Forming and Pouring the Footings
Thick, solid, and strong, concrete footings evenly spread the weight of the house (and everything inside it) over a wide area, ensuring that the finished house does not settle. With some slab foundations, it's possible to pour the concrete footings and slab at the same time. In other cases, the concrete footings and the slab must be formed and poured separately. In many areas, a concrete footing inspection by the building department is required. This must be done before the foundation walls are built. The inspect for proper dimensions, rebar, and a final check on property set-backs according to your city regulations.
Concrete Footings must be set on solid, undisturbed earth, not fill dirt. Local soil conditions determine the depth and design of footings. In areas of the country where the ground freezes solid in the winter, footings must extend below the frost line, which can be 4 ft. or deeper in northern regions. This prevents the freeze-thaw cycle from moving the foundation, cracking it, and causing damage throughout the entire house. The building department in your area will know the footing depth that is required. They'll also know about local soil conditions that may require a larger size footing or the use of rebar (steel reinforcing rod) in the concrete footing.
Forms for Concrete Footings
Concrete Footings are typically poured in temporary forms constructed from plywood or 2x lumber. Wood stakes and braces hold the forms in position. The top edges of the footing forms must be level, and the concrete footings must be set to the dimensions specified on the plans. A concrete truck is called in to fill the footings with concrete. If the site conditions make it difficult or impossible for the truck to get close to the forms, a concrete pumper can be brought in to pump the concrete from the truck to the forms. Check with your building department about code requirements for footings. It also may be necessary to embed vertical rebar in the concrete footings so that the upper part of the reinforcing rods can extend into the foundation wall.
Concrete Footings for our Cabin
For our cabin, we created forms with 2x8s attached to wood stakes. Since the footings needed to be 10" deep this allowed us to level them up without having a perfectly level base. The concrete is thick enough is doesn't waste too much out the bottoms of the forms.
Here's another picture of our forms when almost complete. Footings were required to be 24" below final grade to prevent frost damage from occurring. Remember to check and double check your forms to make sure your layout is square.
We used 1/2" diameter rebar throughout the forms and suspended it with wire. Rebar should be centered top to bottom in the form and also spaced within the forms as much as possible. We used 20' lengths of rebar and cut or bent them as needed. Rebar should be overlapped a couple feet and wired together whenever a splice is made. At this point, you will need the inspector to OK your forms before you pour the concrete. In our case, the inspector mocked us for the nice new lumber we used for forms. I guess he usually sees old stuff that is used over and over again by concrete companies. We ended up making good use of our 2x8s on the deck so it was not a waste in the end.
Here's another picture of the suspended rebar.
The one tricky spot on our footings was a two-foot stepdown we had to make toward the front of the cabin. We created a box with waferboard and supported it with 2x4s. We also tied a long piece of rebar to the rebar in our footings which would eventually become our Ufer ground for the cabin. This piece of rebar runs through the foundation as well and is tied into the main electrical meter panel for ground.
The concrete arrived and we began pouring the footings.
Before long we had all the concrete poured for the footings. At that point we waited a half hour or so for the mud to start setting up a bit so it would be ready to hold the vertical rebar that ties to the foundation.
"L" shaped rebar is placed vertically in the footings every 18-24". This rebar sticks into the foundation about 2 feet and will tie the two together. Before inserting the vertical rebar, it is helpful to run a string between nails in your forms to make sure the rebar ends up in a straight line and will land in the middle of the foundation.
Once the concrete footings had setup for a few hours, we pulled the forms off that evening so the foundation crew could get going the following week.
A final look at our concrete footings. Although it was a long day, we formed up the footings, poured them, and removed the forms in one day.