Building 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

  • Architects designs and plans
  • Get tools. Pick axes, spades, hoes
  • Mark out foundations
  • Dig to an appropriate depth (we dug to 1+ metre depth, 1m wide at the top, 0.7m at the bottom). Digging through any soft soil to get to the harder surface when needed
  • Plan and dig drainage canals around the house, connecting to foundations (50 cm wide)
  • Starting from one point, which will be the highest corner, start to level the bottom of the foundations so they are as smooth as possible, ideally also sloping towards the outside of the house.
  • The gradient should be about 2-3cm down for each metre along the foundation
  • Once all the ditches are done, compact them all down
  • Cut plastic sheeting to cover all interior floors, place over interior floor places
  • Add 30cm of small (3-4cm) stones and compact. Repeat till foundation is nearly full.
  • Before the stones reach the top, lay the underground electricity and waste pipes
  • Fill the foundations to a few inches from the top with small stones and compact.
  • Place large stones at the end of the exits to stop the small stones being washed away
  • 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.
  • When building the walls make sure the each rock is touching at least two other rocks. Only use minimal cement. The strength comes from the rocks touching
  • Whilst building the foundation walls, cut grass and leave to dry for using in the cob
  • Turn the grass so all of it is dried in the sun
  • Make some test cob blocks whilst the foundation wall is being finished
  • Make some ‘dead men’ to put inside the cob walls. These hold in the windows etc, they provide a surface to drill in to
  • Make the first batch of cob using 4 parts clay soil and 1 part sand, mix it using tractor or feet and sprinkle hay over the top occasionally.
  • 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. 
  • 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.
  • Creates floor joists to sit the first floor on
  • Get the bathroom plumbing in place
  • On the ground floor, pour in clay puff ball things to insulate and cover with cement or maybe somethin more ecological, I dont know what
  • Fill in holes between stone walls with plaster. Keep filling with new layers till a smooth surface is achieved
  • Cut channels in the walls for wires for wall light fittings


  • Final internal paint layer. Mix 6 cal and 1 clay 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. 
  • Plaster walls with a mix of lime plaster and clay, increase percentage of lime plaster till you achieve whiteness you want. May need up ten layers on roof
  • Outside 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. 
  • As a tip, you can add a trough it makeshift cob-catcher when cobbing, plastering etc. to catch the falling material and then reuse it 
  • Tadelakt. Final bathroom layer. 4 parts cal, 4 parts marble, 1 colour (we used clay). All Very finely sifted 


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