How to jack up a deck that's already 6 feet high??

Same friend who is getting a new roof, I looked at her home.
She has a deck held up by 6x6 posts. There is a 10 or 12 foot span that sags and needs another post in the middle.
I could use a 6"x6"x8', but I'm not sure how to keep it from going further into the ground in a few years, or even tomorrow. I could pour redi-crete around the base, but I don't see how that would do more than slow its sinking. (I would use a jackpost to lift up the deck to put the new post in.)
Or I could use a 6 to 8 foot jackpost, and build a 6x6 box around it. Then if it sagged, say the post sank into the ground, someone could just open up one side of the box and make the post longer. The box would still be ok at its original length. Because of the grain and warping, I don't think this box could look exactly like the other posts.
Unneeded? details: This time I'd actually do some work and she offered to pay me. :)
Her deck was repaired a few years ago, and the repaired part is the part that is already sagging. This part is about 3 feet wide, 6 feet off the ground, and 15 feet long, with 6x6's holding it up maybe every 5 or 6 feet except one part 10 or 12 feet long with no post.
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I know a guy who can jack up any project...

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It ain't a fencepost. On a deck, wood and dirt should never touch. You need a proper footer, sized for the frostline in your area, and the weight that will be supported. Same store that sells deck parts will have the plastic cone things to form up footers. Just bury them as indicated, fill with concrete, and with whatever bolts local code calls for to hold the metal or plastic spacer plate the post actually sits on. Same concept as a carport post or centerline beam post in a basement. If the existing posts are in the dirt, you may as well fix them too, while you have the jack there and are mixing concrete. You can probably saw them off and reuse them, so the cost will mainly be time.
Might I recommend one of they DIY deck books at the big box, or Google for a deck building web site? The pictures will show what I am talking about much more clearly.
aem sends...
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Correct advice. You can safely block up the deck using appropriate sided supports and remove the post to dig the hole for the footers, then replace it after it is cured. I extended the patio under my 12 x 16 deck and put temporary supports while I removed the four on the outer perimeter.
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mm wrote:

Im pretty tired at the moment, but I've built a few decks. Just wanted to throw my 2 cents on this.
I read some of the replies. about keeping the structure beefy.
Far as the design etc...I just drive down the road, when I see a deck I like I stop and notice how they set the band, the posts etc.
That was more like 3 cents worth.
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wannabe wrote:

Jacks:
Any any any kind of jack will work. It can even be a 6" bottle jack from the car or truck.
make a T out of some 10 footers, or whatever you need.
sturdy up the jack, set a block on, set the long ass T on the block.
crank away! you can lift much with a T like that...
I replace 8 columns on a super old home, my T's were about 16 foot long ea. with two jacks, I worked my way down the powch!
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You got some good advice on how to correctly anchor the base of the posts in concrete. Re jacking the sagged part, I just use a 20 dollar hydraulic bottle jack from home depot to raise a really heavy elevated deck, by the main crossbeam. Used a 4x8 x 4 ft long, laid on its side for a base, put the bottle jack on it, then cut a 4x4 PT fir to about 6 feet long, and attached 4x4 sheet galv caps on each end, so wood would not split under compression.. Cranked it up with ease.
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Thanks to all of you for the help.
I told her to get a price from the roofer, who I think does this kind of thing too, and if I can beat the price by much without feeling used, I'll do it. As you suggest.
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