Structural Considerations for Cob Buildings

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 The Cobber's Companion: How to Build Your Own Earthen Home (Cob Cottage Co., 1998) and co-author of The Art of Natural Building: Design, Construction, Resources (New Society, 2002) and The Hand-Sculpted House: A Practical and Philosophical Guide to Building a Cob Cottage (Chelsea Green, 2002). He teaches practical workshops and provides consultation to owner-builders on a wide variety of natural building techniques, site selection, and design. He lives in an intentional community in Northern California. His informative website is www.strawclaywood.com.

Questions and Answers

Q: My concern is the size. As we have rented and owned homes we seem to be cramped in anything less then 2500 square feet (we have 4 small children ages 7,5,3,and 1). Obviously we would love to minimize our housing costs but I need to know if cob is even possible for us with the size of our family and needs. Will you please advise us on the best and most cost efficient green home for the size of our family.

A: There is no inherent size limitation on cob or any other building system that I know of. If you look in the literature you will find cob mansions in England and cob towers in South Yemen nine stories high! That being said, cob walls can take a lot of work to build, as do most other natural building techniques. One reason many people build small cob houses is to get them done faster (and thereby reduce costs.) Another even more important reason is to reduce the amount of energy required for heating and cooling the building over its lifetime.

There are some natural wall systems that can go up faster than cob.. Do keep in mind that if you plan to build your own home, you would be best advised to start with a small project first to learn the techniques before taking on a large house. The same recommendation would apply if you end up hiring a builder who is not highly experienced with the building techniques you choose.

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: 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: 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 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: 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: 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.

Q: How long will a cob structure take to settle in and can a vault / dome be built on cob walls?

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.

Another solution is to build a cob dome and put up a roof over it to keep the rain off. This is a very important safety consideration, as people have died in earthen domes that collapsed because they were not well waterproofed.

I'm not sure I understand the first part of your question about settling. Cob will shrink vertically as it dries. Drying time is extremely variable, based on weather. It could take anywhere from a month to a year for the cob walls to dry completely.

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.

Earthen domes are very dangerous in all but the very driest climates, as it is difficult to waterproof them well enough to prevent any moisture infiltration. Over time, wet cob overhead could get soft and collapse. Furthermore, since cob is not a very good insulator, it's not a good choice for a roofing material anywhere with cold winters.

Cob is technically simple and quick to learn. However, building a cob house requires a huge investment of time and work unless it is very tiny. The materials are usually very inexpensive, but there is a great deal of labor involved. The way people build inexpensive cob homes is by doing all the work themselves or with unpaid labor sources such as students or volunteers. If you are hiring people to build your cob home, I would expect it to come out similar in cost to a custom-built home made out of conventional materials.

I would recommend reading "The Hand-Sculpted House" and I believe it will answer most of your questions.

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: 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: My friend and I want to raise a doorway in his English cob wall house by approximately 2 feet. The wall is about 3 feet wide and 16 feet high and made of earth and stone - could you give us a step by step guide on how we go about installing a lintel, in-situ or pre-made, or is this beyond our good DIY skills? Perhaps we should get an expert to do the job?

A: The prudent response would be for me to refer you to a local expert. I have no experience with remodeling historic cob buildings, and it's very difficult to assess the situation without being there to see and feel the wall. The only relevant group I know of is called Cob in Cornwall (see www.cobincornwall.com) but there may be other local builders with relevant experience.

Whether this is a DIY project or not depends in large part on the quality and soundness of the cob. If the cob is strong and sound, the solution could be as simple as carving away the 2' of cob above the old lintel and inserting a new one. However, if the cob is cracked or soft or contains too many stones to hold itself together, the whole wall could collapse above the doorway. It sounds like there is a lot of weight up there! In that case you would want to slowly carve away the wall at the level of the new lintel and stabilize it as you go, which would be quite tricky.

The first thing I would do if I were there would be to remove the render on both sides of the wall above the door, from the top of the existing door frame up to and past the height of the new lintel. That would allow me to assess the quality and state of the cob, and make a plan from there.

Q: I am living in a cob built house I have recently bought which is requiring a lot of work to make it livable. I am looking to create some additional openings, for another doorway, and a window, etc. Can you advise the process for how best to cut a hole in the wall? And also how to support it before and after. I am used to brick built houses, and it's really confusing me as the walls are 2ft thick and approximately 200 years old!!

A: It's hard for me to offer advice without being able to see the condition of the cob wall and the way it was built. If it is sound, well built cob, and the width of your new openings is not too large, you can probably just cut the openings and insert a suitable lintel after the fact. But if the cob is at all crumbly or loose that could cause the wall to collapse. I'm guessing that you are somewhere in Britain, very likely Devon. I would recommend finding a local builder who is experienced with renovating cob houses and ask their advice. If you have trouble locating a suitable local professional, two organizations you could ask for recommendations are The Devon Earth Building Association (www.devonearthbuilding.com) and Cob in Cornwall (www.cobincornwall.com).

Q: Is it possible to hang artwork, cooking pans, or mount a TV to the wall in a cob house? I can't find any information about nails or screws securing to the house. If you implanted it in the cob while it was soft would that work or is it just not designed to take the stress of hanging things?

A: It's possible to hang fairly light objects from a cob wall just by putting screws into the wall. This works for pictures, wall hangings, etc. You can also screw wooden hangers to the wall (for coats etc.) as long as they won't have too much weight on them. For heavier objects like overhead cabinets and so on the best approach is to implant a wooden "deadman" into the cob that you can then screw or nail onto it. Any cob manual should explain how to do that. Another approach for hanging pots and pans, hats and coats, etc. is to embed sticks into the cob wall with part of the stick projecting out from the wall at the desired angle for hanging.

Q:  How are we supposed to join our roof with our walls?

A: The answer depends a lot on what forces you are trying to resist. What seismic zone are you in? Do you have hurricanes or high winds to deal with? In the cob code (https://cobcode.s3.amazonaws.com/RB299-19_IRC_ProposedAppendixU_CobConstruction_PublicComment.pdf), you will find a table of different reinforcement options for your cob walls with recommendations of which to use based on your seismic zone. You will also find specifications for wooden and concrete bond beams (which are required by the code for structural cob walls) and type and spacing of anchors from the bond beam into the cob wall. There are also specs on how the roof framing must be attached to the bond beams.

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