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I've read that deck posts have to 4ft underground to get past the frost line. With a 6' x6 [ed: not sure if this mean to refer to 6x6" or something else?] which means you only have 2ft above ground.

What if the deck needs to be in the air 5ft -- would I have to get 10ft post?

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You should use concrete footers in concrete form tubes buried in the ground deep enough to meet the code in your area. ( below frost line ). You need to use the apropriate size for your soil and the weight of the deck you are building. The concrete footer would extend above the ground by a few inches, you would then use a Simpson post bracket anchored into the concrete to set your post on/in. This keeps the bottom of the post from sitting in any water that may accumulate on the top of the footer.

The height of the post will be the distance from the top of the bracket on the concrete footer to the bottom of the beam that caries the deck joists. You will need the determine that length once you you have the footers in place and you have the height of the deck joist and the dimensions of the beam you are using.

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Last time I built a deck in Alaska I buried concrete footings at each corner.

They were pyramid-shaped, and I buried them in compacted dirt so the brackets were right at ground level. Then all the wood was above ground level.

So no, you don't need 10 foot posts, nor do you want your structure to touch the frost layer lest it make it melt.

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