Deck Footings (3)
Deck Footings (phase 3 of 7) are the concrete foundations that provide a base for the vertical posts that support our 10' x 10' deck example.
The size and depth requirements for deck footings vary depending on your location, soil composition, and load. The Prescriptive Residential Wood Deck Construction Guide, by the American Wood Council (AWC) (link at the bottom of the page), contains a table (Post Height for 6x6 and Footing Sizes for All Posts) that explains the sizes of footings based on Beam Span, Joist Span, and Live and Dead loads. In our 10x10 deck example, the 16" x 16" concrete footings are square (406 x 406 mm) with a depth of 7" (178 mm).
Deck Footing Requirements
- The bottom of each footing must bear on undisturbed earth.
- Deck posts must be attached to the center of each concrete foundation.
- The deck posts must be secured to each concrete footing using approved post anchors.
Creating Deck Footings
Building code suggests that all footings should be at least 12" (305 mm) below undisturbed earth or below the frost line. I recommend you contact your local building code authorities to determine frost line and footing requirements in your region.
You can build a simple form out of 2x material to support the freshly poured concrete until it sets up (as in the diagram below). Alternatively, you can use the ground itself as a form for your concrete footing.
Three Concrete Deck Footing Options
- 1. Cylindrical Footing - You can use a concrete forming tube for cylindrical footings for your deck. The concrete forming cylinders come in different diameters and lengths. Our 10x10 deck example would require an 18-inch minimum diameter tube cut to the desired lengths.
- 2. A 16" x 16" square footing with a 12-inch diameter "stem" built on top with a concrete anchor installed for mounting the post.
- 3. A 16" x 16" x 7" high footing where the entire footing is below the ground.
Once you have determined the depth of the footing and have built or dug the necessary hole beneath the location of the 6x6 post, the next step is to determine the length of the 6x6 post. Using the 10x10 deck example, use a tape measure to measure the distance from the bottom of the footing to the underside of the deck framing. Subtract 7" from that number and cut/notch the post so that, when attached, it will float 7" from the bottom of the footing hole.
Attach a galvanized post base to the bottom of the post so that the concrete will form around the post base and secure the 6x6 in place. Before pouring the concrete footing, double-check the deck for level and square. Finally, pour the concrete up to the bottom of the post base.
Let the concrete harden for about 24 hours. Remove any wooden forms and the temporary supports used to prop up the deck.
For building code information related to deck building, check out the Prescriptive Residential Wood Deck Construction Guide by the American Wood Council.
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