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I noticed some guidelines are mandating 6x6 posts regardless of deck height, etc. Ex: "Prescriptive Residential Wood Deck Description Guide based on the International Residential Code". What gives? IMO 4x4 is usually sufficient.

I'm a newbie here. Started learning the ropes to Dad's business earlier this year as he claims he's going to retire one day.
 

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Super Moderator
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We only use 4x4's when the deck is near grade. Any post more than 8 feet long we use 4x6 or 6x6. 4x4's are subject to some serious bending as they dry. On my own deck, I had to replace a 6x6 after a year, when it bowed almost 1 1/2 inches
 

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Sean
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Welcome to CT foosman, how about an Intro in the proper area & adding your location to your profile?

The codes change due to changing conditions - the wood we get now is no where as good as the wood we used to get 10 to 20 years ago, etc... I personally won't use 4x4's unless the deck is within 3' of the ground & even then I seriously mull it over

Also as an FYI - the codes are the minimums, most of us don't build that way here
 

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We just finished replacing a deck that was 5' off the ground. It was built with 4x4's

Steel roof, the snow came off the roof, totaly took out the deck, and snapped the 4x4's like toothpicks.

We always use 6x6, unless it is close to the ground.

FYI in Ontario, Canada, we now are required to built decks to the same structural code as a house. There is actually a seperate code just for decks. And to snow load of course.
 

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I use 6 x 6's for decks even at grade level. Not that a 4 x 4 can't support a suficient amount of weight, but.....

I also like 6x6's. Mst of the time it wouldn't be cost prohibitive and it allows more room for notching to seat rim joists, headers, etc
 

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The Deck Guy
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We almost never use 4x4 (even low to the ground). Just dont seem right. Also, we bury our posts and I like a 6x6 in the ground. Imagine if a 4x4 rots any at all.
 

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Last time I built a deck here in BC, the inspector wanted a full two inch bearing on any beam that sat on a post. So if you have to splice the beam you're into 6x6.
 

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Number cruncher
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On the west coast, we don't notch anything. Beams bear full on posts, with appropriate hardware for the connection. I generally will use 4x4 posts, if they are less then 6' tall, 4x6 or 6x6 when taller.
Yeah, I've been wondering about that...that is notching for a beam. The pic above is for a rim joist so it'd be ok on the north west coast (BC). Wondering which would be better for beam on post; a T strap with bolts or notch cut with bolts, assuming 3-2x10 beam on 6x6 post. I do know code specifies that a beam must seat at least 90mm (3.5").

To answer the original question, 6x6 is used up here...4x4 is used only for railing posts or WHY. One thing the BI note is that deck wood is usually pressure treated and as such isn't graded like untreated dimensional lumber for homes, so they want to see a deck overbuilt.
 

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topsail's trimcat
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in my region the building authority wont accept 4x4's for supporting decks only 6x6's. the only time 4x4's are allowed is for handrail posts.

generally we will set the 4x4's at a height where they are about 2 or 3 inches down into the ground, not act as bearing but so we have nailing for the framework of lattice or fence board backers. then of course 36" or 42" up from the deck depending on the height,

we have our beams sit on 6x6's or we do 1 ply let into the 6x6's if theres a porch roof which needs to be supported then the roof beam sits on top of the 6x6
 
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