A lot of the home we've been building over the past 7 years is made of concrete. The downstairs rooms have ferro-cement walls and a concrete roof, upstairs has a latex concrete roof, with concrete mortar used in the brick walls. Plus there's countless examples of poured concrete throughout, like floors, posts, stairs, benches, shelves and foundations.
The thing about poured concrete, unlike latex or ferro cement, is that you have to have forms of some kind. When we first came to this property, there was nothing here - no water, electricity, building, etc. We started from scratch. What that meant in practical terms is that we had a lot of demand on both our time and our wallet, and we had to prioritize on both.
Solid concrete forms are often expensive, bulky and heavy, and at the beginning, we just couldn't afford them. So we started playing around with fabric forms. We used black plastic a lot, and vinyl billboard for ones that we knew we'd re-use over and over again. And to put fabric forming in a nutshell, we love it. It's cheap, easy, quick and versatile. With solid forms, you have to work with whatever length the forms are, but with fabric, you can customize your length and even shape for each project, at almost no extra cost.
Within this article, we'll run you through using fabric forms to make foundations. We also have an Instructable about Fabric Formed Posts. If you want more information, please visit the shelter section of our site, and especially the part about concrete.
Step 1: Materials and Tools
- Black plastic
- 12" wooden or metal stakes (reusable)
- 1" x 4" Lumber (reusable)
- Metal reinforcement
- Sand (does not need to be screened)
- Tape measure
- Stakes and hammer
- String (preferably something that doesn't stretch much)
- Stapler and staples
- Screwdriver or drill
- Mixer (or hoe/shovel and wheelbarrow)
- Coffee cans (for pouring the concrete)
Step 2: Preparation
The foundation acts as the base for your home. You cannot go back and redo it once the house is built, so it pays to do it right the first time. Reinforced concrete is perhaps the strongest option available to you.
The foundation must be wider than your walls, and sufficiently thick to withstand the weight of your building. Exact depths and widths depend on the house you intend to build on it, but one thing that will not change is the need for metal reinforcement. Always put metal in the concrete, as it can crumble without it. When using several pieces of something like rebar, make sure that they overlap each over at least a foot on each join.
You will need to have an overall design of your house in your mind before you pour the foundation. Know what material you will be using, where the windows and doors will be, where you need gas and water lines, etc. Only with all these things in mind will you know where you need to place pipes under, or tie-ins coming out of, your foundation.
You'll then need to prepare the ground under the foundation, as you don't want it shifting under the weight of the building that's to come. One of the best ways is to put a 12" or so layer of sand over the whole site and then compact it. You can often rent a compacter for a day or so, which is all you'll need to go over the area several times.
Step 3: Layout
Once you have the building site prepared and an overall plan of the building (preferably on paper), you'll need to mark the ground to lay out your foundation.
- Place a stake or rebar in the center of one corner. We'll call this your northeast corner.
- Measure away from this corner the distance to the northwest corner and drive in a stake or rebar. This one line determines the orientation of the building, so adjust it now if you wish, moving the NW corner a little north or south.
- Tie a string between the two corners and put a stake in the line for any other intersecting walls.
- From the NW corner, you'll be putting in the SW corner. You want to get the corner roughly square, though at this point it doesn't have to be perfect. To do so, we will be using the 3:4:5 rule: in a right angle triangle (which is perfectly square), when one side (A-B) is a multiple of 3 and the other (A-C) is a multiple of 4, the diagonal (B-C) is a multiple of 5.
- So, take a piece of non-stretching string, make a loop on one end and put it around the NW corner's stake. Then mark the string 9 feet (this is 3x 3, but you can use any multiple of 3 you want, so long as you multiply the 4 and 5 by the same number) from the loop.
- From the NW corner, measure 12 feet (3x 4) along the NW to NE line, and put in a stake or rebar.
- Take another piece of string and make a loop in it. Put the loop over the stake you've just put in. Make a mark 15 feet (3x 5) from the loop.
- Take both pieces of string in your hand and find where the two marks meet and put in a stake or rebar. This line is square to the NW-NE.
Now that you know roughly where all your corners are, we're going to lay out where the footing will go, while also making sure that it's all exactly square. At each corner, you are going to construct something that's kind of difficult to explain and the only photo we have of the method is around a post, not a foundation - still, look at the photo for this Step to see what we're about to explain.
- Place a 24" board on the ground, about 12" or so (it needs to be clear of where the foundation will be) east of the NE corner. The board should run parallel to the NE-SE line, with it's center positioned roughly 12" east of the NE corner.
- Place another 24" board on the ground at right angles to the first, so that it runs parallel to the NE-NW line about 12" away from it.
- Screw or tie the two boards together at the corner and then raise them up on legs (stakes), so that the top of the boards are at about the height you want the top of the foundation to be. You can use a water (spirit) level to level across to each of the other corners.
- Do the same for each corner.
- For any places where there's an intersecting wall, put one board 12" away and parallel to the line.
- Attach four strings - one for the north, south, east and west lines - to the boards. Each string should touch two corners and there is a corner at the intersection of two strings.
- Measure the two diagonals, one between the NE and SW corner, the other between the NW and SE corner. They should be exactly the same if the corners are all square. Move the strings until the diagonals are the same.
- Now that you have the exact location of the corners, put a screw in the boards to mark where the strings go. By using the string and the screws, you can now relocate the position of your corners even if you dig up the actual ground. You can now remove the stakes that you placed to mark the actual corners.
- The strings mark the center line of your foundation, so measure off each side of the string to find the location of outside of the foundation. For example, if your foundation is 12" wide, make a line 6" above the string and 6" below it.
- Look at your plan again and put markers around the area that will help you remember things that need to be added at this level of the building.
Step 4: Form
- You now have the lines of your foundation marked. If you wish to dig it partially into the ground, do so now.
- Hammer in wooden stakes periodically along both the inside and outside lines of your foundation.
- Attach lumber (1"x4" works well, but it doesn't really matter what you use) to these stakes at the height you wish your foundation to be. Make sure that each side is level from side to side, and even with the other side of the foundation. Alternatively, (especially if you have curves or crazy shapes in your foundation), you can hammer in metal rebar and attach strips of lathing to it at the height you want your foundation.
- Lay black plastic in between the lumber (or lathing), so that it forms a U where the foundation will be. Leave some slack in the bottom, so that the concrete can fill it.
- Attach the plastic with staples to the outside of the lumber. If you are using lathing, you'll have to tie the plastic to it every once in a while.
- Periodically put some chunks of wood, the width of your foundation, in between the lumber. These can be taken out as you fill with concrete, they are just there to ensure that your foundation stays the width it should be.
- Lay your reinforcement in the plastic, lifting it off the ground so that it will be in roughly the middle of the concrete. When using several pieces of metal, make sure that they overlap each over at least a foot on each join.
- Add any features that need to be included now (uprights, pipes, etc.).
- Fill with concrete, tamping it as you go. Mix it as dry as you can, for that will make it stronger. We use a mix of 3:1 sand to cement. You can add concrete fibers for extra strength.
- Level off the top by troweling the concrete in between the two sides of lumber.
- Once the concrete has hardened, cut the top of the plastic away from the lumber and remove the lumber and stakes. The plastic will remain in place, under the concrete, as a vapor barrier.
- Cure the concrete (keeping it moist) for several days. You can then begin on your walls - a very exciting step!