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Q: Can you give me any information on the type of footer I would need for an earthbag building? What about a stemwall if you are building in a wet climate?

A: There are several options for making a foundation for an earthbag building, depending on the type of soil and climate in your locality. One of the simplest is to make a "rubble trench foundation", which is described here and here. A frost-protected foundation is described here.

The need for a stem wall depends more on the building design than on the climate; the foundations described above would work fine in wet climates.

Q: I am interested in building an earthbag home for my mother and stepfather however we are kind of stumped by the foundation portion. We can get up to 2 weeks of rain at once and then go 2 months without any rain, the soil is kind of unpredictable.  I assume we would need to build up the foundation so it will not gather water but should we dig below ground level and use small rocks in the bags for the first 3 rows? 

A: I would recommend digging below frost level with a trench that is as wide as the bags and filling this with enough rubble or drain rock so that the first course of bags will be embedded several inches below grade level. And yes, filling the first few courses of bags with gravel is a good idea to assure that moisture does not wick upward. If it seems likely that this rubble trench will fill with water during heavy rains, then a French drain arrangement, where a perforated pipe embedded at the bottom of the foundation can collect the water and run it off to some "daylight" location away from the building, might be a good idea.

Q: Does anyone install any kind of land drains from the foundation trench?

A: Depending the soil type and the contours of the grade at the site, this might be recommended. The soil at our site is pure sand and drains quite readily, so we didn't bother with drains.

Q: As I thought about your recommendations concerning the rubble footing/sand bag stem wall, something occurred to me that I wanted to check with you. You mentioned it was best to place a layer of large (4 inch) rocks at the bottom of the trench and then layer(s) of progressively smaller gravel until just above grade - then start the bags. At first I thought this was just for drainage, but then it occurred to me that tamping these layers would wedge progressively smaller stones between larger stones, thus stabilizing and distributing the load in a horizontal as well as vertical plane. Does that make sense or am I inferring something that wasn't intended?

A: I think you are absolutely right in your thinking about the distribution of aggregate for a foundation...this is what Mother Nature would like.

Q: You built a house with a sand foundation. How is this done? Do you have some details on foundations?

A: Actually I built an earthbag house that happens to sit on a huge bank of sand, since that is the native soil at this site. Earthbag structures do not really need a traditional concrete foundation, since the bags themselves become a sort of foundation. Earthbags can also be used as a foundation for other sorts of buildings, such as strawbale buildings. The sand is nice because it drains readily and therefore does not present a problem of frost upheaval, as would more expansive soils. One way to deal with these kinds of soils is to utilize a rubble trench foundation.

Q: Would there be a water run off problem building in England where there's lots of rain?

A: Our experience is in the fairly arid Southwestern U.S. As with any building, adequate drainage around the house would be essential.

Q: Is that a case against having the floor inside the building lower than the ground outside - wouldn't that encourage dampness to seep in from the outside?

A: I would usually advise keeping the interior floor level above the outside grade where the doors are. It is possible to create covered entrances and special drains to keep the water from entering the house when it is dug in lower than the grade, but this must be carefully done.

Q: I want to get your thoughts on drainage issues. Berming a home into a hill obviously raises some concerns here, perhaps particularly where our sandy soil means VERY RAPID water movement over and through the soil. I think if you can channel that water effectively, it very quickly moves on, but if you don't, I could imagine some problems.

A: Obviously, drainage is very important. First of all you would want to create a circular swale around the back of the house to direct surface water around to either side and not into the house. Then the water that percolates into the soil near your house needs to be kept from entering the house with a moisture barrier. I used two layers of 6 mil polyethylene draped over the earthbags before they were backfilled to accomplish this, and it has worked out well. In very wet climates, you might also want to place a French drain at the base of the plastic to direct water around the house, with "daylight" exits for the water to run out.

Q: If the foundation bags are laid on a rough stone foundation, doesn't the weight of the walls puncture the bags?

A: Perhaps rounded drain rock would be best as a foundation material in the trench. A smoother gravel would be easier on the bags. Also I used two bags (one inside the other) on the first row. Once the bags are packed into place there is little stress on the bag material.

Q: Would gravel be best for bag fill in the lower rows to prevent wicking in a wet climate?

A: This would likely be an excellent idea. Such bags with gravel have been used successfully for foundations for strawbale building.

Q: I am planning on building an earthship and am researching the possibility of building it using earthbags. I would be doing a multiple U and hut design so most of the walls would be curved for strength. The soil at the site is a heavy red clay that gets rock hard and is very rocky. Most of this area (central NC) seems to have similar soil. I would like to do a partial berm on the north side for cooling. My first question is about foundations. What kind of foundation do earthbags require? Could I place a couple of layers of tires down and use those as a foundation?

A: I think your idea of building an earthship using earthbags instead of tires is an excellent one. Earthbags are much more versatile, easier to use, just as durable as tires, much less work to "pound", require much less plastering to make a smooth wall, and are less likely to off-gas over time. Earthbags themselves provide their own foundation. In fact earthbags filled with gravel are often used as a foundation for other types of construction, such as strawbale walls. Starting out with tires is unnecessary and would create much more work.

Q: Do we need to dig out a perimeter trench? I read that you did not do this. Is this because you are in a more desert-type climate?

A: You're right that I didn't use any particular foundation other than the sand that is on our lot, because the sand drains so readily and isn't really subject to frost upheaval. In other types of soil it may be advisable to prepare a kind of rubble trench as a foundation. Another possibility is to fill the first few courses of bags with gravel to created a kind of raised foundation that won't wick moisture. Recessing the first row of bags below grade would be a good idea if there is any concern about the soil washing away.

Q: My question is about earthbag foundations and strawbale buildings in a seismic area. Any thoughts on this subject? The structure is going to be a small, pole built shed with a bale wrap, so the bales won't be "structural" in the sense of holding up the roof. I'm guessing that the concern in a seismic area would be the multiple forces on the foundation....up down and side to side, effectively taking the foundation apart at the seams. I really appreciate your help on this as down here in Costa Rica.....I'm kinda on my own!

A: I know of several strawbale projects built upon earthbag foundations quite effectively. In Costa Rica you wouldn't have frost upheaval issues to deal with, so it should be pretty straight forward. I would suggest filling the first course or two of bags with gravel, so that water will not wick upward towards the bales. As far as seismic concerns, I think bag foundations would likely hold up way better than concrete, which has no give whatsoever. At least the bags can flex some without failure. You might be surprised how strong a bag wall is, once the two strands of barbed wire are placed between each course, the whole thing is tamped into place, and some sort of plaster is applied to the exterior to protect the bags. Seismic testing that was done at Nader Khalili's CalEarth site demonstrated considerable resistance to failure. If the bales are simply infill material in a post and beam structure, I don't see why you should be concerned, even if the walls shook a bit. You might somehow attach the barbed wire from the bags to the poles, to help tie the foundation to this rigid structure.

Q: My son has just returned from an earthbag building course. We live on a Caribbean island. Our plan is to build a house onto an already existing concrete foundation and we have been advised to drill and insert rebar vertically into the concrete with a layer of gravel between the concrete and the first row of bags to anchor them.

A: Pinning like this is a good insurance that the bags will stay put, but I see no need for the layer of gravel; in fact this gravel might actually act in the opposite way of causing the bags to slip.

Q: I am trying to find a formula for materials ratio in bags for an earthbag foundation for a strawbale home. Specifically what materials and what ratio.

A: I recommend the use of washed 3/4 minus gravel to fill earthbags for strawbale foundations. This will keep any water from wicking upward and will provide a bit of insulation at the same time. Of course the bags need to be well protected from the sun and physical damage with a good plaster...and rather soon.

Q: How much sun is too much? We put the bags up in October and covered the site with a tarp. At some point the weather exposed some of the bags, I would say the bags have seen as much as 4 months of a Vermont winter sun. We'll stucco the bags in the next 2 weeks.

A: You can tell if the bags have deteriorated too much by poking them with your finger. If you can tear the fabric at all this way, then it is usually too far gone to be very dependable. Obviously, it depends on how extensive the damage might be as to how serious it is. A spot or two of this softness won't affect things much, but a large area is not good. I have seen bags rot from the sun within a month of very intense Southwest sunlight. Even with a larger area of damage, you might be able to save the situation by wrapping them with chicken wire or other mesh reinforcement material before you apply the plaster. This is not a bad idea regardless of the state of the bags, since it will make your stucco more permanent.

Q: There is a high spot on the property that we have chosen as the home building site. It just so happens, that it is a high spot due to being on top of solid lava rock. Here is where my question starts. How important is the trench for the sandbag foundation? Is the trench mainly for drainage or also for stability of the structure? I guess I could get a jack hammer, but it will be quite a job to cut through. Also to level the site, do you think we can just bring in some gravel or cinder?

A: I built my earthbag/papercrete house in the natural sand of the site without any trench foundation and it has been fine, showing no tendency to shift whatsoever. If you were to build on solid rock, I think you would be fine. Rubble trench foundations are mainly needed where the soil doesn't drain well, or frost upheaval is a problem, but this would not be likely where you are.

You could level the site with gravel fill, as long as it is sufficiently compacted and contained so that it doesn't shift over time.

Q: My site has hard packed soil and ledge and rocks below surface. This is in Jordan and it is quite arid, getting rain only in winter. Would it be possible to build in such a site using only a foundation of gravel filled bags for the first few courses?

A: I think that a gravel-filled bag foundation would do fine in the climate of Jordan.

Q: I live in Mozambique and am about to start building, and over the last few months have been introduced into sandbag building. Firstly, my land is on a sandy small slope. Can I build a retaining walls with sand bags? Do they need foundations with vertical support beams? Do I need to lay down a solid stone foundation/slab before starting work on the sandbag walls?

A: Earthbags, or sandbags, make an excellent retaining wall. I would suggest leaning them at a bit of an angle against the hill, so that they want to fall into the hill, and then they shouldn't need any other support. You can backfill the wall as you go up with it to keep it from falling. You do not need any other foundation for this wall...it should serve as its own foundation. You might dig the first course down into the ground to make sure that it doesn't slide on the surface...and fill this first course with gravel to help with drainage. Be sure to plaster the bags to keep the sun off!

Q: I have decided that earthbag/superadobe is for me. I have found a retired building inspector who will draw my plans for a building permit. I need information on the foundation.

A: Earthbag buildings do not really need the sort of continuous concrete foundation that most other building forms require, but, unfortunately, the building codes are not likely to agree with this. There is an example of a simple rubble trench foundation here that might help visualize one method. Your retired building inspector should be able to help you determine what the authorities will allow.