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I was going to build a free standing deck at the back of my house. The first set of footings would be 1'9" from the concrete basement/foundation for the house. The house is a new construction built one year ago.

reading this site: http://www.finehomebuilding.com/how-to/articles/top-10-deck-building-mistakes.aspx it says

To install deck piers properly, the piers must bear on undisturbed soil and, depending on the region, be set below the frost line. This could mean the need for 3-ft.-deep footings in some areas. However, if the piers are in a backfill region, as is the case with piers nearest the house on a freestanding deck, the footing depth may have to be as deep as 10 ft. to reach undisturbed soil and to comply with code.

I am now worried that these footings may be less stable. Our area requires 18in diameter footings, 30 inches deep.

Some people on this site suggested against using a ledger if possible. The deck is only 3'6" feet above the ground.

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What are the dimensions of your deck? Can you provide a picture? My initial thought is that your footings do not need to be that close to your house. –  DMoore Jun 21 '13 at 17:57
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Generally, I prefer a ledger. BUT: Local code requirements will almost certainly require a permit and inspection for a deck attached to the house via a ledger. Also consider deck blocks such as: ideas-for-deck-designs.com/image-files/deck-post-block-200.jpg –  Chris Cudmore Jun 21 '13 at 19:11
    
DMoore, The deck is 12ft by 21 ft. One footing will go near an inside corner so the most I can cantilever the beam off of it is 2 feet, I wanted a little wiggle room so I was going to do a little less. You can see a picture I posted in this question: diy.stackexchange.com/questions/28529/… –  Michael Jun 21 '13 at 19:29
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I am a fan of the ledger too. However I think that having two exterior walls might shift my mind on this. The short wall here is the most important since it has the door but your ledger would be on the longer wall - so I wouldn't do ledger. –  DMoore Jun 21 '13 at 22:01
    
But then what should I do about the footings being where they backfilled when the dug the basement? –  Michael Jun 24 '13 at 11:18
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2 Answers 2

It is hard to say how disturbed your soil was during the construction. Some companies dig out a ton and backfill after and some are pretty specific to the basement area.

Either way I would plan for the worst case scenario. I would personally go 6 feet. I would suggest 4 feet is the bare minimum. I would strongly strongly suggest using something like bigfoot deck footings. Your main issue is upheaval since the fresh ground will not be as dense. You need to make sure that the bottom is wider than the top no matter what kind of system that you use.

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I was going to do 18 in diameter footings, with 8 in diameter piers. Are you suggesting going 6 feet deep, or 6 feet from the house? –  Michael Jun 24 '13 at 11:19
    
Just six feet deep –  DMoore Jun 24 '13 at 14:37
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Contact the building inspector for your area. They will know whether the whole area around your house was dug and backfilled, or whether the ground is undisturbed. They will also know what is under the dirt and how far down you need to go. I live in a very rural area and my building inspector remembers a TON of details about my house to the extent of not needing to come out for a site inspection pre-build because he remembers from a previous visit. But even in the city there is documentation that the inspector can look up. They want you to be safe so they will help you do what you need to do.

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