«The Cob Builders Handbook You Can Hand-Sculpt Your Own Home Table Of Contents: Acknowledgements and Warning INTRODUCTION What is cob? Why build ...»
As soon as you get above ground level, the shape of the foundation will be permanently visible. Stand back and look at your artwork from a distance. Do you want an interior wall that is perfectly vertical all the way down to the floor? If so, build the foundation so it will line up with the vertical plane of the wall. On the outside, do you want the same angle as the exterior wall taper (2 inches in 3 feet) or do you want the super stable look of a steeply tapered base? Build accordingly.
If you want to see exposed stone, use ones that create the look that you want to see on the sides. Save the prettiest stones that you want to see forever and use them above ground near the entrance or patio areas. To highlight these you can stick them out a little from the plane of the rest of the wall. Use your artist's license.
Continue building until you reach the height you've decided on. The top needs to be rough so the cob has something to hold on to. It's best if the upper facets of the top layer of stone angle either into the center of the wall or into each other. If you are using mortar, do not make any places on the top where water will pool.
42 To Mortar or not to Mortar?
The purpose of mortar is to discourage the stones from moving.
Options No mortar If you have good stones and you're good at fitting them together, you can use no mortar at all. Many of the old cob homes in England are built on dry stacked stone foundations. You can cob in the gaps between stones from the outside to keep wind and critters out.
Cob mortar Many of the ancient Anasazi buildings (in the Southwestern United States) were built using cob or earthen mortar, along with a lot of attention and skill in placing the stones. Some of these earthen mortars have a little lime added to them.
Concrete mortar This is the modern version of earth or lime based mortars. It consists of sand, lime, cement and water.
Concrete and rebar wall with a stone facade This is what stone walls have devolved to. The library will have books on how to make these modern fake-stone walls.
to mix up concrete mortar The recipe is as follows: one part premixed mortar cement to three or four parts sand of various sized particles, and water. (You can make your own mortar cement by using 2/3 portland cement with 1/3 dry builder's lime instead of premix.) Use three parts sand if you follow instructions well, four if you are conservative and want to stretch the cement a little farther. The production of cement is very bad for the environment so the less you use, the better. Caution: cement burns skin and eyes!
Wear gloves and eye protection.
When you make up the mortar, mix small batches and use immediately. First mix the dry ingredients well, then add the water. It will start to cure as soon as the cement gets wet. Mix it in a wheelbarrow using a hoe or trowel, or on a piece of plastic or tarp by shifting the stuff back and forth.
You can use a wet mortar, or a dry one. When making a wet mortar, watch out because it will go from too dry to too wet very quickly. Add the water carefully until the mix makes stiff peaks. If it does get too wet, it's OK to add a little sand to stiffen it up. When using a dry mortar, barely moisten your ingredients. Mix well. Then it's ready to use!
tools you'll need for mortar
--wheelbarrow, cart or plastic sheet for mixing
--hose and spray nozzle, good for lightly wetting the curing mortar
--hoe for stirring, there are special hoes for this job with holes in them
mortaring the stones Squish mortar into the cracks between the stones you've already set, wiggling it in with a trowel or stick until the holes are full. You can add small stones too. Do this after each course of stone is laid. Clean your tools, gloves, and wheelbarrow before the mortar hardens on them.
For a dry mix, barely moisten the ingredients and wet the surfaces of the stones generously before troweling the mortar into place. If you are using a dry mix for your foundation, you may decide to make up a wet mortar for the places between the stones on the very outside surface of the foundation - the dry mix tends to fall off the sides of the foundation before it cures.
curing the mortar
Cover the fresh mortar with a tarp or with damp burlap bags. If you've laid the mortar in the morning, you can come back in the evening or the next day and tidy up any joints and stone faces that will be visible. A wet whisk broom will do the job unless the mortar has hardened too much, in which case use a wire brush. Keep your mortar work covered for a week or so. Spray water on it every day so it doesn't dry out too fast. This is very important to the strength of the mortar.
When you add more stone to a partially cured section of the foundation, be gentle so you won't dislodge yesterday's work.
Some Other Foundation Options Regardless of which type of foundation you choose, remember to tamp the ground under it and put in the pipes for the plumbing and electricity.
Poured concrete Think twice about using a lot of cement! Its production requires mining, transportation, and heating materials to very high temperatures twice. This entails
It's common to add reinforcing steel to a concrete foundation. The library will have lots of information about how to reinforce this type of foundation.
Remember to add steel anchor bolts to attach the door frame to the foundation. Be aware that wood meeting concrete forms a potential rot spot. Wood treated with poison will fight off the rot longer than untreated wood. Some builders also put a moisture barrier between the concrete and the wood.
A standard concrete mix is made of 1 part Portland cement, 2 parts sand and 3 parts gravel. You can mix it yourself or get it delivered already mixed to the site. Wet concrete does not stick well to dry concrete so ideally it should be poured all at once.
Embed sharp-edged stones on the top before it cures. This will make a rough surface for the cob to hold on to.
Make sure the concrete cures slowly by keeping it wet and covered for a week or two before starting to cob.
Soil cement Soil cement is otherwise known as 'poor people's concrete'. Soil cement is weaker than concrete but it is a good option for areas with limited rock and no road access, or for people who want to minimize their use of cement.
Depending on your soil (sandy is best) add up to 9 parts soil to 1 part cement plus water. Stir well. Use immediately. Make up some test batches with different amounts of cement and from different soil types until you find a suitable mix. Remember to put some stones on top of the soil cement for the cob to hold onto.
Soil cement can also be used for floors or made into large flat bricks for stepping stones or floor 'tiles'.
forms for poured concrete or soil cement Any poured foundation will need forms to hold the cement mixture in place until it's cured enough to hold itself up. The library will have information on building forms and pouring concrete.
Conventional forms are made out of wood. If you'll be building a rounded house, you'll need a rounded foundation which is very difficult to form using wood. Scrap sheet metal held in place with stakes driven into the ground, or straw bales with stakes holding them in place, are simple, inexpensive forms foundation walls.
Tie the tops of the stakes to each other to ensure that the concrete doesn't push the forms apart.
To curve a straw bale, lean it up against something at a 45 degree angle and apply weight to the middle of the bale until you have the shape you want.
Concrete blocks mortared together The library will have books that can tell you about this common way of building a foundation.
Railroad ties and gravel As with any type of foundation, remember to tamp the gravel well and to make an adequate drainage system around the house. The creosote used in railroad ties is toxic, so you may want to use these for outdoor walls, sheds and furniture.
You may want to set these on a tamped, gravel-filled trench to prevent possible rising damp. Pound earth into the tires with a big sledge hammer. Stagger the tires on top of each other like you would bricks. For more information on this, look up "Earth Ships" at the library.
Agricultural bags filled with earth and tamped The white woven plastic seed or fertilizer bags are basically forms for rammed earth bricks. If you're building where it's very dry, this type of foundation can be laid right on the tamped ground. If not, it's best used as an above ground addition to a stone or concrete foundation. Or build it on top of a gravel filled trench, because the earth in the bags may wick moisture. Bags are filled while in place with slightly dampened, sandy earth. The tops of the bags are secured by folding them over and pinning them with a nail, or folding and leaning them against the last filled bag. Then tamp them well. Pound sharpened sticks or bamboo into the bags, so that the sticks protrude out for the cob to hold onto. The bags will disintegrate quickly in the sun, so cover them with plaster as soon as possible. You'll be surprised how well plaster will stick to the woven plastic!
I have heard that you can buy rolls of continuous bag material before it's been cut up for bags to use for this type of construction.
Building with these and with agricultural bags are systems developed by Nader
Khalili. For more information on this contact:
0376 Shangri-La Avenue, Hesperia, CA, USA 92345.
General Info to Consider Accessing the underground temperature The temperature in the ground below frost line is the average yearly air temperature. (The temperature in the ground is the same all year round.) Find out what that temperature is and if it's a comfortable temperature you'll want to connect your floor to the ground. If your average yearly temperature is not pleasant, you may want to move to a more reasonable climate or consider insulating your floor from the ground. To get an idea, call the local concrete folks and ask them what the insulation requirements are in your area.
If some insulation is added under the floor, the heat absorbed by the floor will reflect back to you quicker, but you then lose some of your connection to that average yearly temperature. There may be data on optimum thickness for an insulated thermal mass floor in your climate.
When you build, you will be creating a protected zone in the ground under the structure. It will be shaded from the heat and protected from the freezing by the house. The indoor heating and cooling and the sun shining through the windows will also affect the temperature in this area. If you live in a climate with extreme temperatures, you may want to insulate around the perimeter of the foundation down to the frost line to keep the heat from escaping through the frozen and/or wet ground surface. This is usually done by burying a special water resistant styrofoam (sometimes called Formula) between the foundation and the ground all the way around the house. (2 inches is rated at R10, 3 inches at R15.) The styrofoam stuff 50 needs to be protected from the ultraviolet sunlight. Check with local builders or the building department about how deep, and how much to insulate in your climate's conditions.
Collecting passive solar heat with the floor The floor is one of the best areas to store the sun's heat, because it's where most of the sun rays hit after coming through the windows. This is why it is valuable to make the floor out of something that has plenty of thermal mass. That means make the floor out of something heavy and dense like earth, brick, or stone.
When I lived in an old house with an uninsulated concrete slab in a temperate climate, I was amazed at how comfortable it was. In the hot summers, that house was cooler than all my friends' houses. And in the winter, the floor was cold to the touch but that house needed a lot less heating than I expected. I put down rugs in the areas where my feet touched the floor and was cozy enough. In the rooms where the sun shone onto the floors, I left the floors bare so they could soak up the heat from the sun.
Cold air is like water. It will flow to the lowest point on your floor. If you design your floor with a low spot, the coldest air will flow off the floor where your feet hang out and into the lower cold air reservoir. Put the cold, sink spot on the cold side of the house. Because this will be the coldest place in your home, it is an ideal place for cold food storage if you don't have electricity, or if you'd rather not have a refrigerator.
You might be delightfully surprised, with a little adjustment to your habits, how easy it is to live without an electric fridge. If you live where it's very hot, you may want to make the cold sink area a living space, a cool bedroom, or summertime living room.
Stepping the floors to different levels is an easy way to deal with a sloped site. The differing floor levels define and separate indoor spaces and enhance the interest in a room. Steps from one floor level to another must be at least 4 inches high and at most 9 inches to prevent stubbed toes. The step(s) where the floor level changes can be flowing organic shapes or straight edges. Take this into consideration when planning your foundation. During construction, make temporary ramps where the steps will be.
This will make life easier while you are dragging the tarps full of cob around the floor area during cobbing. Make the actual step later when you've finished the walls.
Planning the floor at the door threshold
The door's threshold limits your choices about the height of the floor at the entrance. When designing the foundation and threshold, it is helpful to estimate floor height(s). Ideally, the threshold will be at the same height as the floor surface.
Your floor can be built directly on the ground. Read on to help you decide how thick your floor will be.
Plan for the finished floor to be at least 2 inches above the outside ground level.