After you select your site, and have a plan and design for your building, then you are ready for site preparation. The first thing to do is make sure you clear the site where the building will be located. This means removal of trees, shrubs, large rocks, stumps, or anything that may be in the way of the construction. If there is a lot of plant life, you may choose to transplant it, or if it’s just grasses you may just mow it down or weed whack it. If you are planning on having a living roof, you could use the top soil and the plants on your roof, as long as they don’t require lots of space for deep roots.
Laying Out The Building Lines
For the purpose of keeping things simple, lets look at two options. A round structure and and rectangular structure.
For a round structure; First, pound a stake deep in the ground where you want the center of your building to be. Make suer it’s deep enough that it won’t move easily if it gets hit on accident. Then, nail or screw a string on the stake that is the length of the radius of your building. Pull the string tight and walk around the building marking the circumference of the building with sticks or marking paint.
Then remove the top soil and set aside for future use. At this point repeat the above process to get the circumference of your inside wall. Make sure to use the same central stake. Try not to remove the stake until the foundation has been completed. You may want to leave it until the roof is done, so that you know where the center of the roof should be. If you are leaving your stake in for the duration of the building construction, make sure to build a cover for it, so it doesn’t get knocked around.
For the thickness of your inside wall, measure the size of your straw bales first. Next, add the thickness of your cob, scratch coat plastering, and finish plastering. For the cob class room, we made the foundation 18 inches wide, because we used 12″ bales (half bales), and added 2 inches of cob on the inside and outside walls, plus 1 inch for plastering.
For a Rectangular Buildings; First you need to make sure that your building lines are squared. The best way to do this is to start out by setting up some Batter Boards. These are usually made from 2×4 stakes and 1×6 ledgers that get screwed between the stakes. Locate the batter boards 4 feet or more away from the building lines. In most cases you want to make sure your batter boards have enough room for you to move your lines while you try to square the building. So, place your stakes at least 4 feet apart from each other and use batter boards that are at least 5 feet long.
The other important thing to remember is that you can also use the batter boards to help you level the tops of your stem wall. So build all the batter boards at the same height as you want your stem wall to be. Since we don’t have a transit or a laser level we used a water level to make sure all of our batter boards where at the same height.
Water levels are easy to use and inexpensive to buy. All you need is a clear tube at any diameter that is long enough to reach all your batter boards. The reason water levels work is because water always finds its own level. You can test it yourself, fill the tube with water making sure there is at least 6 inches of space at each end. Set the two ends next to each other and move one of them up then watch the water level clime on the other end. It works like magic.
Squaring Your Building Lines; Locate the four corners of your building, using a measuring tape or line, to make sure the corners are spaced out exactly the distance specified in the drawings. Use temporary stakes and don’t hammer them too deep. Start with one corner as your permanent stake. We usually start with the northeast corner, but you can start with any corner that makes sense to your site. The other three corners can be moved or adjusted to help make the building square. Using your permanent stake as your bench mark, pick the next stake to make permanent. In our case we choose the northwest corner, to make sure we have a true south facing building. If the site allows, use a compass to help you locate the northwest corner. Now that you have a straight line, the next step is to make sure the 3rd stake is at 90 degrees from your permanent stake. To do this we use the pythagorean theorem.
The pythagorean theorem is something we learned in school, you may remember it.
If you don’t have a calculator handy, and the distances are not easy to square, then you can use the “3” “4” “5” rule. First you need to find a common unit. For example say your building was 12 feet long on side A, and 24 feet on side B, the common denominator is 6, because 3×6 =12 and 4×6=24, which mean that 5×6=30 which should be the distance of side C. Check it, see if I’m right.
For our building we wanted side A to be 12 feet, and our side B to be 17 feet. No common denominator there, so we used the pythagorean theorem.
Once you know what the distance of line “C” is (or your diagonal line), then you can move your third stake to make sure both distance “A” and distance “B” are still right, and that your diagonal line (line “C”) is also right. In our case we knew that the north line (which was our B line), was correct and all we needed to do was find our C line and make sure that the distance was 12 feet.
Then you repeat the process on the other side and get your building lines squared. check your diagonal lines to make sure they are both the same.
Once I have my corners set on the ground, then I build my batter boards. Others use the batter board to find their corner stakes. Try it both ways, and see what works best for you.
Now that you have your building lines, mark it with marking paint or sticks and remove the top soil from your building site and set it aside for later use. At this point you can mark your inside walls using the same processed mentioned above, or use the batter boards to mark them. Again make sure your wall thickness is correct before you move on to the next step. You don’t want to dig out your foundation trench the wrong size, and have to do it again.
In our case, we knew that we where going to use 12″ straw bales, (half bales) and 1″ cob plaster on the inside and out side. Plus we wanted to have some extra room in case we wanted to make a thicker plaster coat. So we went 16″ wide for our wall thickness.
Now on to the next step, the rubble trench foundation…..