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Old 04-15-2012, 11:49 AM   #1
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Floating Deck Posts in Overdig


In my area, I can build a deck less than 24" from grade if it is not attached to the house without a permit. However, I have a few concerns with a floating deck. My concerns are basically with the posts close to the house.

Without doing a large cantilever, I need to set posts in the foundation overdig. Im sure once I start digging the gravel will keep filling in on itself making it impossible to get a 42" deep footing hole. How do people accomplish this?

Also, is the gravel in the overdig a good base for a post footing? I thought it needed to be undisturbed soil?


Last edited by ordonyez; 04-15-2012 at 12:15 PM.
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Old 04-17-2012, 08:56 AM   #2
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Floating Deck Posts in Overdig


There are a few things about your post that puzzle me. You say you don't need a permit if the deck is less than 24 inches above grade. Then you say you want to build a floating deck. Then you have questions about installing support posts. Maybe you are using the term floating in an unusual way. A floating slab is one that is directly supported on the soil. If you were to build a floating deck, I assume it would be directly supported on the soil, not that this would be a good idea, I simply am confused about what you are attempting to do and why.

As to installing posts in gravel, for a deck you would install a concrete footer, typically a sonotube. The required dimensions are generally in the code, but you say you don't need a permit, so I don't know if you plan to follow code anyway. Regardless, the gravel will hold up a 42 inch deep concrete footer fine, as long as the unacceptable soil was removed during installation of the house.

As to how to dig in gravel so the hole does not collapse, it can be done with an auger inside a metal sleeve. You fabricate a metal sleeve just a little larger than the diameter of the auger, duct metal works well. You advance the auger into the ground through the sleeve, and push the sleeve as you go. In the end, you fill the metal sleeve with concrete, you have a perfect form. The cardboard sonotube does not work well because it is too soft to push into the ground as you advance the auger.

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Old 04-18-2012, 09:55 PM   #3
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Floating Deck Posts in Overdig


By floating deck I mean a deck not attached to the house by a ledger board. It is a deck supported completly by posts. This is why the posts would need to be placed in teh foundation overdig because teh overdig for the foundation is 3 ft from the foundation wall and that is to far to cantilever.

Typically when I set posts I dont use sonotubes. I will auger a 48" deep hole. Put a 6" footing at the base of the hole, put the post in its desired location and fill the rest of the hole with concrete. That is why I am worried about the gravel in the overdig. Although I dont think a sonotube would be much easier.

I see your point with the forms in teh gravel.
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Old 04-18-2012, 10:03 PM   #4
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Floating Deck Posts in Overdig


It's called a free standing deck.
Why does the post need to be that far away from the house. It only needs to clear the footings, and that should only be a few inches away from the house.
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Old 04-19-2012, 12:08 PM   #5
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Floating Deck Posts in Overdig


It dosnt need to be that far away, either way I am in the overdig of the foundation. Does anyone know what Im talking about? try augering a 4ft deep hole a foot away from the foundation and see how far you get.
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Old 04-19-2012, 02:30 PM   #6
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Floating Deck Posts in Overdig


ordenyz -
Are you sure that you have gravel in the overdig or do you have something better that could have been compacted or settle to provide better support.? Gravel is not always good in the long term.

joecaptain -
The OP is not in Virginia and needs to go deep since a "free-standing" deck is best to avoid the many problems with ledger attachment and drilling 42" deep is not as easy as it is in VA. It is probably a basement that may be dug to 6' or 8' deep and the over dig width is much greater that a typical VA foundation. The width of the excavation is usually half the strip footing width (24"/2 = 12") plus the width for drain tile and increase depending on the natural angle of the native soils.

2' is a common cantilever amount, but deeper framing members will allow more cantilever.

Often, if native soil in not available, it is cheaper to auger a larger Sonotube in and not use a separate footing since this opens up the option of ready-mix for concrete to make it quick all at once and for all supports (depends on deck size) and avoid the hidden problems, time and costs of mixing on site.

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Old 04-19-2012, 04:36 PM   #7
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Floating Deck Posts in Overdig


Maybe I'm missing something, but if we're talking about a free-standing deck that is less than 24" above grade, why are we talking about needing to put footers below the frost line? Why can't they be shallow footings? Sure, frost heave will move the deck up and down a bit, but it's not attached to the house.
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Old 04-19-2012, 08:07 PM   #8
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Floating Deck Posts in Overdig


Quote:
Originally Posted by cortell View Post
Maybe I'm missing something, but if we're talking about a free-standing deck that is less than 24" above grade, why are we talking about needing to put footers below the frost line? Why can't they be shallow footings? Sure, frost heave will move the deck up and down a bit, but it's not attached to the house.
If it moves a little bit here and there, couldnt that mean lots of loosened nails, hardware squeek squeek squeek.
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Old 04-19-2012, 09:45 PM   #9
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Floating Deck Posts in Overdig


Quote:
Originally Posted by cablerailing View Post
If it moves a little bit here and there, couldnt that mean lots of loosened nails, hardware squeek squeek squeek.
It depends on how much your soil heaves, and how uniform it is. You can mitigate the situation by using the maximum spans possible. Think about a worst case scenario. Say one footing rises by five inches and another only one inch. If those footings are only 4' apart, that's bad. If they're 12' apart, the stress on the fasteners is much less. If you use beams that are three 2x12s sitting on 8x8 posts, and 2x12 joists, you can greatly minimize the number of footings and get those spans pretty large. Sure, you're going to experience a lot of cross-grain shrinkage, but it'll be consistent, and harmless (no drywall to crack, e.g.,).

Anyway, it's just a thought. Drilling 48" through gravel doesn't seem like fun. Just throwing out a possible alternative.

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