Cob Building Tutorial

Chapter 7: Cobbing the Inside of Your Home

 Cob is such a strong and versatile material that it can even be used to build furniture for your home! Benches, chairs, shelves and alcoves made of cob can easily be incorporated into your final home design. By adding a cob couch or bench to the wall of your home, you maximize use of space. If the couch is actually built into the wall, more floor space is freed up, allowing you to get by with less space overall.

Similarly, if you create alcoves and shelving directly into a cob wall, you maximize your use of vertical space for storage, reducing the need for closet spaces. Cob can also be used to add relief sculptures to the walls of your home. In fact, it is such a versatile material, you can even build a fireplace in your home or a bread oven outside your home! Not only is cob fireproof, but building a fire inside a cob oven or fireplace actually cures the clay, turning it into a pottery-like finish.

If you’re considering adding a cob couch, bench or chair to your home design, remember that cob is durable and sturdy. You can use it to build cantilevers up to a certain extent (a cantilever is a protrusion from a wall or bench…basically just an ‘overhang’). If you’re going to build a cantilever for any reason, add a little extra straw and a little extra clay to the mix. This gives it added strength. While building a cantilever, go a little at a time, allowing the mix to dry before adding more.

If you’re going to use a cantilever for a bench or a shelf inside your home, it’s a good idea to practice first, so you’ll know how much of a cantilever will support itself before collapsing. A good way to practice is to build a bench out in the yard first. As you gain experience and confidence, you’ll be able to get the mix right for building indoors.

Another alternative is to just build your bench or couch without using a cantilever. In this case, you’d just stack the cobs to the right height for a couch, then smooth out the top by shaping with a machete and filling in the low spots with a wetter cob mixture.

If you want to take advantage of vertical space, you can add shelves or alcoves to your walls. To add a shelf to a cob wall, you simply imbed a plank into the wall itself. This is one of the reasons why I like to start out with cob walls two feet thick. If your wall is two feet thick, when you get to the shelf part, you make the wall one foot thick, and embed the shelves in the alcove created by the reduced wall size. One-foot thick cob walls are still enough to provide some insulation and thermal mass in more temperate climates. If you live in an area of more extreme temperatures, you may want to add some more thickness to the walls in the shelving area.

To add an alcove, the process is similar. Simply embed a board for the ‘floor’ of the alcove, and then shape the alcove from the clay surrounding the board. Since the alcove will also be bearing the weight of the wall above it, I probably wouldn’t make an alcove bigger than about one square foot without using some sort of support.

If you’d like to include a bigger alcove in your wall, just frame a box out of 3/8” plywood. You can leave it rough on the outside, since this part will be buried in the cob. Smooth the inside. You can cover the inside with a thin layer of cob, or you could just plaster over it when you plaster the rest of the home.

Small alcoves, used for candle or lantern niches, can really add beauty to your home. You can back such alcoves with mirrors (just bury the edges of the mirror into the cob) so that they reflect the candlelight or lamplight.

If you’re thinking about adding a cob fireplace to your home, remember that fireplaces, while beautiful, aren’t necessarily the most efficient way to heat a home. Stoves are, in general, more efficient ways to heat. In particular, there’s a type of stove called a ‘rocket stove’ (Figure 7A) that is very good for home heating.

The rocket stove (so-called because it resembles a rocket) has a few main components: a chimney, a fuel magazine, a fuel shelf, and a heat exchanger. Rocket stoves are estimated to be twice as efficient as a regular fireplace. They’re easily built from inexpensive materials, and use much less wood than a regular fireplace. There are plans all over the Internet for building rocket stoves of various sizes for a variety of purposes. If you’re interested in using this method to heat your home, you shouldn’t have any trouble finding plans for free online.

If you just like the idea of the romantic look and feel of a fireplace, you might consider incorporating both a fireplace and a rocket stove. You can light the fireplace for those special romantic occasions, and you can use the rocket stove for general heating (and possibly cooking) purposes.

If you decide to add a fireplace, do some research before you begin. There are several good books available on fireplace design, and there are plenty of online resources to give you the proper dimensions for the right fireplace size home you’re building. You can wrap a metal stove or chimney pipe with cob, but it’s probably a good idea to insulate it a little by adding a layer of wood ash between the cob and the pipe. You would do this by cobbing around a pipe larger than the actual stovepipe itself, then filling in the space with wood ash.

Stoves and ovens aren’t limited to the indoors! Many, if not most, cobbers got their start by building a cob oven out in the back yard. Cob ovens are relatively simple to build, and are a great way to gain experience with cob building. Not only that, but you can cook a pizza or a loaf of bread in five to ten minutes in a cob oven!

To build a cob oven, you first build a pedestal out of stone. If you don’t have enough stone for an entire pedestal, you can build a sort of ‘well’ that you would then fill with sand or clay. The oven would then be built on top of this sand, or on top of the stone base.

The next step is to place enough firebricks on top of the pedestal to define the floor of the oven. On top of these firebricks, you pile wet sand, shaping it to define the interior of the oven (a dome shape). Cover this pile of sand with wet newspaper. The cob goes on top of this wet newspaper. Shape the cob over the sand and newspaper, making sure that the cob is at least six inches thick all the way around. When you reach the top, insert a stovepipe, about eighteen inches to two feet in length. This allows the hot gases to escape, and allows for circulation.

Once the cob is dry, cut a door using a machete. Make sure the door is wide enough to accommodate your cooking utensils! Now, remove the sand from the interior of the oven. Don’t worry if the newspaper sticks to the cob. The first time you build a fire in the oven, it’ll burn away.

The best thing about building an oven of cob, is the fact that you can let your imagination run wild in sculpting the exterior. The pictures here show just a few of the many variations.

To cook in your oven, you first build a fire inside using either wood or charcoal. When the wood has burned to embers, remove it from the oven using a metal bucket. Be careful not to spill any of the hot ashes on yourself! I find that a hoe with the handle sawed off to about two or three feet in length works well for this. Once you’ve removed the ashes, immediately place the food you intend to cook directly into the oven. Keep a close watch on it! It’ll cook faster in a cob oven than you’d expect. Once you remove the wood or charcoal, the oven temperature is around 900 degrees F!

You can remove the food much easier if you’re able to find an oven peal or a pizza bib. This is just a big metal spatula used for removing food from ovens. It’s basically a flat piece of metal, about 12” x 16”, with a handle on it.

The final touch for your home that can be constructed of cob is relief sculpture. Since cob is such a plastic and flexible medium, you can mold it just like clay. This means that you can add sculptural qualities to your home. The best thing about cob is that if you make a mistake, you can simply start over by cutting away the parts you don’t want with a saw or machete. If you’re going to add a sculpture to a cob wall that has already dried, you can help the new cob to stick by driving nails into the old cob. If you’re worried about the weight of your new sculpture, you can add a little mashed up wet newspaper to the cob mix to make it lighter. Once you’ve gained a little experience in working with cob, you’ll know what you can and can’t get away with.

The best thing about cob is that you can mix other media with it. For example, if you like mosaics, you can place tiles into the cob. In fact, if you like mosaics, some building supply stores will give you their broken tiles for free. I have a building supply place that fills up a giant blue plastic tub for me about once every three months. I pay them for the tub, and the tile is free.

You can also mix wood, seashells, pottery, glass…virtually anything you can think of…into a cob wall. The possibilities are only as limited as your imagination!

These pictures will give you some idea of the fantastic artistic effects you can achieve!