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Old 10-16-2012, 10:29 AM   #1
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Setting posts for deck - Concrete Amount Question


I spoke to my local building inspector about setting deck pilings. I plan to use 8x8 square pilings and he said they must be 48" deep with a 24" diameter hole filled to grade with concrete. The finished floor of the deck would be about 4' 6" above grade. Doesn't this seem like overkill? I am in the coastal region of NC so the soil is somewhat sandy. I know whatever the building inspector says goes but I thought I would get some opinions. My concrete calculations came to:

.4 cubic yards/10.8 cubic feet per post-hole

1.6 cubic yards/43.2 cubic feet for 4 total post-holes

66 80lb. bags of concrete

I have read there is about 2/3 of a cubic foot per 80lb. bag. Is this an accurate estimate?

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Old 10-16-2012, 11:22 AM   #2
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Setting posts for deck - Concrete Amount Question


How big is the deck? Is it a free standing deck, or is one end fastened to a ledgerboard on the house?

I built my 16' x 16' with a ledgerboard and 3 4" x 4" posts.

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Old 10-16-2012, 12:28 PM   #3
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Setting posts for deck - Concrete Amount Question


Your sand in NE NC can be highly unreliable and variable, especially on your proximity to the coast. Mother Nature gives and takes sand in the coastal plain.

I lived in SE VA and when people described ocean front was highly variable depending on the timing since everything was considered to eventually be ocean front.

The elevations required are dictated the possible storm surge (worse than a hurricane wind).

Considering the depth required in the poor soil and the height above grade, the requirements seem to make sense.

People in the surge path of Katrina wish they had to protection of some decent local codes and code enforcement.

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Old 10-16-2012, 12:38 PM   #4
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Setting posts for deck - Concrete Amount Question


I would ask the building official did he mean a 24" diameter footing with a 10" round concrete tube foundation from the footing to the finish grade, something along this line http://www.bigfootsystems.com/?

Sometimes it can be easy to misunderstand construction. This is typically how they are done in my area in Massachusetts.

depth of footing is typically based upon several issues:

1. frost protection depth required by code
2. depth to undisturbed virgin soil
3. engineered structural fill in lieu of deep footings
4. closeness to existing dwelling (see #2)
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Last edited by GBrackins; 10-16-2012 at 12:40 PM.
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Old 10-18-2012, 10:39 AM   #5
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Setting posts for deck - Concrete Amount Question


Quote:
Originally Posted by MisterSteve View Post
My concrete calculations came to:

.4 cubic yards/10.8 cubic feet per post-hole

1.6 cubic yards/43.2 cubic feet for 4 total post-holes

66 80lb. bags of concrete

I have read there is about 2/3 of a cubic foot per 80lb. bag. Is this an accurate estimate?
Not to be that guy, but there is an error in your volume.

V=()r(L)
V=3.14 (1) (4)
V=12.56 ft per hole

Multiply your volume (in ft) by 1.5 to get the number of 80 pound bags.
I get 76 bags.
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Old 10-18-2012, 10:54 AM   #6
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Setting posts for deck - Concrete Amount Question


Quote:
Originally Posted by 47_47 View Post
Not to be that guy, but there is an error in your volume.

V=()r(L)
V=3.14 (1) (4)
V=12.56 ft per hole

Multiply your volume (in ft) by 1.5 to get the number of 80 pound bags.
I get 76 bags.
Ok That guy, you forgot to subtract the volume taken by the post itself.
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Old 10-18-2012, 11:36 AM   #7
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Setting posts for deck - Concrete Amount Question


Hand dug hole or are you using Sontubes? There is a world of difference between the vague amount needed when pouring against an over sized rough face in comparison to pouring inside a rigid tube.

I assume they do not require concrete in the above grade, since you are talking about wood posts.

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Old 10-18-2012, 11:50 AM   #8
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Setting posts for deck - Concrete Amount Question


Quote:
Originally Posted by Jackofall1 View Post
Ok That guy, you forgot to subtract the volume taken by the post itself.
Thank you Jack for pointing out my mistake. I over-looked the fact that the 8x8 are piers, not posts anchored to the concrete.
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Old 10-19-2012, 09:27 AM   #9
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Setting posts for deck - Concrete Amount Question


It will be a pain but I plan to dig the holes by hand. I dug a 90' trench 24" deep for a feeder wire to my garage in the spring so I'm thinking four holes will be a walk in the park compared to that. Mixing all this concrete I 'm not so sure of...

Would it be possible to pour and mix the concrete in the hole???
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Old 10-19-2012, 11:49 AM   #10
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Setting posts for deck - Concrete Amount Question


Just some rough numbers to put things in perspective:

66 bags of pre-mix @ about $4.00 per bag, plus delivery = About $300-$350.

2 yards of ready mix concrete delivered to site: 2 yards @ $120 a yard = $240
cartage/partial load charge of $50-100
Total for ready mix = $290-$340

If you calculate in incidentals like chiropractor visits and such, I think it's an easy decision..........
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Old 10-19-2012, 10:29 PM   #11
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Setting posts for deck - Concrete Amount Question


Quote:
Originally Posted by jomama45 View Post
Just some rough numbers to put things in perspective:

66 bags of pre-mix @ about $4.00 per bag, plus delivery = About $300-$350.

2 yards of ready mix concrete delivered to site: 2 yards @ $120 a yard = $240
cartage/partial load charge of $50-100
Total for ready mix = $290-$340

If you calculate in incidentals like chiropractor visits and such, I think it's an easy decision..........
We rarely use bag mix anymore, even when it's "cheaper" it's not, I'd rather be done pouring in 30 mins than have 1 or 2 guys mixing all day long, plus they're worn out afterwards. Call in a truck.

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