Cob Building Tutorial

Chapter 5: Foundations

The beginning of any good building is a firm foundation. In keeping with the idea of building using natural materials, a cob home relies on a stone foundation. Stones harvested from the building site are usually the best; however, if your site is one of the rare ones that doesn’t have a ready supply of stone, you can also use cement blocks or leftover bricks in a pinch. You could even use a poured concrete foundation if necessary, although in principle I try never to use poured concrete for the reasons mentioned in Chapter 1. The goal is to be as sustainable as possible, and concrete simply isn’t a sustainable material.
I’m of the ‘if in doubt, build it stout’ school of thought when it comes to construction. Therefore, while some cob builders would recommend a foundation of only 8 inches in depth, I tend to want a firmer foundation. I want a stone foundation that is at least as deep in the ground as the walls are thick. For example, if you’re building walls two feet thick, you’d want to make sure the trench you dig for the foundation is at least two feet deep. Additionally, you’ll want the stones to rise at least eighteen inches above the ground in order to minimize moisture seepage into the walls from rainwater runoff. So you can see that you’re going to be moving a lot of stone.
As you dig the trench for the foundation, make sure that the stones will be resting on a firm bed, preferably clay or even stone. Cob walls are heavy, so you don’t want to place a foundation on sandy or unstable soil. If you dig down and hit a patch of sand or peat, it’s best to move the building and try again. 
Once you’ve got a good bed for your foundation, start laying the stones. Look for fairly flat stones for the bottom layer. Placing flatter stones on the bottom helps to create a natural moisture barrier, preventing ground water from leaching into the walls. As you add height to the courses, gradually work towards more jagged rocks. When you lay the final course, the idea is to tilt the stones inward in a ‘V’ shape so the cob won’t slip off easily. Leave the final course jagged so the cob will have something to adhere to. Remember to stagger the stones as you go so that you don’t have any weak spots. See figure 5A.

Before laying the foundation, make sure that you have considered the runoff from rainwater. If necessary, include a layer of sand and/or gravel at the bottom of the foundation to improve drainage. It may also be necessary to add drainage pipes to assist the ground water in escaping from the base of the foundation. Your building inspector will be able to give you advice on proper draining for your foundation walls.
A final consideration is whether or not to mortar the walls to add strength. I tend to like to keep the design pure, meaning that I don’t like to use cement or mortar for environmental reasons. I’m a purist, and I like to use as many materials from the site as possible. Bringing in concrete and mortar products reduces the environmental soundness of the building, so I try not to use mortar if possible. Mortar doesn’t adhere the stones to each other. Instead, once the mortar sets, it simply provides a stable base for the stones to rest on. A little care and consideration when laying the stones provides all the stability you need without having to resort to using mortar.
If your building will have any plumbing or wiring that will need to pass through the foundation walls, consider this necessity before you begin work on the foundation. I’d avoid having such things pass through the foundation if at all possible, as openings interfere with the structural integrity of the walls.
Before beginning the foundation, you’ll want to think about what sort of floor you’d like to have in your home. It is possible to have a raised floor in a cob home, but I don’t like to use them for the expense involved. If you’re using a raised floor, with a crawlspace underneath, you have to buy plywood or other flooring. You also have to include vent spaces in the foundation wall to allow moisture to escape from underneath the house. These vents introduce weak spots in the foundation. Also, once you’ve added a crawlspace, you have to insulate the floor underneath, and this gets to be expensive. You’ll also have to finish the floor above.
I much prefer laying a floor directly on the soil. This is similar to building a slab foundation, except instead of pouring a concrete slab, you use paving stones, brick, or tile. If this is the method you’ve chosen, there will be more details in the chapter on flooring concerning how to level the ground prior to laying the bricks or paving stones. Since heat tends to ‘go to ground,’ there’s some concern about losing heat through the floor in colder climates. Some of this can be offset through the creative use of passive solar design. There’s also a technique of using a geothermal heat exchange, consisting of flexible plastic tubing filled with water. Such a system collects heat from the ground outside by absorbing sunlight from the soil. This is also covered in greater detail in the chapter on flooring.
If you decide to build using a crawlspace, the best way to do so is to make the foundation wider than the cob wall on the inside, with at least a six-inch shelf. This shelf will be used to hold up the floor beams. If you add a crawlspace, you’ll have to install vents in the foundation wall to allow for moisture evaporation and to allow heat to escape in the summer months. Check with your local building code to find out how many of these vents you’ll need and what their placement should be.
The critical factor in building your foundation is to make sure it is level. If you’re building a fairly small home (less than 500 square feet), you can level the foundation fairly well using a line level and a tape measure. In this procedure, you drive stakes at the corners of the foundation trench, then tie a twine between the stakes. Level the twine by attaching a line level and moving the ends until the bubble is centered between the lines. A line level is a small plastic level that clips on twine using two hooks. They can be purchased at most building supply stores for a dollar or two. Get several and your work will go much quicker.
Once you have a level line, use a tape measure to dig down to the same depth all the way around the foundation line. This can be tricky if you’re building curves walls, but with a little creativity it can be done. Another alternative is to use a transit level. These can be fairly expensive to purchase, but you may be able to rent one from a store that rents building supplies. A transit level is a level mounted on a tripod. The level contains a scope. Once the tripod has been positioned and the bubble is centered (making the scope level and parallel to the ground), an assistant ‘sights’ through the scope, while another assistant holds a stick marked with numbers. You keep digging and resetting the stick until the desired level is reached. The rental store will be happy to explain the fine points to you, but the procedure is pretty straightforward once you’ve seen it demonstrated a time or two. 
You’ll also want to make sure that the walls don’t bow inward or outward while building your foundation. This can be accomplished with a simple device known as a plumb bob. A plumb bob is just a weight with a point on the end. It hangs from a string. Gravity pulls it straight downward. By matching the line of the wall to the string, you can be assured that you’re building a straight wall.
A final consideration for building your foundation is placing the openings in the wall for doors. If you’re using doors with wooden frames, you’ll need to elevate the frames a bit so that water doesn’t run in under the threshold. Your best bet is to raise the door frame up even with the height of the foundation wall, or slightly lower. If you do this, you’ll probably need to build steps and/or a porch to make entering the house easier. You may also have to build steps inside.
Now that the foundation is laid, you’re ready to move on to cobbing!