I'm trying to figure out the best mounting layout for installing 20×16 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:
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How should I go about calculating loads I should be expecting, and what kind of load can my roof take before damage occurs?
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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?
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Is there a way to estimate tree trunk flex in strong winds, other than observing when such an event occurs?
Best Answer
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:
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.
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]
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#