Cob Earth Building -Ancient Construction Principles Low Cost Buildings made with labour of love from out of the earth and to return to the Earth
Why Cob? • Cob is cheap. You may have it onsite. • Cob is laid up in handfuls. You don’t have to dry, turn, lift, and mortar bricks. • Cob shapes into curves and arches that you will live in much more happily. • Well-made Cob is stronger than bricks as the ‘mortar’ and ‘brick’ are all one. • Cob requires no formwork or ramming • Its rough straw-coated surface gives plaster a surface to key to firmly
Why Not Cob? • You will not be a big consumer of toxic building materials, and will not be helping the unsustain-able inflationary growth of the economy. • Your building materials will be 100% reusable. • You will not be buying many carbon credits. • You will not be helping the rich get much richer • You will not be able to boast how expensive or unnecessarily large your building is. • Your building will not rot or age • You will get fit and not be so interested in TV!
Heating • Cob’s massive thermal mass will store summer’s heat keeping building warm in winter. • About six months later it will have cooled enough to absorb the excessive heat of summer • Double-glazed windows facing the sun and an insulated roof will enhance this effect.
Breathing • Cob is as comfortable to live in as your skin. • Cob, and natural paints and plasters soak up water-vapour in damp periods and dry out when its warm and dry. This dehumidification keeps your home warmer. • Moisture does not become trapped rotting attached or inbuilt timbers. • Clay absorbs many toxins like an air-filter. • Cob does not isolate you from the health benefits of being in the earth’s magnetic field.
Cob • Cob is mixed by adding water to 20-25% clay and 75-80% gravel chips. • This wraps clay around each chip binding it tight when the clay dries, bonds, and shrinks • Cob softens and oozes away if it gets wet so it needs a hat and boots (a dry roof and a raised wet-proof foundation) • Sharp gravel of different sizes locks together and withstands compression reducing shrinkage and bearing weight. Too much clay and cob shrinks and cracks excessively.
Sit On It Test • David sits on a hollow cob chimney over-hanging the door of a cob-oven. • Chimney is reinforced with long straws. It’s attachment to oven was ‘weak’ as it was added on after the oven had been used. • To demolish it once dry, it and oven had to be hit full force with a long-handle 8kg (18lb) sledgehammer.
Clay Tests • Place sample of soil or clay in a jar, add water and shake. Leave to settle, note proportion of clay in middle. Organic matter will be on top mixed with silt, or just above it. Clay next. Gravel and stones form the bottom layer. • If 70-80% gravel and 20-30% clay use as is! • Make a mix. When dry you should be able to hammer it or drop it without it breaking. • Cob made with straw can be stronger than manufactured bricks or concrete blocks with no reinforcing.
Straw • Straw STEMS are hollow and strong like rebar. Rye is best. Any grain stem helps.\ • Many straws 5-10 cm long reinforce cob • Structural cob is a few per cent straw • Too much straw creates fracture lines through the clay and gravel weakening the cob • Absorbs water so a mix should be slightly wet before it is added, or sprinkle it with water • Straw helps cob bend without breaking • Straw helps cob breathe air in and water out
Foundations • Topsoil, roots and other material that would decompose and leave cavities under foundation are removed • Rocks are laid on firm subsoil • Large rocks are nestled into ground firmly and over-locked with each other • Foundations are raised to reduce rain splatter onto cob • Rocks are generally impermeable enough to water. If concrete is used add a damp-proof course to stop water migrating up into cob.
Gumboots? or socks spare the feet when coarse gravel Cob is made • Here the author is mixing 80% sharp AP20 (Aggregate particles 20mm and smaller from local quarry - It will make strong Cob). • Sand or crusher dust are alternatives. They create more shrinkage cracks during drying. • Clay (20%) spread on tarp beneath the AP20 is starting to show through as stomping proceeds. • Water is added as needed
20% clay 80% gravel or sand Sprinkled with water Stomped Folded by dragging corners of tarp up and over Repeated until blended
Folding • Once the mix holds together well when folded over by dragging the corners of tarp, it’s time to splat test and add straw
Splat Test • Take a handful of cob • Press it together to make a lump • Drop it from waist height • It should stay together with slight cracks • If it breaks apart it is too dry or has to much gravel. Add clay and/or water. • If splays out flat it has to much clay and more gravel needs to be added.
Straw In Straw makes cob much firmer and therefore hard to mix and fold so add it once cob is ‘pre-mixed’. For about three foldings sprinkle straw on top and work it in with the feet until folding shows straw is throughout the mix. Straw absorbs water - so more may need to be sprinked on top if cob is dry. If cob is too dry straw will not stick enough to mix in. Straw must not be mouldy when added.
Straw adds reinforcing • More straw and a little sprinkling of water needs to be added to “bare” patches and worked in with feet.
Thumbing • Throw handful of cob hard onto previous layer • Press one layer of cob into the next by pushing thumbs in all over • If you find soft air pockets push the cob into the softness until it firms up • Thumb tight joints around the edges • Leave the surface rough (don’t pat) so that the next layer has something to bind to • Make thumb holes to thumb the next layer into if it is going to have time to dry out a bit.
Bonding • If cob is built up too quickly lower layers will splooge (bulge) out sideways. • This can be trimmed off and re-used • Ideally cob will have dried out enough on its sides to stand firm when the next layer is added • While being moist enough on top to stick to the next layer
Layering • If the top surface of a previous layer is drying out before you get your next batch mixed keep a wet towel on top to keep it moist • Keep layer edges square or even slightly raised so next layer will not slide over the side • Build layers unevenly, like steps in a wall to help lock layers together • Thumb layers and joins together firmly • Use nails or timber to bind layers together or to windows, doors, and other timbers.
Cob binds Foundation Rocks • Cob stuffed in gaps and laid across rocks binds foundation together • Thumb holes help bind next layer • Next layer will link all rocks as one • Post on right (to hold a gate) is tied in with cob pressed around nails driven into it • Roof will be held up by wall itself
A broken window is recycled • A broken pane has been removed • Sash has been trimmed leaving two stubs that will set window inwall
Window is set on wall • Cob is thumbed firmly to make two balustrades to hold window in place • When this cob became firm enough nails were driven into frame to lock window to wall • On left is a round base for an in-built oven • A pitched top board will be place above string line for roof to nail to.
Wall and Oven Grow Sill under window has been added too! - with anchor nails of course
Team Activity • 80 litres (about 100kg) on a 6’x8’ tarp is easiest to turn with two. One person pulling over each corner of the tarp. • Cob builds a wall quickly with more willing hands • Heavy materials are close at hand