 |
|
|
Structural Considerations for Cob Buildings |
|
|
|
|
| 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? Q: How long do cob structures last if you just build on top of the existing earth? Q: Are footings that are dug about 12" - 18" deep below frostline best for structural stability? 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) 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. Q: My daughter has just moved into a 150 year old cob cottage in Devon, England. She wants to attach curtain rails and shelves directly onto the cob walls but is unsure how to do this without damaging the walls. Are the usual commercial plastic plugs and steel screws OK? Or should she employ specialist fixing products or procedures? A: It's hard to answer your question without seeing the condition of the building. It is likely that commercial plugs would work, but that assumes strong cob and render in good condition. It also depends on the amount of weight she wants to put on the shelves. I am certain that your daughter is not alone in wanting to attach shelving to old cob walls in Devon. As I understand it there are 10,000 historical cob homes still inhabited in Devon. I would suggest that she ask her neighbors for their opinions. Find someone in the village who is knowledgeable about cob buildings, and ask their advice. Q: I have been looking into building with cob for awhile now. I would prefer to build a structure from scratch, and still hope to do so one day. However, we currently live in a brick home built in the 1940's. For the most part, it is in great shape. We were thinking of adding on a couple of rooms onto the outside structure. Do you know if anyone has built a cob addition onto an already existing establishment? I imagine the main concern would be the attachment of the brick and cob, as well as the settling of the original house over time, pulling away from the cob---although there has been little to no major sinking since it was built. Do you think that this might work? What would your suggestion be in doing so, or in going about it in another way? A: Because of the short history of cob in this country, I don't know of any examples here of the sort of cob addition you're describing. I have seen people make cob additions to small cob buildings. Larger cob additions of the sort you describe are probably more common in Britain and elsewhere in Europe. Q: I am interested in learning more about Cob building and as my first project want to build a garden wall between my house and my neighbors house. It will be approx. 15-20 ft long, and 4-5 ft tall with a gate. I plan on making it a very organic, flowing shape, taking advantage of the characteristics of the Cob. How thick should the wall be? A: The stronger your cob is, and the more curved the wall is, the thinner the wall can be. You could probably start at 8" thick at the bottom and taper down to 5" or 6" at the top. But it also depends on what sort of roof or top covering you are going to use. You may need the wall thicker at the top to anchor or support your roof. Q: Is it possible to frame rectangular windows then set them into an opening in the cob wall then cob around them to produce a different shape, such as a gothic window? We've been building a cob structure here at the Nature Center. Last year we put in recycled truck windshields as windows and cobbed them in but they cracked. We're trying to find the best way to replace them that will allow for future replacements if necessary. A: Yes, it is absolutely possible, and in fact commonplace. See the chapter on windows in "The Hand-Sculpted House" for recommendations about how to avoid cracking as the cob dries. A: Yes, it is certainly possible to make domes and vaults out of cob. This is only recommended in the very driest of climates. It's very challenging to seal a dome or vault well enough to eliminate the chance of water getting into the structure. If enough water gets in, the cob could get soft and collapse. Q: We are contemplating building with cob in south central Kansas State. My husband is English, and well familiar with the cob and thatched houses of England. I like the rounded, more natural shapes of some of the contemporary cob buildings myself. Usually the weather is hot and dry in the summer and cold and dry in the winter. Occasional snows; humidity when a storm blows in. So here I am, really keen on building a cob home, yet full of contradictory advice. I've heard that cob stands up to the elements great, and that cob will disintegrate quickly. I've heard that cob stands up to wind velocity well, and that it does not. I've heard that cob is easy and inexpensive with which to work, and that it costs as much if not more to build than traditional stick built housing. I've heard that dome homes are great when using cob, and that dome homes are impractical to the point of being dangerous when using cob. I've heard that you can plant moss on the outside of cob to help weatherproof it, and I've heard that planting anything on the exterior of cob will shorten its life span. I really want to do this, but I want to do it right (and yes, I have 'The Hand Sculpted House', 'Build Your Own Earth Oven', and also the DVD 'Natural Building and a New Sense of the Earth'). A:
As one of the authors of "The Hand-Sculpted House," I would stand by the vast majority of what was written there. Cob certainly does not deteriorate rapidly in the weather as long it has an adequate foundation and roof protection. The Devon Earth Building Association estimates average horizontal erosion rates of an inch per century in unplastered cob walls. This rate will be reduced practically to nothing by a well-maintained lime plaster. There should be no concern about wind damage to a well-constructed cob wall. Definitely don't try to grow moss or plants on the cob walls. For the plants to grow they will have to be watered, and the whole point is to keep your cob walls as dry as possible. Q:
When I contacted the Director of Auroville Earth Homes at Pondicherry in India, he opined that cob / earthbag buildings could not bear the load of earth domes / nubian vaults. Of course the problem of water seepage can be avoided by using plastic sheathing over the domes and extending long eaves over the side walls but I am not quite sure about the load bearing capacity of the mud walls in respect of earthen domes / nubian vaults. A: (Kelly) Yes, Khalili's seismic testing with earthbag domes was quite positive, and you can read about the results at http://earthbagbuilding.com/Testing/superadobe.htm . As for earthbag vaults, even Khalili did not attempt to construct them; his vaulted structures switched from earthbags to ferrocement or fired bricks at the spring line where the vault starts to curve inward. I did make a small vaulted entryway to my earthbag house in Colorado, but this spanned only about 8 feet at the base, and the walls were nearly a yard thick. I would not recommend anything larger than this. Q: Can you build interior cob walls on a second story? Would it be more technically difficult to build a large (2000+ square feet) 2 story structure or would the techniques be the same as a smaller 2 story? A: Cob is a very heavy material. It is possible to make an interior dividing wall on a second story, but this is usually not a good idea from an engineering standpoint, unless there is a very strong way to carry all that weight. Larger buildings are always more complicated to build than smaller ones. This complication is mostly related to the roof, because the loads on a beam increase exponentially as the span increases. There's nothing special about the cob walls that would change as the size of the building increases, as long as they are thick enough and strong enough to carry all the loads. This can be a difficult thing to judge, as there has been very little engineering research done on cob. Q: Can a cob building have a basement? I would like to build 6 houses in a semi circle, each made of cob with thatched roof. I would prefer them to be two story plus attic plus basement. I've seen/read about cob houses with attics but not basements. Also I would not want to use concrete in the basement as it can be toxic. I would want to use something natural and healthy. Is this impossible? I would also like to build underground passages linking the houses. I was thinking this might be possible with stone? A: Your plan certainly sounds possible in theory. Stone has historically been used as a material for lining basements. I don't know where you are or whether you are intending to get building permits for these buildings, but in most areas of the US it might be challenging to get permits for a stone-lined basement. This is especially true in earthquake-prone areas, for good reason. When the ground moves, you need an extremely strong and solid underground wall to retain the earth around the basement and keep the building above from falling down. Another major concern with building underground is water infiltration into your basement space. To me, a rock-lined basement would only make sense in a very dry climate with no earthquakes. Q: I've been wondering if alternatives to the tipi structure exist, such as ones that try to get a bit more solid, such as attaching a lattice to the vertical poles upon which cob could be applied and then waterproofed? The reason I ask is that I am drawn to the simplicity and strength of the tipi in its structure, but craving as, I always do, a solid wall around me rather than cloth, thought that surely this could be done? A: I'm not aware of any buildings meeting this description. I'm sure it would be possible to make a tipi-like structure and infill it with earth in some fashion. Wattle-and-daub is the wall system that occurs to me. Of course the poles would have to be sized much larger to support the significant weight of earthen walls. And obviously the structure would no longer be mobile. The bigger problem would be waterproofing. It is a huge challenge to get a waterproof seal on an earthen wall except in very dry climates. A: (Kelly) Waterproofing an exposed cob tipi could be problematic. Usually cob is used in situations where it is protected by a roof with good overhangs. Exposed cob sculpture usually needs frequent repair work. It might be that a combination of cob with thatch on the outside would work. Another solid-walled tipi idea I've encountered is using earthbags, such as the one I made shown at http://earthbagbuilding.com/articles/riceland.htm , or the one that they are making at http://recycledrice.org/rice-hull-tipi/ . Q: Can cob walls be built around steel beams? Do they work with steel in the same way concrete does? Is it possible to create a reinforced cob slab using formwork? A: Steel is not as compatible with cob as it is with concrete. There is a greater discrepancy in the rates of expansion and contraction between the two materials, so cob with imbedded steel tends to break apart over time due to cracking. (The same happens with concrete and steel, but on a longer time scale.) It's much more effective to reinforce earthen materials with softer, lighter fibers such as straw and bamboo. Even so, I'd be surprised if you could get the same sort of strength out of cob that you can with concrete, either in compression or tension. But on the other hand, those sorts of strength are unnecessary for most aspects of small-scale construction. Q: Can I make a cob wall next to my traditional concrete basement wall and basement concrete floor as I transform this basement area into a living area that will also contain a mass heater? I live in Maine. A:
Earthen materials like cob need to be kept fairly dry in order to hold up well over time. I would be concerned about moisture penetrating through the concrete wall of your foundation or through the floor of the basement and getting into the cob. Do you ever see dampness in the basement? Q: We are planning to build a cob house in the UK. To speed the build we are thinking of building the ground floor walls in cob and then the first floor walls in strawbale. The stawbales wouldn't be structural so a wood frame would be required to sit on top of the cob wall to support the roof and the strawbale walls. Do you think this would be possible if the load from the frame entered the cob wall through spreader plates on top of the wall and held in place by lumber buried into the cob similar to what is suggested in the "Hand Sculpted House"? A: What you're proposing would probably work. I've been living and working in California for the last 12 years, where the likelihood of high-intensity earthquakes makes such a design questionable. Here, I would use posts running from the foundation to support the first floor, and then posts above those from the first floor to the roof. That may not be necessary in the UK. I would get the opinion of local cob builders. Q: I just found out about cob after I have hand built my house with wood and a raised concrete and post foundation. I am hoping you can help my dream of cob still come true with answering just a couple quick questions. The plywood decking that is on my raised foundation, do you know if I am able to do a cob floor directly on to this? Also what do you recommend to seal this in my house like if it was a kitchen floor etc? And do you know if the weight of cob would be OK. Should I be nervous to do cob structures on a raised floor plan? Am I able to use cob for my countertops..and what seal? A: A cob wall weighs much much more than a conventional stud frame wall. My estimate is that it would weigh about 50 times as much, depending on the thickness and exact makeup of each wall. It is theoretically possible to build a cob wall on a post-and-pier foundation, but it's almost never done. Because the cob weighs so much, you would have to use extremely strong beams and joists, which usually seems like a waste of wood. It's unlikely that the foundation that you have already built will be up to the task. If you want to get the look and feel of an earthen wall into your structure, you could use earthen plaster on your interior walls. Countertops? Hmmm..... I've never heard of anyone building cob countertops. I don't know how well they would wear. It would be interesting to find out, though! You could try sealing the cob surface with food-grade flax-seed oil, and see how that holds up. Q: I'm helping some friends who are building a dome with a sort of amended cob (it is Arizona soil with newspaper and lime mixed in). They are applying gobs of this mixture onto a dome-shaped framework that is made of chicken wire stretched over re-bar. The chicken wire sags when we put this coblike stuff onto it, creating surfaces that are convex (versus the dome's concave) to the interior. I'm concerned that the wire-and-rebar framework will prevent the earthen material from settling into a pure compressive load. Am I right? Should I try to convince them to abandon this framework and build the dome in a more traditional way? They are finishing another dome that has no such framework and is made of adobe bricks, with a scaffolding that they will remove (as in any block arch or dome). So that's one option that they're already aware of. A: (Kelly) It is a bit hard to say without actually seeing the project in person, but it seems to me that even if the interior of the dome has some dips in the surface, if the exterior has a good dome shape that it should be pretty sturdy. I would be somewhat more concerned with the longevity of either dome, since earthen domes of this sort are vulnerable to failure if they get soaked with water (and with the paper in the mix, this will wick even more moisture), which even in Arizona is a possibility. The wire mesh and rebar would help to some extent, but it may not be enough for long term support. A: I share all of Kelly's concerns. The only thing I have to add is that the structure your friends are building sounds disturbingly similar to the one that collapsed and killed Ken Kern, the seminal back-to-the-land-er and DIY builder, author of "The Owner-Built House" and other classics. The last house he built was an earthen dome supported by concrete ribs, over which was a network of rebar and chicken wire, and then that was covered with some sort of earthen mix. I don't know exactly what he used as a waterproofing finish, but whatever it was failed during a heavy rainstorm when Ken was asleep inside. The dome absorbed a great deal of water and became so heavy that it broke the concrete supports, bringing tons of earth and steel down on top of Ken. On the other hand, I do know of some adobe domes in dry areas such as New Mexico which seem to be doing fine after many years. The one I am most familiar with belongs to Carole Crews outside of Taos. After multiple attempts to find a waterproofing sealer, and trying every "natural" solution she could think of, she ultimately settled on a synthetic sealer. I believe it was some kind of latex, but I'm not sure. The details may be in her recent book, "Clay Culture." Obviously, it is imperative with an earthen dome to keep moisture out of the structure, and I don't know of any way to do that with entirely natural materials. Another solution could be to build a separate roof above the dome, which sheds rain away from the structure while still allowing the dome itself to "breathe." Q: I have got a recipe of cob with cement so that I can put cob in forms and it is solid the next day...I raise it 18" at a time. My mix is cement-lime-clay (pure, good quality), crushed rock (0-3/8"), sand, straw (1-1-4-4-6-16). My wall is 2x8 structure load bearing (filled with cellulose), outside and inside of it is 4" of cob "poured", but held to the structure. Questions: What do you think of that? How solid is the cob? I want to build 2 stories high. About windows openings-- weight is transferred beside the openings. Is the cob solid enough? A: I'm not sure I understand your proposal well enough to offer useful insight. It sounds as if you want to attach a 4-inch thick layer of stabilized earth mixture to both the inside and outside surfaces of an existing framed building. Is that right? Whether or not it will work probably depends a lot on how exactly you are connecting the earthen mixture to your existing wall. One danger would be separation between the two materials over time, either as a result of differential expansion/contraction rates from changes in temperature and humidity or because of earthquakes and other movements. To evaluate whether your system will hold up, I would need to see details of your foundation plan and attachment methods. Above windows and doors, you will almost certainly want to install strong lintels to help carry the weight of the earth. Another quick comment is that, while I understand the value of a thick earthen layer on the interior of the wall, which will get you a lot of valuable thermal mass inside the insulated building envelope, I don't know why you would want to add so much earth on the exterior of the wall. It won't help you thermally much at all. If you are merely wanting to protect the framed wall from drafts, fire, mice, etc. you could probably do so much more simply with a thick coat of earthen plaster, say 1" to 2" thick, that could probably be applied more simply and cause fewer structural concerns, as well as obviating the need for lintels. |
|
|
Michael
G. Smith
has a background in environmental engineering, ecology, and sustainable
resource management. In 1993, along with Ianto Evans and Linda Smiley,
he started the Cob Cottage Company, a research and teaching group focused
on reviving and improving traditional forms of earthen construction. He
is the author of 
