This article offers strong foundation information for different parts of the country.
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How Are Footings Protected?
Typically the foundation is 2' to 3' in the ground, low enough and depending if you're in a frost area, below what's called the "frost line," which is the depth at which the earth will freeze and swell. The United States is laid out by certain areas where the frost line is already designated. If you aren't aware of the frost line in your area, this is the beauty of getting a building permit. Ideally a city will make you aware of the frost line and the requirements for your footings.
If you're up in Maine and have a 40" or 42" frost line, this means the ground has the potential to freeze in the winter time 48" down. In Florida, for example, you won't need to dig down because the frost line there is 0.
Note: For those of you living in colder climates, keep in mind that you're going to need to dig a bit deeper than usual in order to avoid the frost line.
Of course, this extra depth is going to cost a bit more, but it's well worth it because placing footers above the frost line can cause the concrete to crack and shift, which means costly repairs — and massive headaches.
Moisture also is a primary concern in preserving the footings. The best way to keep moisture out is by applying a sealer to the concrete. A sealer is a tar-like material that you apply to the footing and foundation. The coating "seals" out the moisture, protecting your footings for the life of your new home. This is the goal of any flat or horizontal surfaces — to use a high-quality sealer to keep water from penetrating into the dried concrete.
Pouring the Concrete
Some people may think that it's best to lay the foundation and footing in a single day, but this is "not" true. In fact, commercial jobs have multiple pours all the time because not everything can be done in one day, so the design allows for this multiple-pour process.
Using quality suppliers enters into this picture. Many ready-mix operations utilize the same materials for the same type of pouring applications because it's virtually impossible to always do an entire job all in the same day.