Foundations for Adobe

Quentin Wilson and Associates, specializes in solar adobe design and construction. He grew up in the South Valley of Albuquerque, New Mexico where he watched adobe bricks being made. In the fifth grade, he made miniature adobes on cookie sheets in his mother’s oven in order to construct house models for a class assignment. By age thirteen he made full-sized adobes in the back yard and ruined the grass. Later, he traveled a bit, went through the Army, and graduated eventually from the University of New Mexico with a major in physics, minors in math, chemistry, and education in 1970. After teaching high school two years and community college math for three more, Quentin moved into professional solar adobe construction in 1976 as the Project Manager and Instructor for the Sundwellings Demonstration Project at Ghost Ranch, Abiquiu, NM. He became a licensed general contractor in the State of New Mexico in 1982. He has been building homes and teaching seminars and workshops ever since. In the fall of 1995 he established and taught the full-time Adobe Construction Program at Northern New Mexico Community College. His website, quentinwilson.com, lists the course schedule and many other resources related to working with adobe.

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.

Q: I plan to use a compacted gravel trench foundation, with a 14" wide and 8" thick bond beam sitting on top of that to support the exterior walls, post and beam, with straw clay infill. I wonder what type of foundation is required for the interior adobe walls.

A: To begin with a 14 or 18-inch x 8-inch gravel trench. The NM Earthen Buildings Code specifies that the adobe coursing should start at 4 inches above the floor level. That means a grade beam on top of the gravel, or cmu's of the knock-out or "U" shape to carry rebar and concrete. Alternatively, the first two courses of adobe bricks should be fully stabilized.

The finished adobe floor will come to the top of the exterior wall's bond beam.

(Which I call a grade beam.) This might not be permitted by the Earthen Bld Code, as above.

If I use gravel trench for interior walls, how deep, and will I need an equally large bond beam? Is a bond beam necessary to tie the buildings foundation together?

The gravel filled trench does a fine job of spreading the building load and tying the building together at the bottom as long as the local soil is stable and not moving. The Code does not necessarily recognize this fact.

Q: I am designing a small house to be built in Torrance County NM. It will be passive solar design, post and beam, with light straw clay infill for exterior walls, adobe floor. At least two major walls on the interior are to be 10" thick adobe brick wall. These are in the south rooms of the building and will serve as solar mass. These are non bearing walls. One runs parallel to the roof trusses above. This connects with the south exterior wall. The other will be midway beneath the trusses, and will run perpendicular to them. Loading, however, will be on adequate post and beam structure for that wall. I plan to use a compacted gravel trench foundation, with a 14" wide and 8" thick bond beam sitting on top of that to support the exterior walls, post and beam, with straw clay infill. I wonder what type of foundation is required for the interior adobe walls. The finished adobe floor will come to the top of the exterior wall's bond beam. If I use gravel trench for interior walls, how deep, and will I need an equally large bond beam? Is a bond beam necessary to tie the buildings foundation together?

A: For your interior walls a gravel trench that is four inches wider on each side will be fine. There should be 8-inches of gravel and as long as you are on undisturbed earth, there is no need to go down to the frost line. Myself and PG McHenry never did the concrete grade beam as we call it but just started with the adobe coursework right on top of the gravel. It would be advisable to have the adobe walls start about four inches above the mud floor for protection from the possibility of a burst pipe somewhere in the future or the possibility of moisture wicking up from the floor. Solid ground ties the building together well enough at the bottom so the concrete is optional or something to get adobe above exterior grade and interior floor levels.

Q: Is it possible to establish a lawn around an adobe house? I'm considering using native, drought resistant grass (blue grama) but have been told that a lawn and an adobe house are not compatible.

A: The traditional adobe house enjoyed flowers and other plants at some distance from the house and kept the soil around the house free of plants, well packed and sloped to drain water away from the house. If your house has a modern foundation that is waterproof and has a membrane or tar on top of the cement to stop upward migration of water into the adobe, then a lawn will be fine. I have a small lawn that abuts one adobe wall and has shown that my foundation is not 100% waterproof.

Q: I used to live in Logan in Northern Utah and the land moved a lot causing many broken foundations. One old house across the street had a fine foundation. When I asked the elderly Lady who lived there about it she told me that it had had some work done now and then. She also said that the thing she found most interesting was the last contractor had told her that the interior part of the foundation was adobe brick. She didn't know what it was covered with. The house itself was built around WW1 and was a stick built house. Was the contractor just confused? If not how would this be done?

A: That's a great story and I hope its true. Adobe bricks can tolerate some movement without a house being totally damaged but we would rather is did not have to move. If the adobes were used below grade there might have been stucco or parging as it is called on a foundation. Logan is a fairly dry climate and if there is not much water in the surrounding soil then adobe bricks would hold up over the years. Modern codes don't allow us to place adobes below grade and that is a reasonable rule. Sometimes we find old houses that broke the rule before it was written and the houses have survived just fine.

Q: We are planning to build an adobe home on our farm in Montrose, CO. The soils on our farm are "expansive clay", meaning that there is not a single concrete pad in the county which has not buckled (unless it was poured in the last decade using post-tension concrete, which we will use for out pad).

A: Montrose! Friends don't let friends live in Montrose. Delta, Gunnison, Rifle maybe. Paonia has citizens building grade beams on gravel trenches. If Montrose has such expansive soils that post- or pre-tensioned concrete is needed, then the brutal weight of adobe walls might even do in those high tech solutions. Over-excavation with tamped engineered fill might be a solution. Look around for a contractor over 65 years old to see what he did in the good old days.

Q: Is it possible to build a basement in east Tenn. mountain area out of adobe stabilized bricks or is it better to just use a concrete basement with the adobe brick structure above.

A: It has been done in two houses that I know of that did not have to deal with the NM Earthbuilding Code. They are still standing but I think it is a risk. The NM Earthbuilding specifies that adobe stabilized or not should be at least six inches above the surrounding grade level. Adobe walls should be a minimum of ten inches thick. The foundation stem wall or a basement wall need to be the same thickness to support the adobe wall. Therefore, I only built one house in 25 years of adobe construction that had a basement.

Q: We are in Colorado and use the new Mexico adobe building code. If I am making a two story addition, how thick does the brick need to be on the bottom? Do we need to get a engineered foundation?

A: The NM code specifies that the minimum for two-story construction is 14-in thick walls for the first floor and 10-in thick walls for the second floor. Should you want both walls to be the same thickness, then 20-inch walls are required. As long as the footing gets down to the frost line, is 8-inches deep and 4-inches wider than the stem wall on each side (That's the Q-rule and PG McHenry Rule since the Code only requires 2-inches each side), has two 1/2-in rebar with a stem wall the same width as the adobe wall it supports that gets 8-inches above grade and 4-inches above floor level then no engineer stamp is needed in NM.

Q: My Husband and I are in the process of planning and building a cob brick home with a budget of under $150.00 NZ to prove that it is possible with recycled resources. We would like to construct the foundations using used tyres and glass bottles because we feel this would be sustainable in a high seismic risk area here in the South Island of New Zealand. Replacing these recycled resources with the expensive (both planetary and financially) concrete. We are, however encountering some resistance from authorities etc that this would be sustainable and feasible. Are there any examples of how and where this technique has been used?.

A: I would avoid brick bottles under the house where the greatest weight is experienced. They might be useful higher up and are being collected by one of our instructors for a building on his property. Tyres should make a fine foundation if they are filled with sand, gravel, or well compressed soil. You have my sympathies in dealing with building officials. I have no advice on dealing with them in NM.

Q: I'm thinking about building a largish cabin in western MD. Rather than gathering rocks from the local surrounding I was thinking of making sun dried adobe bricks and using them instead of stone on top of a gravel trench for the foundation. I was just wondering if you knew a good way to incorporate the sun dried adobe brick, into a foundation for a wood cabin. Additionally I was thinking of making the molds for the brick with a hollow in the middle for rebar, is that necessary?

A: With sun-cured adobes we work hard to keep them away from moisture. This is especially the case of moisture from the soil. Adobe structures are almost always built on a stone or concrete foundation that keeps the adobe bricks 6- or 8-inches above the surrounding grade. The New Mexico Earthen Building Materials Code does not allow us to use adobes below grade. A few structures have been built with below grade adobes but it is always a surprise if they have survived and no surprise if they have crumbled.

Should you proceed with adobe bricks in the foundation you could certainly create holes in the adobes for rebar. However, I don't see what they would bond to if the footing is a rubble trench and above the adobes there would presumable be a wood mud sill to begin the construction of the cabin. Rare as it might be, I would probably discourage you from using sun-cured adobes in this situation.

Somewhere around 1774 the Hilltop House was built in Washington, DC. It was rammed earth and probably at least three stories high. It is thought to have served as Monroe's interim White House after the British burned the original It sat at 1300 Rhode Island Avenue NE and lasted until about 1953 when it was razed to make way for subsidized housing. To save wood, you might consider a rammed earth or adobe cabin. It will need a good foundation.

Q: My husband and I are building a house with poured adobe floors and we can't seem to find any information on how the interior walls are done. On a traditional cement pad the walls are put on the cement but is adobe too soft for that? Are they built on top of cement footings, then the floor poured?

A: Walls should be on concrete footings, exterior and interior. The adobe floors can then be poured inside the footings. For adobe walls, the footings should extend 4-inches above the floor or else the first two courses should be fully stabilized adobe bricks. If the walls are some other type of construction the rules might be different.

Q: I am currently doing a bathroom remodel and just finished pulling the subfloor, when I discovered there is no foundation in this part of the house. Because it is the bathroom I would like to dig a crawl space, and have a concrete foundation for moisture protection. Is there any precedent for adding a foundation to an existing adobe wall, perhaps via spaced concrete piers and then pouring footers in between? Currently the wall has no cracks, is 18" thick and is in good condition considering its age.

A: Yipes! My own bathroom has little to recognize as a foundation. My shower was just mud plastered walls and it had a nice earth smell after a shower. That seemed too eccentric for some of the family so the shower walls became 1 x 6 aspen tongue and groove. We lost the earthy smell and there are those who still think I am eccentric.

So if you want to proceed, you can excavate half way under the walls from the inside and pour a half foundation with short lengths of rebar pounded into the dirt on the other side at 16" on center vertically and horizontally. Install a couple of continuous rebar runs horizontally, form and pour the concrete and let it set up. Then go to the other side of the bathroom walls and excavate out that side under the walls and form up the other half with several continuous bands of horizontal rebar. The protruding stubs from the first half will tie the two halves
together and then you get a crawl space. Jack Edwards did this on his house by the plaza in El Rito, NM.

If it is too difficult to get access to both sides of the wall, then you can excavate all the way under the adobe wall for about three feet. Skip six feet and excavate another three. Do this until you are back at the starting point. Now pound rebar as far into the six foot columns as is practical, form up and pour the concrete. Then, strip the forms and shift to the next three foot intervals. By the third round of dig, rebar, form, pour you will have a complete footing. We did this on a house at Chromo, NM just by the Colorado border.

That's a lot of work for a crawl space.

Q: I have a double adobe house that was not built on a foundation in Lamy, NM. There have been 2 "hundred year" floods since I have lived here and also copper pipes were used and they all are being replaced because of all the leaks and now black mold. The house is sinking somewhat also. Should it have been built on a foundation?

A: All houses throughout the world and across the eons need a good foundation and a good roof. My own home has a section built in the 1940's before postwar money began to flow where I think they did little more than sweep off loose dirt before they began laying adobes. I am lucky in that the house has remained stable and I have been able to add an attic over that area. I guess the soil here served as a good foundation but I am about sixty feet above the level of the Vallecitos River.

Your house may be a step up with a trench filled with stone or gravel under the adobes. That is not recognized by the 2009 New Mexico Earthen Building Code as a proper foundation unless it had an architect's or engineer's stamp. Houses like this with no permits or stamps have lasted hundreds of years in NM. It is hard to determine what should have been done under adobe walls at particular locations and built at particular points in history. Adobe codes began with Albuquerque a few years before or after 1960. The State of New Mexico adopted and adapted that code around 1967. The Law of the Indies followed most of the time by the Spanish settlers of New Mexico warned against building too close to a river and insisted that all homes should be above the acequias. Floods and leaking pipes is a double punch that's not fair. I am sorry you have to face it.

Q: Do rubble trench foundations work with adobe buildings?

A: Rubble trenches are fine but are not the hill to die on in many jurisdictions. Besides, there have been torrential, Biblical rains in many places. So, a footing with stem wall that gets well above the finish grade level (on a well sloped site) might not be that much more trouble than rubble with a grade beam on top. My own home has some rubble with adobes right on top - no grade beam. That was P. G. McHenry's style. Frank Lloyd Wright came up with the grade beam idea and look where that got him.

Q:  I have a 90 year old adobe in Albuquerque..that I'm adding an addition to. Should I have any issue using sandstone as a stone foundation with bagged mortar mix (for the look of it more than anything). The addition wall would be 7 ft long and 9 feet high. with adobe brick being the wall materials.

A: Sandstone and bagged  mortar will make a fine foundation. It worked for buildings built in the era of your original building. It should be 6-inches above the exterior grade and 4-inches above the finish floor level. Most likely it will not be approved by building officials but perhaps you are not planning to go that route. NM has the Existing Earthen Building Materials Code but now requires that the building be certified as a historic building. In fact, I think the code is now called the Historic Earthen Building Materials Code. It can be found on the NM Construction Industries Division website. It sometimes takes a bit of a search to find. You could print it out and wave it in front of building officials who most likely have not read it and that might make them happy.

Q: I am planning to build my house with CSEB. Can I make a Stabilized Rammed Earth structure my the foundation for the building? If so, how better it could be than a Rubble Trench foundation?

A: My viewpoint is that a Rubble Trench is the better option. Most codes will not allow anything other than concrete or stone or cement based materials for foundations. Rubble Trenches, where allowed either by code or by architect/engineer stamp do not actually have to go down to the frost line. Since they have lots of interstitial space they mostly drain water downward and away and what moisture remains can expand by freezing into the spaces rather than frost heaving the buildings. Should you be in an area where the ground never freezes this point is of no concern. Still codes and Quentin do not want you to do any construction with earthen building materials below the finished grade.

I don't see the point of putting CSEB walls on SRE foundations. One could just as easily put a SRE wall on a CSEB foundation. It would make sense to me to just do CSEB or SRE from whatever the starting point to the top of the wall. If you plan to make your career building earthen houses it does not hurt to learn several techniques but for the once-in-a-lifetime builder, you have two systems to learn not to mention the acquisition of equipment for two very different systems. 

Q: I have a 40 year old adobe home with a few issues. Mainly, the slab wasn't a raised form, it seems the builder dug out the shape of the floor into the grade, before pouring the slab. on the exterior side the adobe begins at grade. The problem now 40 years later is, the road and property around the house is 4 to 8 inches higher than the lowest parts of the adobe wall. In monsoon, there is always standing water. I am toying with the idea of putting a vapor barrier against the exterior side about two feet high and adding a 4 inch thick masonry wall of some kind against the adobe. This would begin below frost line and rise up to two feet above grade. This would be finished tapering back to the original wall and then all re-plastered. At this point raise the grade higher than the road and low areas of the property and steeply slopping away from the house. I have not seen any other way to address this problem.

A: Somehow, moisture always gets into adobe walls from somewhere. It needs to get out and the route is often moisture vapor moving out of the wall through the interior or exterior surfaces. With the system you propose, the vapor barrier may very well keep moisture out but that one little bit that gets in has its exit obstructed. If your interior surfaces are mud plaster, gypsum plaster, no plaster, they have high moisture vapor permeability and then moisture can get out in that direction. However glossy paints or many coats of paint can really cut the permeability. If that happens, then moisture moves up the adobe wall and you may find a moisture level evidenced by efflorescence crystals or just a damp mark just above your vapor barrier and cement based wall.

I don't know why soil levels keep building up around a house, but they do and that includes my own home. Another solution would be to remove the dirt back down to or a bit below the finish floor line so the adobe is a bit above the grade. If the road is far enough away you can put in a reverse berm called a swale where the soil is graded downward at at least 1/2" per foot for eight feet away from the house. Better yet, 1" per foot. The low spot becomes a ditch and hopefully it can be routed around the house in a continuously sloping manner to the steep slope you mention to provide a runoff route for that monsoon water.

You might think that moving the finish grade down would move the frost line down. Unless you live in Angel Fire or International Falls, MN, there really is no frost line below a slab on grade home. The temperature isoclines from six feet down where the year round temperature stabilizes come right up to the slab including its edges. For most of New Mexico that grave digger's temperature is 57 degrees to 60 degrees and that temperature comes right up to the bottom of the slab.

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