Footing diameter v. footing thickness

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Old 08-19-19, 07:21 AM
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Footing diameter v. footing thickness

I do not understand the difference between the terms "Round Footing Diameter" and "Footing Thickness" in the Prescriptive Residential Wood Deck Construction Guide, Figure 4, page 12.

For my purposes, I am building a 12 x 12 deck, attached by ledger, about 3 feet above ground level. The Guide would prescribe a Round Footing Diameter of 21" and a Footing Thickness of 8" (3 footings; beam span 6').

Would be appreciative for an explanation of what these terms mean. Thanks!
 
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Old 08-19-19, 08:08 AM
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The footer thickness is measurement from the bottom of the footer to the top of the concrete.

Square footer versus round is self explanatory, one use a square form while the other is round [often using a sonotube]
 
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Old 08-19-19, 09:04 AM
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OK - thanks marksr. Want to make sure i understand correctly: If my footer is in the ground 42" (to get below frost line) and it is 4" above ground then the footing thickness for this is 46", yes?

So the Prescriptive Residential Wood Deck Construction Guide says i must have footings either 21" diameter or 18"x18" square, and in my case, a thickness of 46" for each of three footings. Sure seems like overkill and a lot of concrete. I assume you can get sonotubes in that size?

I attached Figure 12 from the Guide, "Typical Footing Options". Does their second from the left option mean i can put a 21" footing at the bottom only and a smaller diameter to above grade?
 
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Old 08-19-19, 10:33 AM
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Your footer doesn't have to be 46" thick, just the 8" or whatever local code requires. You could set the posts on top of the footer [below grade] but most of us feel that isn't a great idea. Even though the PT post is rated for ground contact it will be survive better/longer above ground. You can have a narrower 'footer' above the prescribed footer to get it above grade. I've never personally used sonotubes but have been on jobs where they were used. I believe they come in different lengths and diameters.
 
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Old 08-19-19, 11:34 AM
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Thanks again marksr. I am slowly getting there. Below is my attached drawing assuming hole depth of 42", footing width (diameter) of 21" and footing thickness of 8". Is this what is intended by the Guide?
 
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Old 08-19-19, 11:37 AM
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I think so, but remember your local inspector has the final say.
 
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