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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 6x6? 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?

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  • A 12 foot post is a 12 foot long rigid lever: a device for multiplying the force applied to the bracket. A tensioned cable is a device for multiplying the load imposed by a downward force, eg, the downward component of a gust of wind. (ropebook.com/information/vector-forces). You might find it entertaining to work out just how much mechanical advantage your system will be applying to transform small downdrafts into a force that attempts to destroy the brackets.
    – Eric Lippert
    Apr 26, 2017 at 23:14

2 Answers 2

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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.

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    This crossed my mind as I was typing out my question. I guess I will have to rethink the way I planned on doing this. I will probably end up getting longer posts and sinking them in this case. Am I able to sink a pressure treated 4x4 with no sort of sealant on them?
    – Ronnie
    Apr 26, 2017 at 17:18
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    Yes. They'll last many years if you don't create a concrete pocket that holds water. Set a base of washed rock on which your posts will rest and pour the concrete around the posts.
    – isherwood
    Apr 26, 2017 at 17:22
  • thank you for reminding me I need to add concrete tubes to my list of things to buy
    – Ronnie
    Apr 26, 2017 at 17:31
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    Actually, you don't. They don't do much for you in this situation except create an awkward backfill scenario. Dig accurate holes and fill them up with concrete. An exception would be if you intend to have the footings protrude from the ground and actually need a form.
    – isherwood
    Apr 26, 2017 at 17:33
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    @Ronnie: One option might be to support your posts and shade sail the same way that ships support their masts and real sails -- with guy wires. With properly tensioned guy wires, there should be almost no net lateral load on the posts or their bases. Of course, you do need to properly anchor the guy wires, but that might still be easier than getting thicker and longer posts and sinking them deep into the ground.
    – Ilmari Karonen
    Apr 26, 2017 at 19:30
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I had this exact question regarding a 16x16 sun shade I am installing. I am planning on 8' above ground height and need 12' posts. I reviewed my project with a friend who is contractor, his response was that:

  • a 4x4 will handle the load
  • put 1/3 of the post in the ground (also cited in the other answer here)
  • a 4x6 or 6x6 will look better
    • 4x4s are more likely to warp and twist over time
    • 6x6 or 4x6 is a heavier looking post and most people find it looks better
  • a 4x6 or 6x6 gives you more meat to work with
    • room to make mistakes or move anchors and still have a strong post
    • you can also make decorative cuts or hang plants from the 6x6 and not give up necessary strength
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  • that is a good suggestion, thank you. I do think 6x6 would look much better but the cost goes up quite a bit. I will definitely consider this, plus I like the idea of being able to hang other things from it
    – Ronnie
    Apr 26, 2017 at 20:18
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    Glad I went with 6x6. They look so much better than 4x4s would have i.imgur.com/ZrY2T7q.jpg
    – Ronnie
    Jun 23, 2017 at 16:07

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