the cob walls. The rubble trench below ground level works with the stem wall to further distribute the weight of the building evenly, to provide water drainage around the structure, and to anchor the building into the ground which limits movement of the building’s walls.
The stem wall, which sits above ground on top of the rubble trench, extends up at least 1 ½ feet. The stem wall acts as protection for the walls and floors from water damage. It acts as a moisture break so that water does not seep up from the ground into the walls. Water runoff along the ground is not able to touch the cob portion of the walls, and water splash back from rainfall cannot reach that high.
Its important for your foundation to protect your walls from water since your cob walls will deteriorate if they are soaked by it. Even in arid climates, it is not recommended to build the cob walls starting at the ground level as this shortens the life of the structure.
Especially for massive earthen building, the importance of foundations cannot be overemphasized. Do your due diligence and plan for a solid, strong foundation. If your foundation is well designed, your building will enjoy a much longer life.
From here on, we will be focusing on how to build a simple, yet effective, continuous (monolithic) foundation consisting of a rubble trench and a stem wall.