I am putting a shade sail up over a concrete slab that buts up against the back of the house. I am going to attach the sail cloth to the house (brick), and then put up 2 posts for the anchor points away from the house. I'm wondering if I can just put up 4X4 posts and anchor them to the concrete slab, or whether I need to install the posts by digging the holes just beyond the slab and bury them in concrete? Can the posts being mounted on top of the slab hold up to the tension or is it safer to go with the dig post holes and bury them? Thanks for your help!
1: Can the posts being mounted on top of the slab hold up to the tension ?
Since you are only talking about placing the 4 x 4's above the slab with no other means of support other than the mounting to the slab - your shade sail will eventually fall if not right away.
2: Is it safer to go with the dig post holes and bury them?
Digging your post holes into the ground and bury the poles will work you should dig at least 1 to 2 feet.
Additional Suggestion and Info:
Given you might be using wood, I will make a suggestion wood has many flaws in the outdoors under the sun - if you are in Florida you will notice how rapidly a pretty wood fence turns into an ugly repair project. I will suggest engineered plastic 4 x 4's not only can you bury the 4 x 4 in the ground, but also pour concrete down the hollow center giving you a very stable post. The plastic engineered 4 x 4 will last in the wind, the heat, the humidity and the rain.
How do I know ?
I have made a single pane privacy fence for a home in Florida. The posts have endured 3 Hurricanes and 19 years of grueling summer heat and high humidity. The posts still look good - almost like new and are holding just fine!