Exploring cob

Knead a wet lump of sample in your hand to form a small ball. Stretch out the palm holding it and let the ball drop. If it goes to pieces, there is not enough clay and too much sand. If it stays the way you dropped it, there is too much clay. If it flattened a bit at the bottom and sat where it fell, the mix is just right.

There are several ways of testing the suitability of the soil around you for cob. There is the sedimentation test in which you take a sample in a bottle, add water, shake it up and let it settle. And then read off the clay, sand, silt and gravel content percentages. As long as clay is about 30% and there is not too much silt, the soil should serve. Then there is the ring test where you roll out wet soil into a long cigar and try to wrap it around a finger and see if it will form without cracking. Or you could make small bricks and observe if they dry out as they dry. Or you could make a drop test [see box] which I found to e the simplest.

Once the right soil mix has been put together, a pile of it is wetted and trampled underfoot to knead it well. The pile is left a few days to season well; in this period the wetness is said to encourage beneficial bacterial activity that plasticises the mud.

Coconut coir doesn’t make good reinforcement. I intend trying vetiver grass instead. You could also experiment with reeds and local grasses. Make some experimental bricks and observe their strength.

Reinforcement is added on the building day and trampled into the seasoned cob. Classically it is grain straw cut to about 6″ lengths. But as rice straw is part of Indian cattle’s food chain it may be hard to come by,quite beside the issue of straw’s seasonality. At pointReturn I attempted using coconut coir fibre that is widely available.But I’d advise against it, though. Hair thin, it is difficult to disperse through the mud- it tends to ball-up. I am currently looking at vetiver grass which is a profuse producer of leaves that you need to cut frequently.

The key requirement of a cob building is protection from rain and damp. If this is carefully designed in, a structure can last forever with reasonable maintenance; if not, disastrous failures can occur. One of the simplest ways to achieve this is provide the structure with ‘a boot and a hat’. This is a neglected area in Indian cob dwellings probably because of the extra cost.

3 thoughts on “Exploring cob

  1. Nice blog ! Currently I am building a house in my native village . Completed upto the basement level with stone masonry. Decided to continue with cob for super structure. The one mistake I have done is I have made a 7′ high door frame. And I have the design of having an open space in the middle of the house similar to the old “Agraharam” house and so I Have to raise the centre to 10′ and the walls are going to be around 8′. I have very little space left out in the outside ,so having longer eaves is not possible.

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