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.