We started out with a mixture of 3sand X 2clay, this made the first layer which was in contact with the stone /cement foudation. Later on we decided to change the proportion to 50% each. The clay content in the soil changed with the depht of the ditch, as well as the colour, from vivid brown to reddish/brown, so that we've ended up with a 2sand X 2,5clay mixture.
Rubble Trench - We had a slope round 1,5m, lowest point southeast. rainfall around 2000mm/ year, very humid.
* we dug a dich ranging from 40cm to almost 1 meter at the higher ground.
* covered the ditch with synthetic filtering tarp. I had bought a tarp that was 4,6m wide, i decided to cut it in half (2,3), that was a mistake, my ditch was wide (60cm) and high at some points, it would been better to have the original width, since the house is round you also end up using more tarp and you need to cover it later also.
* we covered the tarp with 10 cm gravel and laid a perforated pipe along that was also covered with the same fabric as the tarp, we covered it all then with more gravel. and Tamped it during the process
For the stone foundation we've hired a mason, they had a team with workers. We've got the stones from a Stone-mine 1 hour car drive away. They were not easy to work with, heavy an very rough in shape, we've ended up using more cement I was expecting to. Basically we had the stones place outside and inside ad filled it with concrete.
We’ve started digging around 8th December, it took us 3 weeks, the rubble trench, we were not working on it everyday though. The stone foundation started after Christmas and took around 3 weeks too. The cobbing and the walls started then only mid April, from this time until now.
It was difficult at first to step away from the concept of an enclosed house, I was used to think about inside and outside, but here being inside or outside is not really important as we are used to in cities or cold places, if the sky is clear you can even sleep outside. So what matters here is more, covered or uncovered, to protect against sun and rain. So actually what we’ve build are wall sections connected with glass doors that can run into the cobwall and leave it as an open structure, that can be closed at night. We will also have openings on the top to create airflow.
Right now its winter (july 23th), we have around 20 Celsius and 96% humidity and it rains everyday. We are working with 4-5 people from Monday to Friday, and we take turns to cut the piaçava in length and take out the fibers that can hurt your feet. Usually we can cob everyday without the walls mushrooming.
The piaçava grows around northeast brazil mainly, its pretty common, for roofs and brooms, here is a piaçava farming belt.
I haven’t seen any cob houses here around, mainly rammed earth, adobe, hiperadobe, wattle and daub which is (or was) the traditional one here. I know from other people that build with cob in similar climates and it seems to work well.
One right thing to do is to build a provisory tarped roof, my initial idea was to cover the walls with a tarp or sheet during the rain, that would have been a disaster, because it rains a lot and in short intervals sometimes, so that the work of covering and uncovering would be too much, also by doing it a lot you would have a hard time trying to fix them on the ground and avoiding uncovered spaces and small water pools during really heavy rainfalls and wind..
During sunny days the cob dries fast and its normally also windy, some sections get more shade and don’t dry at the same rate. Also the outside of the walls drys faster, when carving some niches I noticed that the interior part was not as dry and hard as the exterior, for what I suppose that there will be at some degree the “brownie effect”, crispy outside and smooth inside. Does anyone knows how this can effect the building?
I believe if everything goes right we will be done with the walls after rain season, and use the whole summer to build the roof and apply the plaster after the walls have dried.
Also for humidity during the winter we’ve decided to build a rumford fireplace, I’m still studying the measurements for it and from the videos I saw it doesn’t seem really difficult, my question is how to get the shape for the throat, in most videos people use a prefabricated frame. Has anyone has any good references of rumford f. from scratch?
Thanks a lot for the input, it is a nice experiment and we’re all having much joy building with cob. It’s interesting and nice though to combine with other materials, such as wood and glass, or use wattle n daub for walls inside. All this helps and saves work.
I`m from Brazil as well and I`ve been searching for projects like yours for reference!
Thank you so much for sharing this project. I am currently working on a project to build a modular cob dwelling in Paraty, RJ
I really need to ask you: would accept a visit to the site and maybe even a hand to work on it for a brief period in December?
It would contribute immensely to my project getting to actually see the house you`re building and get to ask you a couple of precious questions!
Anyway, thank you again for sharing this amazing experience!
Gustavo - this is Tammy in Bandon, trying to reach you can you please contact me at the same email address I've always had so we know you are okay down there. PS...the house looks amazing!!! GOOD JOB! You're one of my STAR students *GRINS*