I am building a 12' X 18' foot deck with pressure treated lumber. The 18' will be along the house with 12' joists out from there. I can not find what is required as far as post size. The deck will be 12' high and I planned on using four 4"X6" posts under the beam for support. That will be a post every 6 feet. Will these be sufficient? 6"X6" posts cost so much more and I'm hoping I don't need them that big. Our local code does not require the larger size but I want to know if I need them structurally. Another idea is to use 6"X6" posts on the corners and 4"X6" post between them so two of each.

You should spend the money and use 6x6 posts. Don't skimp on structural issues. Especially 12' up in the air. 4x6 are not as structural as 6x6 post w/ that vertical length of 12'. They have a tendency to warp/bow on the 4" side. They are not intended to be used for vertical structural supports.

Look at it this way. What would you say to yourself if there were an accident caused by there not being enough support?

For 4 posts, the additional cost would be nominal, but, even if it doubled the cost, there is no price on the safety of my family.

Here is some info from decks.com:

It is not that expensive to upgrade to 6”x6” and the extra material allows for stronger connections at the top of the support to hold the beam in place. Taller wood supports have a tendency to bow or twist after the drying process if they are of the smaller sizes. We have seen a 4”x4” support posts bow as much as 6” out, causing the deck to sag 1”. If this has happened to your deck, the supports should be changed. The larger sizes tend to be more stable during the drying process.

Also, here is the link for some info on what heights are permitted for which size posts: http://www.decks.com/articles.aspx?articleid=29

Good Luck, Dan

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Hate to go againist the grain but it might help to know how high in the air your deck will be, 12" or 12'. Your code does tell you how far apart the posts can be and nearly all building codes are set to meet the worst case scenario. It never hurts to over build but if cost is a factor, just follow the code.

I am building a canopy over my deck. What size post should I use. It will be 8 feet total height and 18 feet x 12 feet. I will have a total of 8 post. I will use 9 2x6x12 for cross beams. It will also have a canopy over those for shade. I was planing on using 4x4 will these hold the weight?