Structural Considerations for Cob Buildings
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Q: I am an architecture student, with a project outside of Phoenix, Arizona, made out of earth. I want to know what would be the maximal height for a tower out of earth, and what would be the best structural material. Is it possible to realize one tower (possibly with rectangular base), only with earth for a height of 20 floors for example?

A: It sounds like a very ambitious project. I am not aware of any earthen buildings 20 stories high. I imagine the engineering on such a structure would be very complicated. The highest buildings that I know of made completely of structural earth are the towers in the city of Shibam, South Yemen, many of which reach to 8 or 9 stories.

Q: I have participated in the building of a number of cob structures with Cob Cottage Co. and they have all used "urbanite" (some cob mortared and some not). Being in central CA. I feel the seismic considerations do not warrant using "urbanite" because of the level planes created with each successive layer would not be stable enough to withstand the sheer forces. I have built a Khalili earthbag dome with great success and am now wondering if using the earthbags for the foundation is a good idea. First I would make a good draining rubble trench, then 18" of rock filled bags below grade (clay soil, no bedrock) then 18" of cement stabilized earth filled bags. The big question is this: Being in a seismic zone would the top layers of cement stabilized earth filled bags benefit me or would you just use rock filled bags the whole way?

A: I share your concerns about unreinforced masonry foundations in seismic zones. However, it is possible to reinforce masonry such as urbanite by embedding rebar or other steel members in mortar between each course, and/or by casting a steel-reinforced concrete bond beam at the top of the stemwall. I also like earthbags as a foundation solution, especially in seismic areas. Your plan sounds workable. I don't know what advantages you would get using rock-filled bags below grade, rather than just filling the trench with drain rock up to grade. It may be that a cement-stabilized soil mix in the above-ground bags would be more earthquake resistant than rock-filled bags. However, the rock-filled bags have the advantage of not wicking water vertically through capillary action. I would recommend at least 1 course of rock-filled bags above grade, then you could switch to soil-cement. Remember to use barbed wire "mortar" between each 2 courses. I also like to pound stakes into the top of the final course in order to provide a little more "key" for the cob than the slick bags will give. Remember that polypropylene bags are very susceptible to degradation from UV radiation. You will want to get the bags covered with plaster quickly, especially those that contain loose fill such as gravel. You also want to provide some sort of "plaster stop" capillary break, rather than plastering down to the ground on the exterior of the building. A row of stones works well for this. Otherwise, you can have moisture wicking up into the foundation through the plaster.

Q: We live in northern MI in a traditional 3 bedroom ranch style home. We do massage therapy from our home and are soon going to construct 2 massage rooms and a bathroom in our walkout basement so we can actually use all 3 of our bedrooms again. Our question is this: Would it be possible to use cob to make these rooms in our basement? Our home (including our basement) is heated by Geo thermal heating and cooling. I am thinking that heating the cob rooms should not be a problem as long as each room has it's own heat duct. We have never attempted any building project before. Would the walls of these rooms need to be reinforced with anything? How thick would the walls need to be since they are inside and not outside walls? As you can see we need all the help and info we can get. Have you ever heard of anyone using cob to finish inside rooms?

A: Using cob to finish your basement sounds like an excellent idea. One of the big advantages of cob for healing spaces is that it is extremely sound proof. You could probably have both rooms in use simultaneously with no sound transfer between them. You can also make the walls as beautifully sculptural if you wish. The construction should be fairly straightforward. Read either "The Cobber's Companion" or "The Hand-Sculpted House" for help with the mix proportions, mixing and building techniques. Because the walls will not be load-bearing, they can be quite thin. As an experiment, Ianto at Cob Cottage Company has built free-standing walls as little as 2" thick. However, I would recommend something more in the 6" to 8" range, both for ease of construction (very thin walls are harder to keep plumb) and for sound insulation.

I agree that heating should be no problem as long as each room has a duct. The biggest problem I foresee is with drying. Fresh cob contains a great deal of moisture which needs to evaporate out for the walls to dry. Building in an enclosed space makes that take longer. I hope the basement has an opening window or windows to the outside. If it doesn't, I would seriously consider installing one just for this purpose. (It could also be very helpful for getting clay and sand into the basement. Or you could mix your cob outside and throw it through the window.) Get a large fan, place it in the window, and leave it on as much as possible during the construction and drying process to suck the moist air out, or that moisture is liable to end up elsewhere in your house. To reduce the amount of wet cob you need to mix and speed up the drying time, you can build chunks of rock, bricks, recycled concrete or the like into your walls. You can use quite a lot of this kind of material - easily 25% of the volume of the wall. Keep the chunks buried inside the wall away from the surfaces so they don't interfere with trimming.

Q: Is a cob cone inherently stronger than a dome.. especially if you are going to cap it with a wooden roof to make the top floor an observation room? It would seem that a truncated cone might have a lot of advantages.

A: According to engineering theory, a cob dome should be stronger than a cone. The strongest kind of dome is a kind of pointed one, made by taking a catenary arch (the shape you get when you hold a chain or rope on both ends and let it hang), inverting it, and rotating it on a vertical axis. In this kind of structure there is theoretically no tension - all forces are transferred by compression through the structure. This is ideal for a material like cob or any kind of masonry, which is much stronger in compression than in tension. In practice, I don't know that a truncated cob cone would behave very differently from a catenary dome with a hole in the top. In either case, the structure should definitely be protected by a roof which sheds water away from the cob. Unroofed earthen domes (or cones) are safe only in severely dry climates.

Q: How does one create a top plate for a round cob wall? Is it possible/practical to do away with the plate completely and embed the beam ends directly in the cob? I am thinking of a small one-story house with very curved walls.

A: It is possible to set beam ends and rafters directly into load-bearing cob walls. In fact, that it is how it was traditionally done in Britain and is still done by many renaissance cob builders. I would recommend that technique for small structures in non-seismic regions. Make sure the beams and rafters are securely anchored into the cob by using adequate deadmen (see "The Cobber's Companion" or "The Hand-Sculpted House" for details.) In case of earthquake, a continuous top plate can be very helpful in resisting shear forces which could otherwise crack your walls. A curved top plate can be constructed in several ways: by bolting together short overlapping pieces of heavy lumber; manufacturing a box beam out of plywood; or even casting it from concrete. Remember that the beam need not necessarily be the full width of the wall. Once again, it should be securely fastened to the cob with deadmen.

Q: Tell me for a kids whimsical playhouse made of cob do I need to be sure to work upon stone?, gravel?, or concrete?

A: (Kelly) It depends on how long you want the structure to last, how thorough you might be in providing a good foundation. I would recommend building on some form of foundation if you want it to last for awhile.

Q: How long do cob structures last if you just build on top of the existing earth?

A: (Kelly) This would depend on how well the surrounding soil drains, how much of a roof eave is provided, and how arid the climate is in general...but you might only expect a few seasons under some circumstances.

Q: Are footings that are dug about 12" - 18" deep below frostline best for structural stability?

A: (Kelly) To assure that the building doesn't move in deep frosts, the footing should extend at least down to the frostline. This can be a simple rubble trench foundation, with rocks or earthbags filled with gravel above grade.

Q: I live in San Antonio, Texas. Although we don't get as much rain (or snow) as the northwest or the east, we ARE on a flood plain. Basically, this area is where humid south meets arid southwest. I'm seriously considering building a cob house as my primary residence. Would you recommend plaster or a different material as a finish for the exterior and interior walls? Also, since limestone's available here, would it be suitable to set limestone at the base of the exterior walls to make them more weatherproof?

A: Flood plains are dangerous places to build. This especially true of earthen buildings, which can collapse if they become saturated with water. Try to find out where the highest level is that flood waters will ever reach, and build on higher ground. If you don't have any choice but to build in a flood plain, put your building on a high stone foundation. In Britain, traditional cob houses were sometimes built with the entire first story being made of stone, although a stone foundation 3 to 4 feet high was more typical. Limestone could be a suitable material for the foundation, although it is somewhat soft and porous. It sounds like floodwaters are a much bigger issue where you live than rain or snow. The best protection for a cob building is either an earthen or lime plaster. If you get much wind-driven rain, use lime on the exterior; otherwise earthen finishes are suitable.

Q: I am planning a cob house in Southern Oregon. I attended a cob workshop and have The Hand-Sculpted House which is great. I am considering a "hybrid" structure with straw bales to insulate the north and west walls and then cob for the south and east sides (and an earth floor, of course!). I will probably make a timber frame structure and build up to the roof. I am wondering how the cob reacts with the straw bales without getting them wet. Can they touch, or should I create some kind of vapor barrier between them?

A: I've built a number of cob/straw bale hybrids, and I think it's a great idea. As long as the weather is warm and dry, I haven't been very concerned about building wet cob in contact with bales. I think the cob will dry out before the bales begin to rot. However, if drying conditions are slow, this could be a problem. In that case, you might want to build the cob wall first, let it dry substantially, and then build the bale wall up against it. You will find several suggestions in "The Hand-Sculpted House" on how to get a firm connection between the two wall systems. This is important both for safety (earthquakes) and aesthetic reasons (to avoid plaster cracks). It's easier to make a really strong connection if the bales go up before the cob, or (ideally) if both go up at the same time. But it can be done successfully in any order. If you decide on a load-bearing post-and-beam structure, the problem is simplified. As long as there is a post at the juncture between the two wall systems, all you have to do is ensure a firm connection between each wall system and the post.

Q: We are preparing plans for a proposed first floor extension to be built above an existing blockwork cavity wall structure. This existing cavity wall building is in it's self an extension built on the gable end of a very old farmhouse built of Cob. Our Question is, do you have any advice for linking into the existing gable end cob wall of the farmhouse with the new first floor cavity wall extension?

A: I presume from your question that you are writing from somewhere in Britain, very likely Devon. Here in the United States we have very few historic cob buildings, mostly dating from the 18th and 19th centuries and on the East Coast. Even those would not classify as "very old" by your standards. Most of our known cob buildings have been built in the last 10 years or so, and have not reached the point of major addition/reconstruction. Luckily for you, there is a significant body of experience with exactly the kind of extension you are contemplating in your locality. The recent resurgence in interest in cob in Britain was in fact sparked by the need to do repairs and maintenance on old cob buildings there. A good place to start would be the Devon Earth Building Association in Exeter. (I don't have a phone or email address). Also you might look at www.cobincornwall.com

Q: My friend and I are hoping to build a cob structure this summer. We have planned to make it two stories tall and 15 by 11 with walls two feet thick. We were wondering if the wall thickness is necessary and if there are any ways to speed up the building process?

A: Without knowing a great deal more about your structure, its difficult to recommend a precise wall thickness. Factors that effect wall thickness include the amount of weight the walls will bear (Are they bearing the weight of the second floor and roof? How heavy is the roof?); the amount of curvature or buttressing (Long, straight walls should be thicker for stability); the amount of seismic activity in your area; and the quality of the cob (Different cob mixes based on different soils will have very different bearing capacities.) All that said, 2 feet of wall thickness sounds reasonable for average conditions. The top of the wall carries a lot less weight than the bottom, so the wall could be tapered, say from 2 feet at the base to 1 foot at the top. This also brings the wall's center of mass down closer to the ground, which improves stability during earthquakes etc.

There are many ways to speed up the wall building process. One is to incorporate a fair amount of stones, urbanite chunks, bottles, or other solid material into the wall as you go. You might also consider mixing your cob with a tractor or other mechanical method. For further suggestions on speeding up the process, see "The Hand-Sculpted House." But be prepared to spend a great deal of time (probably months) mixing and building cob on a structure that size.

Q: I want to build a sweat lodge out of cob but don't know how to dome the top. Can you help me?

A: You have several choices. If you want to use pure cob technique, you can simply create a cob dome by corbelling. Any of the cob handbooks out there (I recommend "The Hand-Sculpted House") will explain how. A quicker solution would be to weave a sort of inverted basket with flexible sticks, and wattle over that either with more sticks (and then smear the whole with a clay-straw plaster) or simply with long straw dunked in clay (the technique I call "straw wattle.") Whichever technique you choose, remember that you will have to put a roof over it or otherwise protect it from the weather if you want it to last very long.

Q: I live in the Southwest corner of Washington on an exposed site very close to the beach. There is an old 8x8 ft. stick-frame shed (with a 12x10ft. roof) on the property that I want to use as an experiment in natural building (renovation). Essentially, I'd like to keep the roof and foundation (layers of mortared brick, with a layer of mortar spread over the top) and replace the plywood siding and 2x4 framing with cob. The space would be used as an extra bedroom, and possibly be heated with a rocket stove. I want to ask about the feasibility of this project. First, can I build an urbanite stemwall on top of the 'slab' of bricks or will I have problems with water draining to the bottom and running into the house (the foundation is tilted slightly north, and our driving rains come from the south)? If I decided to build the stemwall (lets say one foot wide) in front of the existing foundation instead (to create drainage), that would cut the 2ft. overhang that I have now, in half. Is this enough overhang? I think with the kind of wind and rain we get around here, a lime plaster will be necessary in any case. Does this sound like a workable project? By the way - I have enough salvaged lumber around to support the roof with posts, so I would like to make thinner walls that can go up faster.

A: I think your project sounds feasible. I don't understand the issue with the drainage. Does the brick 'slab' extend outside of the stemwall? If it does, you could probably cut or chip it back and dig a drainage trench around the exterior. I would recommend maintaining the 2 feet of roof overhang you have currently. That means that even if you build your cob walls very thin (I wouldn't recommend less than 6 or 8 inches for full-height walls even if they aren't load-bearing)
you will not have a lot of space left inside. As another alternative, I wonder whether whether you have considered retrofitting your structure with straw light-clay. That way you could keep the existing load-bearing framing and get better insulation value out of thinner walls. It would also be substantially less work.

Q: I am contemplating how to build a cob dome underground. I am aware of the moisture issue and wonder if this can be mitigated? Given your knowledge what would need be required to accomplish this task?

A: I would not consider this in any but the very driest of climates (e.g. desert). The risk of possible collapse from saturation is just too high.

Q: Any possibilities of other natural materials holding up?

A: Stone would be a good choice if you are in a very seismically stable area. Otherwise, gravel-filled bags ("earthbags") could be a possibility. Nothing based on unstabilized earth or containing a high proportion of organic fiber (e.g. straw) is recommended for underground use.

Q: I've often mentioned to my wife the idea of possibly building a cob house in the future when we move near her family in Chile. She gives me the question of how well do they hold up in different types weather disasters, particularly earth quakes. Granted this depends on the size of the earthquake, but from my understanding adobes don't do very well at all and I was wondering how cob compared to the concrete and stucco structures of today.

A: (Kelly) I suspect that cob would generally fair better in an earthquake than adobe because of all the straw that is used in the mix to bind the material together, the fact that the walls are often thicker, and there are fewer seams when the material is laid into a wall. I would not expect cob to do as well as steel-reinforced concrete, but there are many cob buildings around the world, especially in England, that have withstood the ravages of nature of several centuries.

Q: I have recently purchased a 1851 Cob house in the West Country (England). I have been doing some research into Cob but am concerned how to put up some large mirrors, on the wall in the hall way. These mirrors are around 4 foot by 2.5 foot. They hung in my previous house with a pair of large screws and plugs (but that house was brick), but am concerned this will not be the right thing to do here. Any advise would be most welcome.

A: It's hard to know the best approach without being able to investigate the strength and condition of the cob and the render. If both seem strong and sound, you could use a number of long screws (I would use many more than 2, to distribute the weight widely) to attach the mirror. Try screwing the longest screws you can find (I would recommend 5" or more) into the wall and see how much weight you can hang from them before they give. Then divide the weight of the mirrors by that amount to determine the number of screws necessary, and double that number for safety. Another alternative would be to excavate long horizontal cavities in the wall at the bottom and top of each mirror, set in pieces of lumber (say 4" by 4" by the length needed) studded with nails, and fill cob around them to hold them in place. After the cob infill dries, repair the render leaving one face of the wooden deadman exposed as an attachment surface.

Q: I have been advised by a home buyer's surveyor that the thickness of my cob wall is sub-standard; my wall reads 600mm in thickness and the surveyor has said that cob walls should be a minimum of 1000mm in thickness to support any loads placed upon it! Therefore my building could potentially collapse any moment. Surely its been there 300 years and would remain longer? There are no obvious signs of damage, movement or decay, therefore I can only think he is confused. Please help!

A: Nothing you write sounds like great cause for alarm. Here in the U.S. we frequently build load-bearing cob walls down to as little as 300 mm thickness or even less. However, the conditions and especially the mixes in England (where I'm assuming your home is located) are somewhat different, and I would recommend finding some local expertise. You could start with the "Devon Earth Builders Association" or "Cob in Cornwall".

Q: We live in Tennessee. I am sure that the code department would like to see an engineer licensed in this state. However, if we can find a structural engineer with experience in cob, that may be more important.  Also- do you have any information on where we can send samples of our planned mix to be tested for strength so that we may be able to use a local engineer? 

A: It's extremely challenging to find engineers who are experienced with and interested in cob. We have that problem even here on the West Coast where there are a lot more cob buildings. All I can say is that I wish you luck. Engineers will typically not work outside of the state in which they are licensed, so I would suggest confining your search to Tennessee. Talk to anyone you can find in-state who has built a permitted natural building and ask for recommendations of either engineers or architects. Architects should be able to refer you to engineers. Or call any old structural engineer out of the phone book, ask if they're interested, and if not, ask if they know someone who might be. It may take you a while to find the right person, but with persistence hopefully you will eventually succeed. Any lab that does testing on concrete samples should be able to test cob as well. Your engineer will know where to go.

Q: My dream is to build a natural house, a rammed earth or a cob house, in Taiwan, as that's where I plan to retire and my husband's family is. Taiwan's east coast continuously has earthquakes, usually around 4 -6 on the scale. And typhoons which hit the Island's east coast first before traveling onward. Which would better rammed earth or cob?

A: It's hard to say which would perform better in an earthquake, cob or rammed earth. There is plenty of anecdotal evidence that both types of building perform relatively well in earthquakes (especially when compared with adobe block) but little seismic engineering testing has been done to my knowledge on either cob or traditional rammed earth. Versions of rammed earth using lots of Portland cement and steel reinforcing have been tested and permitted in highly seismic areas of California.

As far as the typhoons go, you will most likely want to protect the exterior surfaces with a lime-sand plaster which will not get soft when it gets wet. Definitely use wide roof overhangs, and if the typhoon winds come from a predictable direction, build a porch or other additional protection on that side of the building.