On Ground Deck Framing & Posts?


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Old 03-18-14, 05:19 PM
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On Ground Deck Framing & Posts?

Looking to build a small onground 10x10 freestanding deck adjacent to a semi-inground pool.
Pool is 24" above grade so I would make 4' wide steps on the deck frame.

I would like to use 2x6x10' pt for joists and rims and not use any beam/girders so its only about 8" above grade (5.5" joists + 5/4" pt decking + 1" vent space?)

How many buried footings/posts do I need?

Can I use deck blocks on ground instead of digging? (outside metro NYC)
Do they support as much load as 4x4 posts in concrete?
 
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Old 03-18-14, 06:27 PM
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You can use 2x6 for that size deck, but it is a minimum. I would use 2x8 for rim joists, and support it using 4x4's on the corners and midspan on the rims. You can't use dek blocks in your climate due to ground heave. You will need to dig and pour sonotube footings and install galvanized post bases to the concrete. The depth of your footings is determined by your location and frost line.
 
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Old 03-18-14, 09:15 PM
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In that part of NY, the frost line is 3'.
 
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Old 03-23-14, 02:19 AM
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Skimp now, and (definitely) pay later.
 
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Old 03-24-14, 10:41 AM
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This is all answered on the Dek Block website, but yes, you can do exactly what you want. 2x6s, no need for 2x8 rim joists, sonotubes, or any digging or pouring of concrete.

Just go to the site, and follow their plans and directions. You'll be building a completely floating deck that will move with the frost heave. The key is not to tie it to anything (obviously no ledger board, but don't attempt to tie it to the pool in any way).

I did this, using 2x4's instead (which the dek block website recommends against) and much shorter spans. The only tricky part is the excessive amount of leveling (the load is split between literally dozens of contact points, all of which have to be properly leveled and sloped). My deck was 20' x 12', with composite decking (Timbertech). 2x4 joists spaced every foot.

If you're only going to have the joists 1" above the ground, I'd order ground contact rated PT 2x6's from your local lumber supplier. I think mine charged me an extra 50 cents each, and I had to wait 6 days.
 
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Old 03-25-14, 05:11 AM
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Well, I guess I have been trumped! Good reading for all contemplating a deck build. http://www.awc.org/publications/dca/dca6/dca6-09.pdf

So good, in fact it is code. Imagine that.
 
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Old 03-25-14, 05:17 PM
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More info from Decks.com website: "A deck pier block is in many ways just a simplified version of a “precast foundation”, a foundation type recognized by building codes. They’re subject to all the same requirements as a typical footing, regardless of not being cast-in-place. They must have a sufficient bearing area (the area of the block that sits on the earth) and be a minimum of 12-inches below grade, or below the local frost depth. They cannot, however, be simply placed at grade level. "

Being 12" below grade nullifies the code specification for no wood to earth contact.
 
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Old 03-26-14, 01:22 PM
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This is an interesting point. The American Wood Council guide you linked is simply a deck construction guide based on the 2009 IRC. The IRC is the actual code.

Here is a link to the 2012 IRC:

International Residential Code for One- and Two-Family Dwellings

The applicable section:

R403.1.4.1 Frost protection.
Except where otherwise protected from frost, foundation walls, piers and other permanent supports of buildings and structures shall be protected from frost by one or more of the following methods:
1. Extended below the frost line specified in Table R301.2.(1);
2. Constructing in accordance with Section R403.3;
3. Constructing in accordance with ASCE 32; or
4. Erected on solid rock.

Exceptions:
1. Protection of freestanding accessory structures with an area of 600 square feet (56 m2) or less, of light-frame construction, with an eave height of 10 feet (3048 mm) or less shall not be required.
2. Protection of freestanding accessory structures with an area of 400 square feet (37 m2) or less, of other than light-frame construction, with an eave height of 10 feet (3048 mm) or less shall not be required.
3. Decks not supported by a dwelling need not be provided with footings that extend below the frost line.

So, per the bolding above, we know a free standing deck does not need footings to the frost line, at least according to IRC 2012.

The next question, per your latest post, is do they need to be 12" below grade?

We know that Dek Brands recommends against burying their blocks (as frost heave will simply push them up over time), and claims their product is approved in 49 of 50 states (no NJ):

https://deckplans.com/faqs/building-codes

Per the IRC (my bold)

R403.1 General.
All exterior walls shall be supported on continuous solid or fully grouted masonry or concrete footings, crushed stone footings, wood foundations, or other approved structural systems which shall be of sufficient design to accommodate all loads according to Section R301 and to transmit the resulting loads to the soil within the limitations as determined from the character of the soil

Basically, DekBrands will give you the structural calculations showing their design accomodates the appropriate loads, and they are claiming they are an approved structural system in 49 of 50 states.

Note, I have absolutely no affiliation with Dek Blocks. The only reason I used them was because the design of my deck necessitated having literally 20 different points where the load was being transferred to the ground, and that would be a lot of digging and sonotubes. I also was installing partially over an existing concrete pad that was already floating (4" depth), so the worst thing I could do would be to make part of the deck floating, and the other part anchored below the frost line.

To be honest, the extra labor of leveling all the blocks was a PITA. If I were to do it all again, I would take their advice and use 4x4 mini posts attached to the blocks, rather than have them supporting my joists directly.

We'll see if my deck holds up over the years. We have had a very severe winter in the mid-Atlantic with a lot of snow and many freeze thaw cycles, and no problems so far.
 
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Old 03-26-14, 03:16 PM
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The link I gave included all 2012 requirement changes, specifically for decks. Granted you are quoting from the IRC directly. You forgot, or omitted one sentence in the footing section that has a direct bearing on your theory.."Footings shall not bear on frozen soil unless the frozen condition is permanent." So if the soil thaws out, the footing shall not bear on it.

403.1 concerns walls, not decks.

One concern I would have with dek blocks is their proximity to frost heave. The soil nearest the house will heave at a lesser degree than it will 10 or 12 feet away from the structure. This will tend to make the structure lift at an angle, and it may not settle back to the flatness originally built.

I wish you luck on your deck, but suggest to the OP that they check with their local building department before they use them. Where we are located, we cannot even use sonotube footings if the location is within x feet from a precipice, which is nearly all our land, since we are in the mountains. We must pour 12" deep minimum x 24" wide continuous footings from one end of the deck to the other and all the posts must rest on that footing with proper post bases. Just a vast difference in the practices as prescribed by local codes.
 
 

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