How To Build a Cob House

Here are the stages I have seen whilst building a cob house in Brazil

Most bio construction I have learned has been through Eco Caminhos, Nova Friburgo. Some photos are from our first building, and some from the second so apologies for the discontinuity. Both were constructed using similar if not the same techniques

  1. Architects designs and plans
  2. Get tools. Pick axes, spades, hoes
  3. Mark out foundations using wooden stakes in the ground, and string to show where the walls are going
  4. Dig to an appropriate depth (we dug to 1+ metre depth, 1m wide at the top, 0.7m at the bottom). Start by levelling the ground. Make sure your foundation is all in hard earth.
  5. Plan and dig drainage canals around the house, connecting to foundations (50 cm wide). You can see one in the top right of the photo above. These allow water to run out of the foundations and away from the house.
  6. For the foundations you start from one point, which will be the highest corner, start to level the bottom of the foundations so they are as smooth as possible, sloping towards the outside of the house.
  7. The gradient should be about 2-3cm down for each metre along the foundation
  8. Once all the ditches are done, compact them all down, using a machine, or by stamping the ground by foot and metal plates.
  9. Cut plastic sheeting to cover all interior floors, place over interior floor places. This stops moisture from climbing up and getting to the walls.floor plastic
  10. Filling the foundations with gravel…Add 30cm of small (3-4cm) stones and compact. Repeat till foundation is nearly full.gravel
  11. Before the stones reach the top, lay the underground electricity and waste pipes
  12. Fill the foundations to a few inches from the top with small stones and compact.
  13. Place large stones at the end of the drainage ditches to stop the small stones being washed away
  14. Build the foundation walls. These are a metre high. This is built with large stones, which are set to an inclination towards the centre of the wall, so if there are earth quakes the stones are shaken in to the wall not out of it. For a full guide on building with natural stones, see the link in this paragraph.
  15. As you go up you are going to have to make sure to keep an eye on your doorways. Use a post and water level to mark your zero level (usually the height of the floor) and make sure not to build above that in your doorways. I may write a full post on making your zero and floor levels, below is a photo of me checking that the doorway has not gone above the zero level, which it hadnt, well done team.received_10155555794002930
  16. When building the walls make sure the each rock is touching at least two other rocks. Only use minimal cement or dry stack with a bit of limecrete. The strength comes from the rocks touching each other and by tightly packing the gaps between the rocks with chips and fragments
  17. Do tests on your soil to figure out the sand and clay content
  18. Once you have done these tests, make some cob blocks (75-85% river sand, 15-25% clay, water) and also test your cob using these suitability tests 
  19. Whilst building the foundation walls, cut grass and leave to dry for using in the cob
  20. Turn the grass so all of it is dried in the sun
  21. Make some ‘dead men’ to put inside the cob walls. These hold in the windows etc, they provide a surface to drill in to. Dead men are just stumps of strong wood with lots of nails sticking out, these can be placed in the walls and the flat wood surface used to attach things like shelves or floor framing etc. They can also be placed at the top of the wall as shown here, with wires sticking out to attach the floors to.
  22. Make the first batch of cob. Mix it using tractor or feet and sprinkle hay over the top as you mix.
  23. Build your cob walls. Make sure not to go up too quickly or the wall can slump or fall if it is not given time to dry. A recommended daily lift is 60cm. 
  24. You can also stab holes in your cob with sticks whilst it is wet. This will help the drying process. 
  25. When you reach the top of the first floor, and want to build another floor on top, you can go from 50cm wide walls to 30cm.
  26. Make wooden frames which sit on the top, inside the walls, these are secured with iron rebar and wires attached to more dead men, and help keep the walls from falling outwards. On top of these wooden frames will sit your floor boards, or your roof if you just have one floor.
  27. Create floor joists to sit the first floor on. If you are just building one floor then these wooden frames can be for your roof to attach to.
  28. Get the bathroom plumbing in place
  29. On the ground floor, pour in clay puff ball things to insulate and cover with cement or maybe something more ecological like a chalk floor (we are resarching this for our next build)
  30. On your inside ground floor walls, fill in the gaps between stones with lime plaster. Keep filling with new layers till a smooth surface is achieved. We left some stones exposed and some covered depending how much the protruded from the wall.
  31. Cut channels in the walls for wires for wall light fittings

Finishes  

  • Make all internal walls smooth with Margarida Mix.  We used 1 clay, 1 sand, short cut straw and water. You may need to do tests on your earth and alter the quantities accordingly. Follow the link in this paragraph for a full run down on making and applying this particular mix.
  • Final internal paint layer. So you have applied your Margarida Mix and made the walls as flat as you want them to be.
  • Plaster walls with a mix of lime plaster and clay. We used a mix of 4 lime, 4 sand and 1 earth (for the colour)
  • For the final paint layer. Mix 6 lime and 1 clay (for colour, use something else if you like) with water. First use a paint brush, sponge or fine sprayer to wet the wall. Then use the paint brush to apply the paint quite thickly. Always paint in one direction. If you paint back and forward it will make the wall rough. One direction makes it smoother and smoother as you go. After paint, use a sponge to wipe the wall again in one direction to make the paint smooth. Here is a quick guide on making a lime wash paint
  • Outside smoothing finish: 50/50 fine sifted clay and sand mix. Add water and short-cut hay. Apply to outside like a plaster. First wet the wall with a brush and rib around till there is a sticky layer or clay in the surface. Then with the brush or by hand add clay slip. Finally add the cob mixture by hand. In thin layers maybe up to a centimetre thick. 
  • Finally, for your outside walls, make a more waterproof render. We used one part Cow poo, one part, clay, 2 parts sand, and water. You can read the full guide on testing and application here 
  • It is good practise to build some sort of catchment device as shown below when working on your walls. You can then re-use the material that has fallen to the ground.
  • Tadelakt – Final bathroom layer. 4 parts cal, 4 parts marble, 1 colour (we used clay). All Very finely sifted

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