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Old 05-30-2020, 12:58 PM   #1
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Decks: concrete column size vs. footing size


Having finished the basement with some great help from all of you, I have been informed I will be building a deck next.

I have calculated the footing size based on my tributary loads and assuming maximum soil bearing capacity of 1500 psf. This calculation indicates 16" diameter footings will be sufficient. The footings will be 8" thick and 42" below grade per code.

But I am having trouble figuring out how large/small the concrete column rising from the footing should/can be. I can just pour a 16" column but to save concrete I would like to drop a cylindrical form into the hole and pour a smaller column if possible. The columns sitting on the piers will be 6 x 6 lumber. Is a 12" pier poured on top of the 16" footing acceptable? Or not? Intuitively this seems ok since I could just put the pressure treated column right on the footings (but that seems like a good way to guarantee I am going to be digging up rotten columns in about 15 years).

Thank you!
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Old 05-30-2020, 01:10 PM   #2
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Re: Decks: concrete column size vs. footing size


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Originally Posted by vaiek View Post
Having finished the basement with some great help from all of you, I have been informed I will be building a deck next.

I have calculated the footing size based on my tributary loads and assuming maximum soil bearing capacity of 1500 psf. This calculation indicates 16" diameter footings will be sufficient. The footings will be 8" thick and 42" below grade per code.

But I am having trouble figuring out how large/small the concrete column rising from the footing should/can be. I can just pour a 16" column but to save concrete I would like to drop a cylindrical form into the hole and pour a smaller column if possible. The columns sitting on the piers will be 6 x 6 lumber. Is a 12" pier poured on top of the 16" footing acceptable? Or not? Intuitively this seems ok since I could just put the pressure treated column right on the footings (but that seems like a good way to guarantee I am going to be digging up rotten columns in about 15 years).

Thank you!
Some places have rules on that but up here I can only remember one time that 12" was called for, our normal is 8" tube with rebar in the pier and a grid of rebar in the footing. But our footing can get anywhere from 24 to 36" and the rebar is to help spread the weight out.
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Old 05-30-2020, 05:12 PM   #3
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Re: Decks: concrete column size vs. footing size


If you can get a foundation grade 6x6, you could set it on the footing and it will outlast you.


But it would be perfectly fine to go down to even a 6" sonotube, though I would probably go with at least an 8" just because it gives you a little more fudge room. You can use a Bigfoot deck footing base if you want. Otherwise you would have to do two pours.



Of course you want rebar ells that go into the footing base and up through the sonotube.
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Old 05-30-2020, 06:23 PM   #4
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Re: Decks: concrete column size vs. footing size


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If you can get a foundation grade 6x6, you could set it on the footing and it will outlast you.


But it would be perfectly fine to go down to even a 6" sonotube, though I would probably go with at least an 8" just because it gives you a little more fudge room. You can use a Bigfoot deck footing base if you want. Otherwise you would have to do two pours.



Of course you want rebar ells that go into the footing base and up through the sonotube.
Thank you! So if I use a big foot form and do it as a single pour do I still need the rebar? IRC would indicate "no", but what would you do?
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Old 05-30-2020, 07:33 PM   #5
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Re: Decks: concrete column size vs. footing size


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Thank you! So if I use a big foot form and do it as a single pour do I still need the rebar? IRC would indicate "no", but what would you do?

We are required to use rebar. Makes sense, and it's very cheap insurance. I see I was wrong about 6".
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Old 05-30-2020, 07:49 PM   #6
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Re: Decks: concrete column size vs. footing size


we just build a 2x4 grid to hold the tube up 8" off the bottom and always use rebar.
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Old 05-30-2020, 08:54 PM   #7
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Re: Decks: concrete column size vs. footing size


If you are in an area that enforces a building code, ask them.
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Old 05-31-2020, 11:56 AM   #8
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Re: Decks: concrete column size vs. footing size


Thank you to all for the very informative words and illustrations. 8" pier with rebar it shall be! (And yes, I'll double check with the inspector when he comes to sign off on my footing holes--but the less I look and sound like an idiot the better! ). Great tip on the 2x4 grid to suspend the tube also.

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Old 06-01-2020, 07:05 AM   #9
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Re: Decks: concrete column size vs. footing size


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we just build a 2x4 grid to hold the tube up 8" off the bottom and always use rebar.
Stupid question, maybe, but what keeps the concrete from escaping from the sonotube when it's suspended like that? Do you wait to let the 8 inch footer cure a bit?
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Old 06-01-2020, 07:21 AM   #10
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Re: Decks: concrete column size vs. footing size


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Stupid question, maybe, but what keeps the concrete from escaping from the sonotube when it's suspended like that? Do you wait to let the 8 inch footer cure a bit?
Once you have filled to 8 inches or a little more so the bottom of the tube is locked in place, back fill some to hold the concrete down.

Or if you have a bunch to do do all the footings first and they will set up a little.
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Old 06-01-2020, 10:04 AM   #11
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Re: Decks: concrete column size vs. footing size


AWC calls out for 12" stem. AHJ confirmation.
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Old 06-02-2020, 05:24 AM   #12
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Re: Decks: concrete column size vs. footing size


You have done calculation but I'm still wondering what kind of deck would want 16" footers. Are you sure about it? Your soil that bad that deep, example? I'm in nj and usually 12" sonotube is fine as a footing. Also, since code is minimum required, you may benefit from going to 48" and fill with gravel for drainage and start the footing at 42.
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Old 06-02-2020, 05:43 AM   #13
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Re: Decks: concrete column size vs. footing size


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You have done calculation but I'm still wondering what kind of deck would want 16" footers. Are you sure about it? Your soil that bad that deep, example? I'm in nj and usually 12" sonotube is fine as a footing. Also, since code is minimum required, you may benefit from going to 48" and fill with gravel for drainage and start the footing at 42.
Gravel for drainage, where does it drain to. and are you trying to keep concrete dry?
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Old 06-02-2020, 06:33 AM   #14
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Re: Decks: concrete column size vs. footing size


good one
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Old 06-02-2020, 08:03 AM   #15
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Re: Decks: concrete column size vs. footing size


Unless your soil conditions are terrible why would you need rebar.?

Why would you need sonatube.?

In my neck of the woods the footer has to be 3 x the width of the post/leg of a residential deck. Example 6" x 6" post times 3 = 18" diameter footing.
No rebar or sonatube required.

Sonatube is a waste of money and time.
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