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In brief - I want to use a 2x4 to resist the tension put on trellis posts ~16ft apart, rather than the other two options.

I'm building a grape trellis with upright 4x4s connected by high tensile wire. As with any wire setup, like a long wire fence, you usually need to brace the ends to avoid leaning. I'm wondering if I can run a 16ft 2x4 between the posts at the top, using brace pins to attach to the posts. This 2x4 wont really hold any load, as that will be held by the high tensile wire. The 2x4 is only there to resist the tension force that the wire puts on the main 4x4 posts, turning it into a compressive force on the 2x4.

Can that 2x4x16 even make that span given it's own load? And if so, is that compressive force (it should be straight and level) going to force it to sag?

FWIW this is really to avoid the other two options - 1) angle the posts at 60deg so the tension forces the posts into the ground, or 2) build large H braces on the ends.

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  • The real question should be your...foundation of those 4X4's. Are they set in concrete, just dirt, are they pressure treated or did you use concrete piers with post anchors? How did you secure these posts in the ground. Would it be tough to add another 4X4 to make 8' spans instead? That is what I would do, bag the 2x4's and use just the wire to train your vines. Use guy wires to secure the posts. 16 feet is way too long of a span. 2X4 will sag and the wire will pull the posts towards each other. 8 feet span with guy wires should do the trick. Tell us how you sunk those post...thanks!
    – stormy
    Commented Nov 13, 2017 at 22:26

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Welcome to the forum. If I understand what you're asking, I believe a 16' 2x4 could work. The 2" side should be facing up to minimise warping (but you should probably expect some). You might consider a 2x6 to give you even more strength.

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  • Gotcha. Yea I definitely figured it would need to hang in the stronger orientation. And I figured a 2x4 that long would eventually warp a small amount, but as long as it doesn't sag enough for the tension to break the board, then I'm hoping it will hold up.
    – The Shoe Shiner
    Commented Nov 13, 2017 at 14:19

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