Deck footings, part 3 (of 7) of the 10 x 10 how to build a deck tutorial, provide support at the base of your vertical posts.
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The size and depth of footings vary depending on your location and
soil composition. For example, in Northern Virginia, a deck's footing
needs to be at a depth of 24" below the ground. In more southern
locations, many concrete footing details indicate that the footing can
be placed much closer to the surface.
You must check the building code for your area to determine the size and depth of your footings.
In Austin, Texas (in 1987) footings could be placed on top of the ground. Generally, they should be placed upon "undisturbed" earth, meaning that if topsoil was brought in to level the ground, etc., you should dig down to where the bottom of the footing is resting on solid ground.
The top of the footing can be above the ground. You can build a simple concrete form out of 2x material to support the freshly poured concrete until it sets up.
Once you have built the form, notch the 4" x 4" vertical post as shown in the diagram above and cut the post to where it will be about 1" below the top of your form. This is so that once you fill up the form with concrete, it will surround the bottom part of the post.
Plumb the post. Mix some ready-mix concrete in a wheel barrow (or other large container), and pour or shovel it into the form until it surrounds the base of the post. When the concrete hardens, it will not only vertically support the post, but it will keep it from sliding from side to side.
Let the concrete harden for about 24 hours. Remove the wood forms and the temporary supports used to hold up the end of the deck. The deck frame and footings are now complete.