We have just about finished our cob floor and it looks and feels beautiful. As such I thought it necessary to start the blog post about how we did it. It was a lot of work but the techniques weren’t all that difficult.
Here we go
First up level the places in your home that you want to be level. Half of our house is sloped and half is levelled so it is not necessary to be completely flat everywhere. In the levelled spots we used a bit of cement to make mini retaining walls between the rooms. I chose to use cement as each of these spaces would eventually be load bearing.
We used coarse sand to level the spaces, at a thickness of at least 5cm
I lightly compacted this afterwards and added a bit more sand where necessary.
Next up we laid our damp proof membrane.
On top of that we put another 5cm layer of sand, lightly compacting again. And then comes your first layer of cob – ‘the slab’. This can be made of unfiltered rough earth. We started out mixing it with some coarse sand but in the end we just took the earth right from the hill and mixed it as it was. Our earth contains 50/50 sand clay so ideally we would want to increase our sand level to 75 but as we weren’t concerned about cracks (as this layer is covered) we just mixed the earth as it was. We also added straw at this point which is said to create small air pockets inside the stems which acts as an additional layer of insulation from the cold winters. We made this mix quite wet and literally poured it onto the floor, and slapped it flat with our hands.
While it is wet and jelly-like you can level it. The more level it is now the easier the next stage is.
The slab nearly finished. Don’t worry if it takes more than a day and some parts have dried before you continue. Just thoroughly wet and break apart the edges a bit to make sure the new floor section holds to the older part
Once that has completely dried you are ready to do the final earthen layer. We made this about 1-2cm thick. The mix has to be right with this layer in order to avoid cracks, annoying stones and bumps and lumps. For our mix we went to the other hill which has a much higher sand content (80:20 sand:clay) and took half a wheelbarrow of this earth, and mixed it with two buckets of our 50:50 earth. The 80:20 alone has too much sand and would not bind properly so adding a bit of the 50:50 earth ensures a strong bond between clay and sand. You must make sure though that the sand content is high enough, otherwise you risk cracks forming as the floor dries. I would recommend a 75:25 sand:clay.
And all of this needs to be filtered to less 0.5cm. If this layer is 1cm thick the filter must be at least 0.5cm in order to stop large stones which will break up the floor as you are trying to apply it.
Then slowly add water. If the cob is too wet it will crack more as it dries so add water slowly until the mixture is wet enough to pour and smear around during application, but dry enough to just about hold its form in your hand
Next up wet your floor, liberally without soaking it through
Then we used a hard bristled brush to mash up the surface and create a nice clay slip for the final layer to stick to
Get you mix and pour it in a convenient spot, and use your trowel or other tool to start working on your floor.
Any bumps will become more obvious as the floor dries so try to make it as smooth as possible.
Next is the final layer: linseed oil. We found that 1 warm litre covered about 6 square metres for the first layer, the third layer took one litre for over 15sqm. We will be applying four coats using a large paintbrush. Linseed is quite expensive here but considering this is one of the only cost so far for the floor (the other being the damp proof membrane) it works out much cheaper than a modern floor.
And here it is after three coats. It gets darker, shinier and smoother each time
And that is our cob floor. Any questions please get in touch
2 Comments Add yours
Hi, the house I have already has a plaster based floor. Can I just lay one finishing layer of cob onto this or will I have problems with it not sticking to the existing floor? Thank you
Whenever applying natural materials to a new surface I will always try to test a small area and leave it for some time (a few months) to see how it reacts to temperature and humidity changes. I feel if you are going to go right on top of the plaster then a layer of clay slip first (fine filtered pure clay with water made into a paste) folllowed by a thick (10cm) layer of cob would be what I would go for. As I say though I couldn´t guarantee it would work, but with such a heavy slab of cob sitting on a stable floor there isn´t anywhere it could go.