Roger Hunt on building with cob

ABOVE (click on gallery image to view larger picture): A cob-built house in the West Country, which has ideal soil for the building method. Cob homes are snug in winter due to their high thermal mass. They can last for hundreds of years as long as they are well maintained, with roofs, gutters and drains regularly checked to ensure protection from rainwater penetration. To avoid any build up of damp in walls it is essential to use plasters, mortars and renders that are vapour-permeable, so choose either clay-based or lime-based products.

Houses built with earth or ‘cob’ can last for centuries if properly maintained discovers Roger Hunt, as he considers the history of the vernacular technique and the current revival of interest in this eco-friendly method of construction.

The first time I discovered a building built of cob I fell in love with the material. The ‘roses-around- the-door’ West Country cottage had the seductive curves typical of cob, and inside it was sublimely peaceful, comfortable and cosy. I admired the simple solidity of the structure and the gentle undulations of the chunky walls. They were built up on a plinth of local stone and stood under the wide overhanging eaves of a thatched roof – design features described locally as ‘a good hat and a good pair of boots’, both essential to keeping cob dry.

Cob is the word commonly used to describe earth walls built using a clay-based subsoil mixed with straw and water. The walls are often 600mm or more thick and they are finished in breathable materials such as lime or clay plasters and renders coated with limewash.

Due to its cheapness and availability, earth has been used extensively as a walling material since early times and is considered an eco-friendly way of building today. The oldest remains of an earth structure discovered so far date from 8000 BC at Jericho, a town made of mud bricks. In the UK clay subsoils are present in many areas but the techniques used vary enormously across the country and have distinct local names and traditions, though they fall into a few basic groups.

Where suitable soil was available, cob contributed to the vernacular tradition with simple cottages generally built by the owner and pooled labour from the local community. The use of earth for building only declined after World War II; now, it is undergoing something of a revival with a new generation of cob buildings being constructed.

ABOVE (left-right): According to cob building expert Adam Weismann, to build successfully with earth you need 15%-25% of clay naturally within the soil; The rest is made up of aggregate, sand and silt; Straw is added to the mix to reinforce the cob when it is dry.

Building with cob
Adam Weismann, director of products and services at Clayworks, is one of today’s cob builders and, with his wife Katy Bryce, builds and restores cob houses. To determine the suitability of the soil, he uses the ‘shake test’, first digging three or four holes to reach the subsoil for sampling and then using a process of shaking and sedimentation to separate the soil’s components and determine its composition.

Adam explained to me that, to build successfully with earth, you need 15%-25% of clay and 75%-85% aggregate, sand and silt. He adds more sand or aggregate, or clay as appropriate to achieve this combination.

Cob can be mixed mechanically or, more traditionally, using a fork and spade but Adam finds the ‘tarp’ method much kinder on the body. This approach involves laying a tarpaulin on the ground and using buckets to measure out the proportions of dry clay and sand or aggregate on to it. With someone helping he then folds the tarpaulin from side to side to mix all the dry ingredients together. To these is added water to moisten the mix, which is then stomped, either barefoot or with boots, until the ingredients are thoroughly mixed; the tarpaulin is then folded and the mix stomped again. Next, fresh straw is added and stomped until each strand is coated in clay. When thoroughly mixed, the tarpaulin is folded to form a large ‘burrito’ of mixed cob ready for use.

ABOVE (left-right): Like making a cake mix, liquid in the form of water is added to moisten the cob blend and make it easier to combine; Fresh straw is added and stomped until each strand is coated in clay; The tarpaulin is folded by hand to mix the ingredients thoroughly.

To build with it, Adam scoops up a forkful and forms the cob into a patty with his feet – this he lifts up and places on the wall before compressing it into place. Once a whole line of patties has been laid he walks on them to ensure that they are thoroughly compacted into the wall below. The excess cob is trimmed off with a sharp spade. Again, he stands on top of the wall, this time compressing the cob from the side with a wooden ‘thwacker’ – a flat, paddle shaped piece of wood. According to Adam, it’s possible to build a 300mm-500mm high section or ‘lift’ of cob wall in one session.

ABOVE (left-right): The finished ‘drip lip’; Placing a patty on a ‘high lift’; Here earth render is being applied to an exterior wall.

Maintenance issues
Provided they are maintained and protected from excessive damp, cob structures will last centuries. Unlike brick or stone, cob softens and loses its structural stability if it becomes damp so water penetrating the top of a wall from a leaking roof or failing gutter can be disastrous.

Cement render is another enemy and often deceives surveyors and homeowners who fail to realise the wall is cob. Invariably this hard render cracks so water becomes trapped behind it and, as the moisture levels rise, the load bearing capacity of the earth wall diminishes.

High external ground levels, where damp ground rises above the masonry plinth and meets the earth wall, is another common cause of damage. Cob walls are particularly vulnerable to rodents burrowing through them and a large network of runs may significantly undermine a wall’s structural stability.

Repairs to cob
Cob walls are relatively easy to repair, although it is worth understanding the vernacular tradition of the area and calling upon a local conservation specialist. Water damage often results in dips and hollows in the surface, but these cannot be rectified by merely applying a new cob mix as it will not adhere. Cob is effectively a masonry construction and is acting in compression to carry the load of the walls and roof. With this in mind, any repairs should be made up in horizontal layers to carry the forces acting from above. One of the simplest methods of achieving this is to make cob blocks.

Once the blocks have dried, the damaged section of wall is cut away to receive them. The blocks are bedded in an earth mortar made from the same subsoil, or from the salvaged cob mix that has been cut away to form the hole. The earth is sieved to remove larger stones and mixed with a little water to form a sticky mortar ideal for bedding the blocks.

When fresh cob is mixed it is best used within four weeks, although it will last indefinitely and it should be covered with tarpaulins in between building to prevent it from drying out from the sun or getting too wet with the rain. If it goes hard, simply add some more fresh water and re-stomp it back to a workable mix.

ABOVE (left-right): A cob cottage in the West Country, where the soil has the ideal proportion of clay required for this method of construction; A cob inglenook, with an attractive uneven finish, houses a wood-burning stove.

Find out more about building with cob in Building with Cob: A Step-by-Step Guide by Adam Weismann and Katy Bryce (Green Books, £25).

Visit Roger Hunt's website at huntwriter.com

WORDS ROGER HUNT PHOTOGRAPHS FROM BUILDING WITH COB: A STEP-BY-STEP GUIDE BY ADAM WEISMANN AND KATY BRYCE
Featured in the November 2010 issue of Period Living