I have a 22'x16' shade sail like the one found here.
I have 4x4x12' (pressure treated) posts that I wanted to use for each corner. I will be pouring cement and using simpson post base brackets and bolting the post to the bracket (I am not sinking the post in cement).
My question is, will the tension on the sail pull the 4x4s inward too much or should I go with 6×6? To get the sail to not sag, you use turn buckles to tighten the distance between the sail and post. I know this heavily depends on how tight I make the sail. Any insight on this?
Best Answer
It isn't a question of post size. Well, it is, but you have bigger concerns: The brackets will not serve your purpose. They're mostly designed to anchor the base of the post, and to keep it off the concrete. They don't provide nearly enough stability to keep a post standing under load.
Your posts must be embedded a substantial distance into the concrete. 1:3 is a good rule of thumb for anything carrying lateral loads. (Heck, the concrete does nothing at all for you in your scenario.) Or they must be supported with rigid crossbars or diagonals of adequate length.
If your posts will actually be 12' above ground, and they're not supported with any sort of diagonal bracing, you will want 6x6s (or at least 4x6s). 4x4s will flex and bounce substantially at that height.