Foundations for Adobe
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Q: How do you build foundations for an adobe house (2 floors) on the sand? The location would be on a stable sand dune. I don't know yet if there's any layer of rock at a decent distance from the surface. But, in the case the layer is very deep, what can we do?

A: Sand is not such a bad base for a house. The foundation needs to get down to the frost level if there is one at your location. The foundation needs to be a minimum of 8-inches thick and should be at six-inches wider than the wall on each side. For two story construction the first floor needs to be at least 14-inches thick and the second should step back to 10-inches. Adobe walls should not go higher than ten times their thickness so if the wall does not step back for the second floor then it needs to be twenty-inches thick all the way up. My best recommendation is to stick to a single story if you have not built adobe before and given that you are on a sand base.

Q: If one followed the 10 to 1 width/height ratio, could we build a relatively self standing adobe wall (used like a fence) at the heights mentioned? We had learned that if building a cement block wall of over 4 feet tall, one has to reinforce with mortar & rebar. We would also love our 'wall' to have some curves. Thanks for any hints on reading material and whether homemade sun baked adobe bricks would support this wall project.

A: It will work just fine. Just stay a bit below the height limitation of a 10" or 12" wall. A good foundation to the frost line or else a Frank Lloyd Wright gravel trench with or without the concrete grade beam will work. Curves in the wall make it stronger. We do have an adobe discussion group at adobe-subscribeATyahoogroups.com I normally don't recruit for that group but you might benefit from it more than most.

Q: Could it be useful to use tires for the foundation of an adobe house?

A: Tires rammed tightly with soil make a reasonably acceptable foundation. I don't think that it saves time or energy. My preference would be large flat stones dry stacked or round stones laid up in the minimum amount of cement/lime mortar.

Q: I am designing an adobe house. Since abode walls have to be protected from rain I was thinking of elevating my house and using rocks up to the plinth height. Will it be structurally safe as the house elevation will be partially in a artificial pond? If yes/no what kind of foundation should I go in for?

A: This sounds like a really interesting project! Actually a stone foundation might be one of the best choices if your house is to be situated in standing water. The reason for this is that because of the spaces between stones, there is little chance that they will transmit moisture upward due to capillary action. If you use a cement/lime/sand mortar between the stones, that might change since that mix might support capillarity. There are plenty of successful buildings around the world sitting on stone foundations.

Q: I am wondering though about the capillary action of the concrete foundation. Our house site is in a Very dry area with the water table down around 200 feet or so, so I'm not worried about that. My question: Is the capillary action of concrete such that, during mild to heavy New Mexico snow melt or summer monsoons, enough moisture could wick up into the foundation from the ground to threaten the integrity of the structure, given that the first course of the house is all of unstabilized adobes?

A: It is useful to paint emulsified asphalt on top of the foundation or trowel on one of the roofing asphalt coatings. Since your adobes are already in place, you might coat the exterior surface of the foundation with asphalt or Bituthene or similar sheet material. In the long run, I don't think you have much to worry about. The traditional response to the snow problem was to sweep or shovel it away from the building.You will do fine. Worst case is that the house might only last 200 years instead of 2000.

Q: Firstly, my plan is to build a small (10' diameter dome) using earth, with an earthbag stem wall filled with pumice (on site). Thoughts?

A: Over the years, I have worked with pumice in a variety of forms. In ground contact it becomes damp and looses its insulating abilities. In walls, it has higher moisture content than the ambient atmosphere and surrounding materials. Pumice is well designed to be a fine capillary action medium and any moisture in pumice becomes well disbursed and uniformly damp or dry very quickly. Therefore you will want to introduce a good moisture and vapor barrier between the bag wall and the adobe.

I am no fan of the use of bag walls at the lower part of the building. Filled with pumice and no binder such as clay or cement, the system depends entirely on the polypropylene material. As evidenced by the short lifetime of blue poly tarps found at most construction sites, the material has little resistance to ultraviolet degradation. Therefore, you will want to have a very good plaster or some other form of covering over the stem wall to protect the bag material. You will want to be careful to maintain that coating for the life of the building. ironically, the old burlap material which has now been replaced by poly for sand bags lasted far longer out in the open.