New build:cob
From Sustainable Building Resource
Contents |
[edit] History of cob
The earliest archaeological remains of cob found in the UK date from Roman times, and the earliest recorded building dates from around 1200AD. The oldest cob buildings still standing in the UK are found in Devon, constructed with oak cruck frames and thatch roofs. Cob was used extensively as a building material until the industrial revolution when bricks and imported building materials became prevalent. Cob is now enjoying a revival, as fossil fuels (and therefore materials and transport) become more expensive and concern for the environment is growing.
[edit] Uses for cob
The most common use for cob is as a walling system, creating thick walls (approx. 2’) with very high ‘thermal mass’. Materials with thermal mass store heat very effectively, absorbing heat from the sun during the day and then slowly releasing it during the night. For this reason it also makes very effective bread ovens and chimney stacks. Cob can also be used to make earthen floors, which (when sited by south facing windows) will absorb the sun's heat, and slowly heat the room to a very comfortable temperature. Bricks, known as ‘adobe’ can also be made from cob. They are very useful for repair work in cob walls, as they are pre-shrunk and very stable. In Asia and South America adobe bricks are more common and very cheap, entire buildings can be built from adobe bricks.
[edit] Site and Foundations
As with any building, choice of site is crucial to the success of a cob structure. A well-drained site with high bedrock will make solid foundations easier to achieve. Choosing a sheltered location below the snow line and out of the reach of floodwaters will prolong the life of the structure. The orientation of the structure is also important, to protect occupants from prevailing winds, and also to make the most of the sun’s warmth.
A traditional saying in the UK is that cob buildings “need a good hat and a good pair of boots”. This means a good roof overhang (minimum 18”) and a 12 -18” stem wall to protect the cob walls from driving rain, splash back from the ground, and rising moisture. Walls should be minimum 18” thick.
[edit] Composition of cob
Concrete, lime and cob all function in the same way, and need a similar mix recipe:
• Aggregate – provides the bulk and compressive strength. For cob use a mix of large stones of up to 2” diameter and progressively smaller stones, down to sand. The smaller stones and sand fill in the gaps between the larger stones. It stabilizes the clay and minimises shrinkage and cracking in the cob.
• Binder – holds the aggregate together. In cob the clay is the sticky substance that washes around, and holds each particle of sand and stone in place.
• Water – washes the binder through the aggregate to form an even mix, and then slowly evaporates to leave a hard strong mix.
• Tensile strength – is provided by lengths of fibre to prevent the finished material from sheering. In the UK fresh long straw is commonly used.
The straw also helps to reduce cracking as the cob dries and shrinks, and provides some insulation due to the air contained in the hollow stalks. Straw can be from wheat, rye or oats provided it is long, strong and dry. Depending on local availability flax, rushes and heather are sometimes used. Cob mixes are usually roughly 15% - 25% clay to 75% - 85% aggregate. i.e.: 1:4 clay: aggregate. This provides a stable mix. Too much clay will lead to excessive shrinking and cracking whereas too little clay will lead to dusty, crumbly cob.
The clay should be collected from subsoil, as topsoil contains organic matter that will rot down leaving gaps in the cob, and a less stable structure. A cob mix will vary from region to region depending on local availability of materials. Chalk is used in Dorset to make very strong and durable walls with minimal mixing and processing needed.
[edit] Soil testing
Samples of subsoil can be tested for their clay and sand content to determine their suitability for cob. If you are lucky you may find a perfect mix all ready to be dug up on site. However you may find you have a sand heavy or clay heavy soil and need to add more clay or aggregate.
• Look – Observe your site to see what kind of soil you have. Water will tend to collect on the surface of clay-rich soil, which will then crack in very dry periods. To find soil suitable for building material, first scrape away the top organic layer and examine what's underneath. Clay has quite distinct colours – red, pink, blue, grey. It looks smooth and streaky as you dig through it.
• Touch – Take a handful of subsoil, clay feels greasy, sticky and smooth when wet. It is easy to mould and shape without breaking and is difficult to wash off your hands. Sandy soil is grainy and crumbly in your hands.
• Taste – Clay feels smooth, not gritty when ground between your teeth.
• Smell – Organic matter smells rich, musty and mouldy. This is to be avoided.
• Shake test – When you think you have a good sample of cob place about 2” of the test mix (with the big stones removed) into a glass jar. Fill the jar to ¾ full with water and a little sugar or washing-up liquid to break up the lumps; shake vigorously then let it settle. If your soil has sand, silt and clay in it, you'll get three distinct layers. Large aggregate is the heaviest and will sink to the bottom immediately as you watch. Sand and silt will settle more slowly, and the clay will stay suspended in the water for a while then settle leaving the clean water on top. This diagram gives a good idea of what you should see. The layers can then be marked, and ratios calculated.
• Brick test – When you think you have a good cob mix, you need to test how the cob will dry, and if it will make a strong, even, dry mix. Make some wooden moulds (about 10 x 2 x 2”). Pack each mould with a different test mix (do not add straw at this stage) and allow to dry. Once the brick is dry, measure (in inches) the shrinkage from the ends. Multiply this by ten to a give percentage shrinkage. More than about 3% shrinkage means there is too much clay in the mix.
• Ball test – Another option is to make a very dry mix, packed tightly into a ball and dropped from waist height. If the ball crumbles there is too much sand in the mix. If the ball squashes there is too much clay, and if the balls remains unchanged you have a good mix.
[edit] Mixing cob
There are different methods you can use to mix your cob, depending on the tools you have available, volunteers you have to help you, or how much you are mixing.
Place the dry materials in the centre of the tarp and mix together by folding the tarp over and dragging it over itself in all directions. Unfold and stamp on the mix, then refold and continue until well mixed. Slowly add water and continue mixing until the clay is consistently mixed through the aggregate. Adding too much water will make mixing easier, but building impossible, so resist the temptation! Slowly add about 1 part straw to 5 parts cob and mix until well incorporated. Putting the tarp into a pit with the edges over the sides of the pit, and mixing as above, helps to keep the cob contained.
A much quicker, but less ‘organic’ way of mixing uses either the bucket or treads / wheels of a digger or tractor to do the mixing. It can also be used to dig up the subsoil, collect aggregate, and deliver finished cob. Large amounts of cob can be lifted onto the wall then stamped in, therefore reducing the labour of carrying the cob. This is by far the most commercially viable method of mixing and building.
Horses or cattle used to be used to trample or mix cob in a pen. This also added manure, which makes the mix more sticky and pliable.
[edit] Building with cob
When is cob fully mixed and ready to build with?
When the cob looks well mixed with no dry bits and the straw is well incorporated and dirty. The mix should not be too sloppy or stiff, and it should be difficult to pull apart. Doing a ‘ball test’ as above, will help to determine if the cob is ready.
To build with cob:
• Take a scoop of cob, about spade sized.
• Roughly shape with feet to make a patty, with one flat face to face the outside of the wall.
• Lift onto the wall, one outside, one inside, one in the middle. Then smash together into one homogenous mass with feet, spades etc.
• You can build 1-2’ in a single lift, depending on the weather (therefore how quickly it dries) and how wet your mix is.
• You can trim the walls when the cob has gone ‘green hard’ ie: firm but not hard. Use a spade or saw. The cob will shrink as it dries so don’t over-trim as it is difficult to put material back.
• Compress the wall with a ‘thwacker’ after trimming.
On the first lift you need to creat a drip line by overhanging the cob patties by 2-3” over the stem wall. Compact the bottom edge of this to 45 degrees using a fork or spade. All subsequent lifts should be vertical above the lip.
Important things to remember:
• Avoid ‘shouldering’ and ‘mushrooming’ where the wall gets respectively narrower or wider as you build up. Keep measuring the wall thickness as you move up, and ensure it is vertical using a spirit level.
• Avoid ‘splooging’ where wall sags and bulges out due to building up too quickly (ie: each lift is too tall) before each lift has had time to dry.
[edit] Niches / windows / arches
• After the cob has dried for a few months, you can cut out a niche using a sharp spade or ‘scutch’, maximum 8” deep.
• Build a wooden form, which you build in to the wall as you go. Allow drying for 6 months before removing.
• Place hardwood bearers on either side of an opening in the cob, then a lintel on top.
• Make ‘corbels’ by folding long pieces of straw into a small patty to strengthen it. Use these to slowly cantilever out to eventually meet in the middle to form an arch. Must allow each layer to dry before building up.
• You can also use corbels for relief work, for sculpting shapes onto surfaces such as walls.
Good reference books:
Cob builders Hand book by Becky Bee Building with Cob, step by step guide by Adam weismann and katy Bryce
