Making Buttresses for Earthbag Buildings
Questions answered by Kelly Hart

Q: When space is wide open and needs buttresses do they need to be on the interior as well as the exterior?

A: Either side will do, as long as they are well attached to the wall.

Q: Can buttress walls can be as far apart as 10-14 feet and be as little as 2 feet deep and 2 feet wide?

A: I wouldn't go much more than about 10' between buttresses on vertical walls, especially if they are over 8 feet high. I would say that 2' is about the least depth to use, but they can be thinner, say the width of a bag should work. I like tapered or stepped buttresses that are deeper at the base.

Q: What exactly is involved in building vertical earthbag walls without buttresses?  Is there a safe way to build vertical walls without buttresses? 

A: The very best design for vertical walls without buttressing is to curve them. Otherwise putting rebar vertically through the bags can help rigidify the plane of the wall, but this will not necessarily keep the entire wall from buckling or toppling. Also, the thicker the wall the more stable it will be in general. Then reinforced concrete bond beams, or wooden box beams integrally connected to the wall, especially at the top will add quite a lot of stability. Such a bonding layer could also be introduced lower down in the wall, especially with rather tall walls.

I noticed that the black South African couple seemed to build without buttresses.  How are they doing it?

They are using a unique system of double bags with a void between them, and then using this void to create lintels and bond beams with reinforced concrete. You can see in one of the pictures that the lintel over the windows actually runs the entire length of the wall, so they are basically creating a bond beam at this level.  

I also was intrigued by the white South African woman's home that seemed to cleverly hide the buttresses in the design (am I right?) but the black couple seemed to not use them at all. 

Using interior room divisions to naturally buttress vertical walls is an excellent idea. She used the "eco-beam" construction described here  to build her house, and this system basically uses earthbags just as infill within a rigid framework.

They also weren't used in the Haiti home.

This is a small home with an L-shaped footprint, so there were only two longer walls that might really need buttressing, and it appears that they used interior wall partitions to accomplish this. Also, they did use rather large bags to begin with.

Is it possible to build without buttresses if you dig a deeper trench for your foundation? 

I  don't think that in general deeper trenches will help much. One approach might be to create periodic vertical posts of reinforced concrete that are buried into the ground, and this could substantially stiffen a wall.  

OR If the bags were stacked side to side instead of end to end, would this MUCH thicker wall make vertical walls without buttresses possible?  I realize this would use a lot more bags, but do you know if it would work?

Yes, thicker walls help, but doing as you suggest makes it hard to create a running bond by overlapping the bags, and it also means that each bag has to be more securely fastened, which can take time. Double columns of bags are also a possible way of making thicker walls and provide the opportunity to place insulation on the outside and denser, thermal mass material on the inside. Another way of making thicker walls is to taper them somewhat by starting with very large bags at the base, and then graduating to smaller ones as the wall rises.

Q: Many earthbag house plans seem built around curved or circular walls with most square walled plans showing external buttresses for lateral stabilization. Is all this buttressing necessary for non-circular walls and will internal walls suffice?

A: Straight vertical walled earthbag structures do often need buttressing, but this can be done with internal walls, as you suggest. Each design is different, and needs to be evaluated independently to determine where buttressing might be needed. Also, it is possible to stabilize earthbag walls in other ways, such as with periodic columns and horizontal bond beams.

Q: I am planning an oval with about 30 foot of vertical wall on both sides. I would think three buttresses on each side should be enough right? As to the buttresses, you only need them on the outside right?

A: Buttresses can be either on the inside or the outside (or both), as long as they are well integrated with the wall. Interior walls can effectively act as buttresses. Conventionally, buttresses seem to be spaced at least every 10 feet, so 30 feet of vertical wall may need 2 of them, dividing the space into 3 sections. A continuous reinforced concrete bond beam at the top of the wall will greatly strengthen it. There is some good discussion of building design here.

Q: On a round house made with earthbags, if you don't build a dome but rather make a different kind of roof, do you need to do buttresses?

A: Round earthbag houses don't necessarily need any buttressing, especially if they have a good strong bond beam at the top.

Q: When you say "sufficiently buttressed", are you suggesting that we place buttress walls on the interior of the house? I have seen diagrams of buttress walls used on the exterior of structures, is this basically the same idea? The exterior house dimensions are 20' x 40', the 40' measurement is the length which is bermed against the dirt at the back of the house. How many interior buttress walls would be sufficient to withstand the pressure?

A: The buttressing would depend on the specific design. Interior walls can be used to perform this function. The amount of needed buttressing would also depend on how high the berm on the outside might be. In general, with a straight wall buttressing (either interior or exterior) is needed every 10 feet. If the berm is only say 2 feet high, then this pattern may be sufficient. If the berm is much higher than this, then all of the buttressing would need to be in the interior. To some extent, extra reinforcement in the wall can be used to stiffen it, such as a bond beam at 2 - 3 foot intervals going up the wall. In any case, you will want to incorporate a substantial bond beam at the top of the wall, all the way around.

The back wall will be bermed to 4 feet above floor level across the entire 40 ft length of the house. Will interior wall buttresses every 10 ft be sufficient here? And yes, we planned on straight walls with a solid bond beam all the way around at a 9 ft height.

If your berm goes up four feet, I would suggest yes, interior buttressing at 10 foot intervals AND creating a bond beam at the four foot level (even if is just a steel or treated wood 1/1/2" X 6" beam embedded in the bag wall. I would also suggest using your vertical rebar at about 2 - 3 foot intervals pounded down through that entire bermed area.

Q: The spring-line for our dome is at three feet high, and so based on the calculations we will need to provide some buttressing. We decided to lay a buttress ring all the way around the dome and use it as a bench. Do we need to excavate below the buttress ring and create footings or would we only do that on the wall of the dome and just place the buttress ring on the ground? If we had to extend the foundation trench to include the buttress wall it would be approximately 3 ft. wide (which is ok if it's necessary, but a lot of extra work if you don't need it).

A: I would think that if you filled the first course of bags for your buttress/bench with gravel, that this would adequately provide a foundation for this.

If we put in that buttress ring and we wanted to have a French drain system at the base of the wall, where would we put the french drain? On the last project we did, we didn't have a buttress ring, and we put the French drain in the same trench we dug out for the foundation. But where would we put it in relation to the buttress ring? I'm particularly thinking where would it go if we don't extend the foundation trench to include the buttress ring.

This is a good question. It seems to me that the primary purpose of the French drain is to keep moisture from entering into the interior of the dome, and that if it were place in the bottom of the rubble trench foundation for the dome that it would still function in that way. The gravel bag layer should take care of the bench itself.

Q: I'm planning on building a square of 8 by 8 meters (26 feet). Do you think it would be necessary to curve a wall this length, or will it be stable? (I will place bond-beams and a heavy roof.)

A: Curved walls are definitely more inherently stable. For straight walls, the rule of thumb is that they should be buttressed about every 10 feet so your 26 foot wall would need some buttressing, although this might be accomplished with interior partitions in the house. You could probably get by with one partition/buttress near the middle of the span, and then plan on pinning the bags with rebar that is attached to the bond beam in between.

Q: I just learned about earthbag construction in a search for root cellar ideas. I currently have a 10 foot deep hole that is 13 feet wide by 20+ feet long. My plan is to build a standard farmed room in the center of the hole (10x12x8'high) and then build an earthbag outer wall around the structure to the top. Then lay (and spike to the wall and earthbag wall) logs across the 10' span on 12" centers and sheet with PT 1 1/4" plywood. Finally, after encasing the wooden walls and the roof in 60mil plastic I'll bury the entire structure. To enter I'll build an earthbag constructed 'porch' and walls leading out of the underground root cellar. Do you think the earthbag walls will provide the strength I need to prevent cave ins from the 10' high walls of the cellar hole?

A: Your plan basically sounds good. Normally I would say that straight, vertical walls of earthbags could not be relied on to withhold the pressure of the soil, but in your case the wood-framed building should act to sufficiently buttress the bag wall. You might place the vertical studs somewhat closer together to help with this. The area of most concern might be the entryway, where the bags themselves will need to retain the soil. Here I suggest that you drive rebar stakes all the way through the bag walls and leave enough protruding at the top to embed in a poured concrete bond beam.

Q: What do you recommend to stabilize a wall against an earth berm? We are using wide bags with a 20% lime content and the fill does set up pretty hard.

A: A straight and vertical earthbag wall needs buttressing, especially if it is will be backfilled or bermed. This can be accomplished with interior walls to some extent, or at the very least, you need vertical rebar pounded down through the bags at intervals and left standing free at the top enough to be incorporated into a concrete bond beam at the top.

Another approach to dealing with this could be what I have been doing with a recessed pantry wall. It is inclined outward noticeably, so that there is no impulse to topple inward, even with the berm. You can see what this looks like in a recent blog post. You will also notice that the plastic moisture barrier has been in place as the wall is built, and backfilled securely against it.

As for stabilizing the fill, I would say that if it is hardening up nicely, then that is all you need. In my case, with the inclined pantry wall, I am not stabilizing it at all...nor am I using barbed wire.

Q: When is it necessary to provide a column or buttress? Will a 6.5x6.5 meter residential building work without columns or buttresses?

A: It depends on the spacing of windows and doors which especially need to be supported. Also, any straight section of wall more than about 3 meters should be buttressed, or at least reinforced with a stiff bond beam at the top and vertical rebar pounded down into the wall that is connected to the bond beam.

Q: Can you go with larger size bags to make buttress walls around doors unnecessary? I have noticed that in some cases no buttress walls have been used. And eliminating them can give you extra space, save you time, and make the design more appealing.

A: I think the only safe way to avoid buttresses around door openings is to use fully cement-stabilized fill, as well as very well reinforced internal pinning with steel rods. But this still makes it difficult to place doors vertically, especially in small domes. Buttresses are better.

Q: How many feet do each buttress need to be spaced out if say a wall that will be 12 ft high and 150 ft long.

A: About every 10 feet is a good rule of thumb. These buttresses can be interior walls if they are well connected, and they don't have to go all the way to the top necessarily.

Q: How far out do the buttresses need to come for a 12 ft high wall and for a 20 ft high wall?

A: For an 8' wall a vertical-edged buttress must stick out from the wall at least 2', and a sloping or stepped buttress 2.5'. For higher walls, I would suggest keeping that ratio, so for 12' go about 3' or 3.5' and for 20' go 4.8' or 6'.

Q: I live in Dallas, TX and the main thing I am concerned about is tornado's. Can the buttresses be columns that extend 2 feet out and connect to the roof, instead of the piers that have steps? Do I need buttresses on the 10 feet long walls?

A: (Owen Geiger) The short buttresses can be vertical without steps. Put them in the weakest areas: on each side of the doors.

A: The rule of thumb for placing buttresses in earthbag walls dictates that you shouldn't go over 10 feet without provided a buttress. You are right at that limit with your design. Strong upper bond beams can help counteract the need for buttresses as well. But if you want to error on the safe side, and since you have some concern with winds, you might add more buttresses.

Q: I am currently doing independent research on the structural behavior of earthbag construction. I write to request your help in understanding the concept of buttressing doorways. Is providing buttressing at doorways necessary even after providing arches above the doorways or providing lintels to redistribute loads? I would like to chalk down a mathematical expression for this issue and am trying to understand how this load redistribution works.

A: I am not a structural engineer, so you'll have to regard my comments in that light. I have designed and built several successful earthbag domes, however. Buttressing doorways in earthbag domes serves a number of functions.

First, there is the practical matter of somehow providing a vertical plane that can accommodate a vertical door. Because of  the curvature of the dome (which is more pronounced the smaller the dome is) some sort of buttress at the doorway is usually the easiest way to provide this. The buttress can either extend inward or outward (or both), and if it goes outward it will require some small roof arrangement.

Then there is the matter of stabilizing the wall of the dome where the doorway has been cut through. Any penetrations to the uniform shell of a dome will naturally weaken the structure. Both windows and doorways are best stiffened somehow to keep them from deforming. To some extent this can be done with internal pinning and the use of rigid frames or bucks that are connected to the matrix of the dome. But for any large openings, like doorways, a buttress is a much more robust solution.

The third reason I consider substantial buttressing essential in domes has to do with the dynamics of how the forces are distributed through the wall. At any given concentric level the weight above it has both a vertical and a horizontal component, so starting at the spring level, where the dome starts to curve inward, there is an outward thrust on the wall that must be counteracted by the tensile strength of the wall itself. In most earthbag domes this tensile strength comes from both the fabric of the bag material and from the embedded concentric strands of barbed wire placed between every course of bags. This is where openings create the greatest weakness in  the shell of the dome, because the continuous loop of fabric and wire is ruptured and can no longer withhold the horizontal thrust, like a tension cable does at the top of a yurt. Therefore, in order to maintain sufficient tensile rigidity, the doorway needs a buttress to serve as an anchor on both sides.

Q: I was hoping I could make an earthship out of earthbags. I figure I would need to place posts every 10' or so along the back wall because of the straightness and length of the wall. is this correct?

A: Yes, you would need to buttress that back wall one way or another, and hefty posts might suffice. Another approach is shown in this Earthship style plan: http://dreamgreenhomes.com/plans/zeroenergy2.htm

Q: I want to build a big circular community space. What is the maximum radius of an earthbag house that I can build? I don't want it to be a dome roofed but I will use traditional bamboo thatched roof. Ideal size for my needs is 14 meters in diameter. Can that be built without much buttressing? 

A: With circular vertical wall structures there is really no limit to how big they might be. Your roofing system will be more of a factor in the size. Circular walls generally need no buttressing.


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