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I'm trying to figure out the best mounting layout for installing 20x16 sail shade over my patio. My plan is to anchor 2 corners to roof rafters, and 2 other to trees. I'm worried about load from winds, and how would all of this hold up to that. Mostly my concern is that trees would be flexing in the wind. I'm planning to anchor at ~10' off the ground to a 10-15" diameter tree. I don't like to use posts (that would not have this problem) since they would end up in the middle of the grass. The trees are ~35' from the corner of the shade. To install post by the tree, I'd need to bury it 5' deep (for 10' above grade), which would be quite a fit.

So 3 questions I'm trying to address:

  1. How should I go about calculating loads I should be expecting, and what kind of load can my roof take before damage occurs?

  2. As an idea to solving roof damage, would you recommend using some sort of fail-safe mechanism that would break and drop the shade in case winds are too strong before doing damage to a roof? If so, what would that mechanism be, and how to properly size it?

  3. Is there a way to estimate tree trunk flex in strong winds, other than observing when such an event occurs?

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You could probably calculate the loads starting with some ideal lift calculations [1]. I would bet that these numbers would be surprisingly high and that your design is driven by how much force you are willing to let the rafer see.

The direction is also important to, but something flapping in the wind will likely be pulling in a wide range of direction.

Here's a few things I'm thinking:

  1. Add a system where you can easily drop/furl the sail shade. This can be as simple as, on any corner you cannot reach easily, add a block instead of a hard point and send the corner line thru that block and into a cleat. When it's not in use, you can take it down easily.

  2. If you want to design in a weak point, perhaps it could be done with rope, making the corner which is tied to the roof rafter, a small diameter with a breaking strength just under whatever you determine is correct. [2]

  3. Maybe another way to add a weak point would be to use a rectangular ring that has a split in the center.

Have you considered what material you have on the shade fabric? Something mesh would still shade you but not become a wing/sail.

Another benefit of having an easy way of taking the shade down is cleaning!

[1] https://en.wikipedia.org/wiki/Forces_on_sails

[2] http://www.neropes.com/new-england-ropes/sail-making-products/braided-luffline.html#

  • Using a rope as a fail safe was one of my options. Taking it down after each use would be cumbersome, but worthy of a plan B. I'll work in this direction, and see what first trial really shows. – Serge May 3 '17 at 2:49
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It's not really clear what you're asking, but 3/8" stainless eye bolts into your rafters and springs inline with your tree lines should do. The springs should be stout enough that they don't stretch except in severe weather and act as a safety measure for extreme tree movement.

  • Thanks for the answer! I restructured the question to hopefully be more clear. The spring suggestion - how would one size it? If the spring just flexes, then sail might flap, and produce stronger dynamic loads (according to other sources I looked at). Is there a standard mechanism that releases when loaded over certain threshold (and is re-settable)? – Serge May 1 '17 at 16:25
  • I agree, springs would help lower the jerk, but not the overall forces here, and In fact, you might be creating more lift as the fabric gets more camber due to stretch. – InstantHouse May 1 '17 at 20:21
  • You can use shear bolts, but they're not exactly "resettable". They snap when their shear force is exceeded and need to be replaced. Think of them as "fuses" for mechanical loads instead of electrical. – Chris M. May 1 '17 at 20:25
  • @ChrisM. this is exactly the direction I was thinking. But looks like bolt will not work with more or less standard mounting. Are there any other "fuse-like" solutions? Like a carabiner or a ring that will break after a certain threshold. – Serge May 2 '17 at 1:50
  • @Serge I can't figure out what they're called, but at work we use what amounts to a double-shear clevis pin rope coupling (fork on one end, loop on the other, joined by a pin through the middle) that uses a shear pin (same idea as a shear bolt). Essentially, once the tension in the line exceeds a certain amount the pin snaps and releases all tension. – Chris M. May 2 '17 at 14:14
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I found what I mentioned in the comments on isherwood's answer. They're called "cable pulling line swivels", used for power lines and other cables to prevent over-tensioning the line when pulling. The two halves of the swivel are joined by a shear pin held in place by a retaining ring. When the line tension exceeds the shear pin's rated load, it breaks, preventing the line from being overstretched.

They do seem to be kind of expensive, though. I imagine they're not really intended for purchase by consumers.

More information on one of the brands available here. You'll need to find a distributor near you for pricing, though.

  • Good stuff, but at $200 a pop is out of the budget :D – Serge May 3 '17 at 2:47

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