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I'm building an elevated kid's play house. The raised platform will be 6' - 8' off the ground. The platform will be 8' x 10', the house will take up half of the platform and maybe less. I'm planning on using 48" 2x6 braces at each corner angled 45 degrees from the platform to the support posts. Can I use 4x4 posts in each corner to support this structure? Do I need more than 4 posts? Should I be using 6x6 posts?

The current plan calls for using 2 x 10s for the joists. The end joists will be 96" 2x10s. These will be spanned by seven side + common joists which will be 117" 2x10s spaced 16" apart. I've been reviewing load tables for the joists and may reduce those to 2x8s if possible. However, I haven't been able to find similar information for the support post requirements.

EDIT 12/9/15 Platform size increased to 10' x 12', with beams running the 10' span and joists resting on the beams the 12' (technically the joist span 11' 9"). All the decking code I've seen supports using 2x8s for the 12' joists (pressure treated #1 SYP). Thanks for the answers and comments.

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  • The concerns here are 1) bending forces due to wind, and 2) ground-level rot. For those reasons I'd be likely to go 6x6 unless you don't have those concerns in your case. 4x4s are adequate to support the weight in a static scenario. I'd also use 2x8 joists. You may have a tiny bit of bounce, but load isn't a concern there, either. – isherwood Dec 2 '15 at 15:08
  • The "rim joists" or "beams" should be 2x10, though, or possibly doubled 2x8s. Obviously they carry far more weight than any individual joist. – isherwood Dec 2 '15 at 15:10
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Decks made for people are to be supported by 6x6's. That being said, my 4' x 8' childhood treehouse was supported by 4x4's, until we tore it down 30 years later, still in serviceable condition.

I'd be more concerned that you do the footings right, then if you skimped out and used 4x4's.

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