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Adobe
Adobe, or cob, is basically a sun-dried mud. It has been used throughout the world, in many different time periods. Several buildings, hundreds of years old, still stand.
For the DIY builder, adobe has a lot going for it:
- It is cheap, especially if you have a good deposit near your building site.
- It is environmentally friendly. It comes form the earth, without the need for mining or processing, and will eventually return to the earth.
- It has great thermal mass, so no need for insulation.
- It is easy to use and work with.
- It is strong and stable, especially if it is supported with buttresses.
- It is beautiful.
The downside is that it takes a lot more time to build with adobe than it does with many other building materials. It also requires some energy – you’ll be moving a lot of mud.
Testing the adobe
Before you start building, it’s a good idea to test your possible adobe deposits. This comes in two stages:
- Put some earth (take out rocks) in a jar, add water, and shake real well. The sand should settle to the bottom, then silt, then clay.
- Make some test bricks out of each sample. Too much sand will result in a crumbly, weak brick. Too much clay will shrink and crack as it dries.
Bricks
The traditional way to make bricks is to set your forms on the ground, pour wet mud into them, and remove forms. When the bricks have dried out and cured, you then lay them as you would any other brick, using adobe as mortar.
We do it a little differently. We set our forms up on the wall itself and pour the bricks in place. After about 15 minutes, we remove the forms and move them along the wall.
We use two types of brick forms:
1) The powerhouse is a small, 10’x10’ adobe building, which we used to practice on and try out new ideas. For its construction, we used a wooden, box-like form, 12”x12”, 6” tall.
- Set up the forms on the wall.
- Pour mud into the forms.
- After about 5-10 minutes of the mud setting up (when it’s firm, but not dry, to the touch), pull the forms upwards, off the brick.
- If the mud is sticking to the form, push down on the corners of the brick as you’re pulling the form up.
The thing with pouring bricks in place is that, when you remove the forms, there is a space in between two bricks, where the forms used to be. On the powerhouse form, we solved this by attaching two extra pieces of wood, 6” tall, to the middle underside of 2 opposing sides of the brick form. We call them T bricks.
- Lay one level of bricks.
- On the next level, shift the brick forms over 6” so that the l part of the T covers the gap of the level below.
2) On the next building, a 16’x24’ room, we had remesh through the center of the adobe wall (for extra strength), so the lift up and off form wasn’t possible. For this room, we used forms that bolt together, so you assemble the bricks on the wall and then bolt them together. These are 12”x12”, 12” tall. The mud takes about 15-20 minutes to set up. You then unbolt the forms and remove them. We fill up the gaps using free-form adobe, which is a little slow, as you can only do half at a time (the mud has to dry before it will take any more weight).
Mixing Adobe
Many people use straw in adobe. We don’t. Instead, we add about 10% (20% when added strength is needed) cement. We find it makes stronger bricks and doesn’t erode as fast in rain, giving us more breathing space before having to do a plaster.
Set up a canvas tarp on the ground (about 5’x5’).- Have buckets of water and a bag of cement handy.
- Put 27 shovels of dirt and 3 shovels of cement (9:1) onto the tarp.
- Mix the two together while it is still dry.
- Pull the edges of the tarp towards the center, so that the dirt is left in a pile.
- Hollow out the center, and then almost fill it with water.
- Scoop dry dirt from the sides into the pool of water, until the pool is covered over and looks like a volcano.
- Jump into the volcano (we started using old boots after the water got cold, and our feet were torn up by small rocks) and start stomping.
- Sprinkle water over the dirt and pull one side of the tarp towards the other side, making the dirt roll up together.
- Stomp again. Repeat the process until there is no dry dirt left and the mud sticks together. You don’t want it too sloppy. Cookie dough consistency is what you’re aiming for.
Curing adobe
Curing is the process of keeping the material wet, or damp, for a period of time after it has set up. The slower it dries out completely, the stronger it will be.
For the first day, we use a paintbrush to paint on water. This helps join the mud at the seams. If small cracks appear while the mud is still a little malleable, use your thumb to mush the mud together around the crack, and then paint it with water. We then spray on water for a couple of days, a few times a day.
Small cracks are no big deal. You can patch them with adobe or just plaster over them. Larger cracks that keep growing are a sign of a more serious, structural problem (like the wall is leaning).
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