Walls – Attachment sites for sun sail on house

Pea gravel for the base of the hole makes sense...for drainage. On the sides, less so. I'd opt for crushed rock which will compact better when tamping. If tamped properly, you likely don't even need concrete.

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#