Advice for building a cob structure for first time builder

Hi all!

I am a person in their early 20s wanting to get into Natural building. A farmer from around my area agreed to let me experiment with building a structure (120 sqr ft) on their land and I am interested in using Cob. I will have plenty of help this summer as all of my friends want to help out and the farmer herself is skilled at building traditional structures and would offer help but she has not worked with natural building.  Do any of y'all have advice for a first time builder?

I have a couple of things that I am curious about:
1) The land that I will be building on is decently sandy. The benefit of this is that water from rain drains really quickly and so water is not as much of a concern but a drawback is that I am concerned about it shifting and cracking the material over time. My plan to go around this is to lay a foundation of concrete (dig 6 inches into the ground and create a 6 inch stem from where I will start laying cob). What would be a good plan of attack for this?
2)  For better insulation and structural integrity, I plan on laying glass bottles (cut in half and connected together to create a cylindrical shape) into the width of the cob. It seems like a decent idea but is there any pitfalls that I might encounter for this?
3) How thick should I make the wall? I heard someone tell me that the foundation stem should be wider than the cob wall. I was thinking of making the cob around 18 inches but I'm hearing different things from different people

Thanks guys (:

Hi Aud.

1. Footings are usually built below the potential frost line and it's usually never less than 12" for warm locations. I would not recommend to improvise there, because foundation is the building element that is most difficult to correct and repair.  If you use concrete, do not skimp on rebar. Four 5/8" bars and 3/8" stirrups every 16" will do it. Make sure you protect it from water intrusion, because rebar corrosion is the reason of reinforced concrete failure.

2. I would think that it would lower the structural integrity by creating voids and adding smooth material with poor adherence of cob. Please provide a sketch. You could insulate the wall like SIRE rammed earth system, where a rigid insulation is placed inside the wall. Then the wall would have to be thicker. You could also build the outside wythe from light clay (straw and clay slip) to provide continuous exterior insulation.

3. Traditionally the maximum height should not exceed 10 times the width. 18" will be sufficient.
The footing width is determined by the load bearing capacity of the soil.

Hey Cristobal,

Thanks for the advice! I'll make sure to use rebar in the concrete. The person who's helping me out should be able to help out this.

I've attached a copy of a rough sketch. I think you are right, it does seem a little sketchy to add the glass bottles. Originally, I was thinking to cover them completely with cob so they wont be exposed and I was thinking that the bottles might add some kind of structural support due to the rounded shape. It makes sense though that the cob won't adhere well to the glass.  I'll look more into the SIRE rammed earth insulation as well as using the light clay.

For round structure it will be challenging to bend the rebar, especially 5/8". A good rebar shop will do it.
Have you thought about the roof? It would be good to have some bond beam. If it's wood then it would also work as the top plate, but again - curved heavy timber would need to start with much larger diameter. In old times for curved timbers -  curved trees were picked. If you pour it from concrete then it will be easier and you could also encase metal L-pins so rafters could be secured.
You could also build a masonry dome, but the walls would need to be thicker or have buttresses or have reinforced concrete bond beam.

Earthbag or hyperadobe is a possibility for creating a domed roof. Make a form in the shape of a catenary arch to guide the placement and the result is a very strong, durable structure. As long as it has a good weatherproof coating, of course.

Have you studied the types of soiul you actually need for cob or adobe.
You need roughly 50 /50 sand and clay to build with.
What books have you read?

Hey all,

Yep, i've been researching regarding soil. I am in the process of testing different soil around my area to look at clay composition. I am from Southern Wisconsin US and so the soil in my area has a pretty high clay content.

So far I've been doing research from the books Building Green: A complete How-To Guide To Alternative Building Methods by Clarke Snell and Tim Callahan, and The cob builders handbook : you can hand-sculpt your own home by Becky Bee. I've also been watching the Youtube channel Natural Buildings playlist, How to Build a Cob house.

For the roof, I was considering doing the Reciprocal roof design from the Youtube channel, Natural Buildings. There's a ton of Black Locust trees in the place where I'll be building this and I'll have access to a saw mill on their property so I wanted to incorporate some wood into the design. However, It seems like the design could be fairly complicated and it may be more wise to pick something for the roof that is not as technically hard. Has anyone here done a reciprocal roof before?

I'll look more into the Masonry dome and using Earthbag for it. This might be more feasible for a first time project.

Thanks all!

If you are in cold Wisconsin, please make deep footings. If the soil has high clay contents, make them wide enough. Such building will last.

For round building I would definitely build a dome. I consider a good wood roof structure to be more complicated than all masonry walls and foundation. Masonry is just easier. Looking for a heavy timber carpenter stalled my construction for months.
You could also make blocks with higher straw contents to make them lighter for the dome. They would be also insulating to some extent. For my barn/coop I have built barrel vault from bricks then covered with perlite concrete  (also excellent insulator) to form gable roof and then laid rooftiles with clay mortar directly on perlite concrete.
You could also make aerated concrete blocks

If you go the masonry dome route, please pour properly reinforced concrete beam, so the (relatively thin) walls will not break apart under the dome load.
Please note that for dome roof it will be more difficult to create eaves protruding enough to protect your cob walls from the harsh Midwest weather. I think this may be one of the reasons why round structures got somewhat popular only in arid regions.

Have you considered square structure?

For sandy soil, I’ve had better luck doing a rubble trench with a solid grade beam instead of jumping straight to concrete, as long as drainage is on point. For insulation, I’d skip glass bottles since they make weird stress points and can trap moisture. Wall thickness usually ends up thicker than you expect anyway, so I’d mock up a small test wall to see how your mix behaves before committing.

I have been experimenting with minimal-cost cob construction for the past couple years, building a few structures which are slightly smaller than what's in your sketch. It's definitely worth pointing out up front that I live in an ideal climate for earthen construction. The people who lived here a thousand years ago all built  their houses mostly with rocks and earth, and some of those have lasted until the present day after being abandoned in the 1200s. Based on all the reading I've done, and my experience working with this material, persistent moisture is the main enemy of this construction technique. So your experience may vary quite a bit, given that you live in a place where it actually rains pretty frequently. Also worth pointing out is that I'm okay with any of the things I've built collapsing, because they really are all just experiments, and having one collapse would be a valuable data point. Your risk tolerance may vary.

So far, what I've seen is that these small structures do just fine over a winter or two without much of a foundation at all. Every source I could find said that it's absolutely required to dig down below frost line and pour a concrete slab that could withstand an artillery barrage, because otherwise the freeze-thaw cycle could crack your foundation and even topple your structure. So far, I have not seen any evidence of this being an issue for me. I suspect that the main difference between what I've read and what I've seen is that the people who write these things are assuming that your building has a footprint of 2000+ square feet, and that what goes on top is a flimsy stack of 2x4s and drywall, which cannot tolerate even the slightest shift underneath. A properly built cob wall is far, far stronger than that, especially if it's in a round rather than square shape.

I wouldn't count on glass bottles to have any positive structural effects; as others have said there will be no bond between the glass and the cob, so the wall will be about as strong as if the bottle areas were just empty space. If you leave them full of air, it might have a slight insulating effect, but I think in a Wisconsin winter that would be negligible. The only reason I'd put glass bottles in a cob wall is the same reason I'd put rocks or logs in there -- just to take up space and save myself the effort of mixing and hauling around that much more cob.

As for wall thickness, I think you could get away with 8 or 10 inches for the structure you've sketched. It will have to be thicker at the base than at the top. New Mexico building code for a single floor adobe structure is 10 inches thick, or for two floors it's 14 inches on the ground floor and 10 inches for the second floor. It will be more difficult to build at a perfectly consistent thickness using cob compared to adobe bricks, so you might want to aim a bit thicker than that so you don't accidentally end up with some sections thinner than you intended.

Bond beams, as far as I understand, are primarily useful in earthquake territory. Neither you nor I have to worry about that problem. As long as your walls can withstand the downward force of whatever roof you put on top, I wouldn't worry about reinforcing horizontally.

Josh,

Bond beams are used in all masonry constructions and not only in seismic zones. It just helps holding walls together, helps resist forces transferred from the roof or wind, equalizes stress distribution in uneven walls. Old adobe, stone or brick buildings had bond beams made of heavy timber. Ancient Greeks were connecting architrave slabs with cast in place lead anchors.
There are many buildings in not seismic zones that have walls cracking, because the foundation shift/heavy roofs and lack of bond beams. Bond beams help thinner walls in greater extent than in thicker. Some old adobe buildings survived earthquakes because their walls were very thick (and braced with perpendicular walls or buttresses).

Of course any building can be built with no foundation, but if such things lasted, people would be glad to eliminate it long time ago, because foundations are costly, but the reality is different.

Flimsy 2x4  actually will be much less demanding for foundation, because it's light and flexible. Mobile homes can be supported by several concrete blocks. A 2000 sq ft stick building structure will weigh around 15 tons. The same size adobe building with 16" walls will be 200 tons just for the walls. It has to be supported by something, because it will sink unevenly and it will crack, especially in loose//not compacted/clay soils.
Locating foundation below the frost line is especially important in sandy soils which will be easily penetrated by moisture and freeze. If foundation was located on solid rock it would be more forgiving to frost line requirement. Also slab foundations are more resistant to freezing problems, especially if insulated. All of it could be determined by geotechnical analysis, but since we are talking about simple, cost efficient building it's better to copy proven solutions.

If the structure is experimental then it can be built any way but if someone agreed that Aud will build some cob structure on her property - the best approach should be taken, not only from ethical standpoint but also to not give alternative building methods a bad reputation. Also wonderful dryness of Colorado is less wearing on buildings than harsh Wisconsin winter and humid summer which is not a traditional place for cob structures. People in such climates traditionally built from wood logs (in later times also from fired bricks).

Cristobal Cristo wrote:if such things lasted, people would be glad to eliminate it long time ago, because foundations are costly, but the reality is different.

For sure, this would be true if there were no such thing as building codes. My main contention is that the actual logistical requirements of making a durable structure is highly context-dependent, and US building codes are meant to cover about a third of an entire continent, so they're almost guaranteed to be overly broad. The structure I build in Southwest Colorado might last my lifetime, but fall apart in a couple years if I built the exact same thing on the coast in Oregon.

Cristobal Cristo wrote:It has to be supported by something, because it will sink unevenly and it will crack, especially in loose//not compacted/clay soils.

Loose and not compacted, I have no clue about. My subsoil may as well be adobe brick. I've never dug into the ground in Wisconsin so absolutely the OP's context may be completely different. On the other hand, my clay soil definitely absorbs a huge amount of water, and therefore freeze/though ought to be a concern. But I have not so far seen any winter damage on the small foundations I've built, and the sketch from OP is not much larger than those. My theory is that a 6x8 foot rectangle of 10 inch thick cob is sufficiently strong as a unit, that it's not likely to crack no matter how much the dirt might shift underneath it.

For reference, here's some of the structures I was talking about that have lasted 8 centuries after being abandoned: https://www.nps.gov/meve/planyourvisit/cliff_dwelling_tours.htm

One thing these all have in common is that they're kinda "cheating" by being built beneath a natural rock overhang, so they will only ever see water if it's raining sideways. Which only happens maybe once every year or two in this region.

But if you look closely (and I have gone on one of these tours and looked closely in person), you'll see no bond beams, no foundations, just stacks of flat-ish rocks with nothing but mud as mortar. I don't want to oversell this, because again, the natural rock overhang is kinda cheating. On the other hand, the people who built these couldn't just go to Home Depot and get galvanized steel roof panels for 20 bucks apiece, so of course they were limited to using the natural terrain.

Josh Warfield wrote:For sure, this would be true if there were no such thing as building codes.

Building codes are recent and people were building foundations for millennia because they make buildings last and most humans are too poor to make disposable things.

Josh Warfield wrote:But I have not so far seen any winter damage on the small foundations I've built, and the sketch from OP is not much larger than those. My theory is that a 6x8 foot rectangle of 10 inch thick cob is sufficiently strong as a unit, that it's not likely to crack no matter how much the dirt might shift underneath it.

It's quite possible and I have no reason to not believe you, but I also know how roads and streets look in Midwest cities after the winter. It's a tough and destructive environment. Personally - I would not dare to build unstabillized clay building in such climate and if I did, it would have oversized foundation and large overhangs.

I think one thing we have learned as a community is improvement.
We dont need to build as they did 400 years ago, but we can pick the good bits out.
That is the way I use alternative building techniques.
I have built a small house entirely with hand tools, I would not do it again, because I could not find or learn the best tools or use methods that were effective.
Some of the techniques described above may not be practical and I wonder what others think?