The one tricky thing about this step is that it is fairly site specific. You will need to know the frost depth in your area. This is based on how cold it gets where you live. The figure below illustrates frost depths for the continental U.S.
Figure 3. Frost Depth (in Meters)
To design your foundation well (so that it will last longer than the house), you will also need to know what type(s) of soil or rock are present immediately beneath your site. The table below summarizes approximate soil strengths for different types of soils.
Table 1. Approximate Strengths of Various Soil Types
| Type of Soil | Load Bearing Capacity (Pounds Per Square Foot) |
| Rock w/ Gravel | 6000 psf + |
| Gravel | 5000 psf |
| Sandy Gravel | 5000 psf |
| Sand | 3000 psf |
| Silt Sand | 3000 psf |
| Silt Gravel | 3000 psf |
| Gravel w/ Clay | 3000 psf |
| Clay | 2000 psf |
| Sandy Clay | 2000 psf |
| Silt Clay | 2000 psf |
You can get information on the type of soil beneath your site through the following ways:
- Drilling soil borings.
- Digging test pits.
- Reviewing soil data from soil borings drilled nearby for other projects, or drillers’ logs for nearby water wells (these are sometimes available on state or other public databases).
- Reviewing soil surveys from the Natural Resources Conservation Service, county government, or other sources.
- Talking with neighbors about the area about their soils and whether they have experienced foundation issues in the past.
The best method is drilling soil borings, but it is the most expensive. Most of the other methods can be used without spending any money, but they will require some effort. In the end, you are trying to identify the approximate bearing strength of the soil at your site (in pounds per square foot [psf]) and whether there are any major geotechnical issues that might cause issues for your foundation in the future (e.g., swelling clays, decomposing organic matter, sinkholes).
You will also need to determine where on your site you want to locate your house. This will be influenced by the following considerations:
- Orientation to the street – usually you want your front door facing the street; driveway design and privacy should also be considered.
- Site drainage – you don’t want water to collect near your house during major rainstorms.
- Major trees – assume tree root systems are twice the diameter of the branches.
- Building restrictions – most areas require that buildings be a certain distance from the front, sides, and back of the lot. In my area, for example, that’s 25 feet from the front and 5 feet from the sides and back of the lot.
- Utility access – you want to minimize the cost of bringing electricity, gas, internet, and other services to your house.
- Orientation to the sun – if possible, you want to orient the long dimension of your house roughly east-west to take advantage of passive solar heating and shading.
For the purposes of our house, let’s assume there are no major geotechnical issues, and the soil bearing pressure is at least 2000 psf.
The three main types of house foundations are slab-on-grade, pier and beam, and basement foundations. Our house will be built using a slab-on-grade foundation, with a rubble trench variation to achieve a high strength with a minimal amount of concrete[1]. The figure below illustrates the rubble trench design (the figure shows the design for a stem wall footer, but the design can also be used for the grade beams of a slab-on-grade foundation).
Figure 4. Rubble Trench Foundation Design
Source: http://buildnaturally.blogspot.com/2011/01/what-is-rubble-trench-foundation.html
To build this type of foundation, it is generally best to start by assembling three sides of the concrete forms first and leaving one of the short sides open to bring material in and out with a skid steer loader or other heavy equipment.
The cheapest way to make the forms is using 7/16 oriented strand board (OSB) plywood and 2 x 4s[2]. Sometimes you can find concrete forms and scrap lumber for free on Craigslist. While you will want straight 2 x 4s to reinforce the OSB, there is some flexibility in the quality of 2 x 4s required for the form supports, since those will be cut into shorter pieces. Figure 5 illustrates the form work assembly.
Figure 5. Illustration of Form Work and Steel Reinforcement (1-inch Grid)
To assemble the forms, start by hammering four stakes that will not bend easily (e.g., tee posts, pieces of angle iron) into the ground vertically where you want the four corners of your foundation to be located. Measure the diagonals of the rectangle formed by your four stakes and adjust until they are approximately equal, and the four sides of the rectangle correspond to the dimensions of the floor plan (Figure 1 from the previous post). Using a laser level or a bucket of water with clear tubing and a tape measure, make a mark at the same level on each stake that represents the desired top elevation of your foundation. At the level of the marks, tie a string between two of the stakes that is stretched tightly. This will be your guide for assembling the first side of the formwork.
To begin, rip one piece of OSB lengthwise into strips that are 10 inches by 96 inches. Attach a 2×4 to each of the long edges of the OSB with a few short nails or screws. Cut a few pointed stakes out of 2 x 4s that are roughly 30 inches long from the straight edge to the tip of the point, and a few that are roughly 12 inches long. Also cut a few sections of 2 x 4 that are approximately 20 inches long (straight on both ends). Standing outside of the area of the planned foundation, hold the OSB panel which has been reinforced with 2 x 4s so that the OSB is facing in toward the foundation area. Hold the top edge of the OSB roughly against the string and immediately next to one of the stakes. Using a sledgehammer, drive one of the 30-inch stakes into the ground vertically so that you can attach the OSB panel to it, allowing the top edge of the inside face of the OSB to line up with the string line. Repeat this process, spacing the stakes approximately every 3 feet. Stagger the stakes at the edges of the OSB panels so that one stake supports the edges of both OSB panels. Once you attach the OSB panel to the stakes (ensuring that it lines up with the string), attach one of the 20-inch-long 2 x 4s to the side of each stake (near the top) and drive one of the shorter stakes into the ground at the other end of the 2 x 4 so that it braces the form at an angle, as shown on the left edge of Figure 5 above. These “kickouts” will support the formwork when the heavy concrete fills them. Continue to assemble the forms in this manner, going around the outside of the planned foundation, until only one short side remains open. You will finish this side after you have prepared the soil and steel reinforcement inside the forms.
To prepare the soil, rent a piece of earth moving equipment, ideally a mini-excavator. Dig a trench that is 16 inches wide by at least 12 inches deep (or 4 inches beneath the frost depth in Figure 3, whichever is deeper) under the load bearing walls noted on the floorplan. Everywhere else, the soil should be four inches below the top of the formwork. All organic matter (e.g., plant roots, leaves, dark black humus-rich soil) should be removed from the planned foundation area, because this material will eventually decompose and leave small voids under the building. Once you have reached the desired grade, soil should be wetted with a hose enough to help compact it, but not so much as to be excessively sloppy or have large puddles of standing water[3]. The wet soil should then be compacted with a vibratory compactor, or a hand tamper used vigorously. Proper compaction is key to building a strong foundation, especially in areas where “fill” soil has been added to achieve a higher grade.
Once the soil is compacted, assemble the rubble trenches as shown in Figure 4, by stretching landscape fabric over each trench, and filling it with the drainage tile (perforated black pipe) and gravel. Ensure that the pipe has a slight slope so that the entire foundation drains to one corner. This simple act will be extremely effective in preventing rain and groundwater from compromising your home’s foundation or contributing to residual moisture in the building envelope.
After the rubble trenches have been assembled and covered with the landscape fabric, line the entire foundation area with 10 mil plastic sheeting and place the steel reinforcement. Begin with the trenches first. Place stirrups every 4 feet in the trenches and lay out rebar inside of the stirrups for easy assembly later. First “shake out” all the trench rebar by laying it out in the right locations and cutting and bending as necessary. Cut the rebar with an angle grinder and metal cutting blade. Bend the rebar with a section of metal pipe (e.g., black iron pipe) at least three feet long and with an inside diameter slightly larger than the thickest rebar. Once you are finished with the “shake out” phase in the trenches, tie all the rebar together as shown in the figures illustrated below. Once you are done with the trenches, take one end of the welded wire mesh roll and tie it to the rebar at one corner of the foundation. Make sure to tie it well so that it doesn’t break loose as you are unrolling the welded wire. Unroll the wire lengthwise until you reach the other corner of the foundation, then cut the welded wire with the angle grinder and tie it to the rebar in place. Continue in this manner until you have covered the slab with the welded wire mesh. Finally, use plastic rebar chairs to prop up all the rebar and mesh to the proper heights in the form (see Figure 5).
it is time to pour concrete! If the concrete trucks cannot pull up near the formwork, you will probably want to rent a concrete pump with an aerial boom to help you place the concrete. This will probably cost at least $1,000. The pump operator will drive the rig to your site before the first truck arrives, and park it where you tell them. Each concrete truck will back up to the pump rig and dump its concrete into the pump intake chute. For this foundation, we want to order concrete with a 5” slump and a strength of at least 2500 pounds per square inch (psi). Concrete comes in trucks that can carry up to 10 cubic yards. It is your responsibility to ensure that the trucks (and pump truck) can drive to where you need them without getting stuck in soft soil. The trucks are very heavy when full (~70,000 pounds), so make sure the path you plan for them isn’t going to cause them to ruin any paving you have on site. Sometimes the pump can reach from the street to your slab so that none of the vehicles have to leave the street and enter your site.
Order 10 percent more concrete than you think you’ll need. Plan on receiving one truck every 30 minutes. One person with a good eye for level (detail-oriented with good depth perception) will hold the discharge pipe to place the concrete. Two others with concrete rakes or flat shovels will help level out the concrete. Proper, even placement of the concrete can make the raker’s job as easy as possible. One person should use a 2-foot length of rebar to consolidate concrete in the trenches by jabbing the rebar gently up and down in the concrete. Outside of the trenches, where the concrete is only four inches deep, a worker should walk around in the concrete (wearing rubber boots of course) to consolidate the concrete and remove any air voids. Once a section of concrete has been placed and consolidated, two people holding a straight 10-foot 2 x 4 on either end should screed the concrete by sawing the 2 x 4 back and forth across the surface of the concrete. The person on the outside of the foundation should ensure that their end of the 2 x 4 is resting directly on the top of the formwork as it is sawing back and forth. The other person inside the planned foundation area should regularly check the 2 x 4 with a torpedo level to ensure that the screed is leaving the concrete level. The purpose of the screed is to further consolidate the concrete and ensure that it ends up at the right finished height. Pour and screed the concrete in strips that are approximately ⅓ the width of foundation. Pour the two outside strips first, and then use them as a guide to pour the middle strip with the right finished height.
Depending on temperature, sunshine, and wind, the concrete surface will be ready for troweling approximately 30 minutes after screeding, when the moisture has mostly disappeared from the surface of the concrete. One person can trowel the entire slab by themselves in 15-30 minutes using a rented 36-inch power trowel (read the instructions ahead of time). This tool can be rented for four hours for approximately $50 from your local Home Depot or tool rental business, and many businesses will rent the hand tools I’ve mentioned throughout. Finally, approximately one hour after troweling, flood the concrete surface with water and cover it with 6-mil plastic sheeting. Tape any seams with duct tape and staple the edges of the plastic to the sides of the form. Leave a hose trickling near the middle of the slab.
Once the slab cures, you will be ready to build the walls. If you can manage to wait 28 days for the concrete to cure, you will effectively have built your house upon the strongest possible rock you could have made. It is certainly possible to give the concrete less time to cure (e.g., one week), but the longer you keep a slab wet, and keep weight off it, the stronger it becomes. In fact, concrete theoretically gets stronger forever if left to cure wet. However, nearly all the maximum theoretical strength will be achieved at 28 days. So give your back a vacation after you finish the concrete.
[1] The famous American architect Frank Lloyd Wright popularized the rubble trench for single family homes, noting that trains which weigh several thousand tons run on a foundation of railroad ties (simple wood beams) over coarse, angular gravel.
[2] Note that you can also use 2 x 12s for the forms to save time, but these are more expensive. You can wrap them in plastic and reuse them as headers and ridge beams during the framing stage to minimize waste, but the wood must be carefully protected from insects and rot.
[3] Workers add water to soil during site preparation to assist in compacting the soil to the maximum achievable density, and to minimize post-construction soil settlement.