I would like to repair a wooden fence, in Florida, similar to the one in the photo below:
Is there a rule of thumb (ratio) for the amount of post exposed to the amount buried underground?
Home Depot has the option for 8,10 & 12 foot posts.
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I always have used 2' underground so use an 8' post for a 6' fence. I don't know if this would be adequate for a taller fence, but it would probably do at least for an 8' fence.
Once while replacing termite eaten cedar 4x4" posts I encountered a soft limestone 14" or so below ground. I dug 4" or 6" deep into the limestone with a steel digging bar and stopped. I put in the post and then cut 6" or 4" off the top.
You also have the option if pouring concrete footer with galvanized bolts, then use a galvanized base to keep the posts up off the ground. When a post needs to be replaced, unbolt it and bolt another one in. Code in Kentucky requires this method for porch posts if they are mounted to a concrete slab.
This also depends on if you have a frost line. If you do, as in NJ, then your 6' post needs 3' underground for a total of 9'. Trim the excess from the top to level the post. Also, no need for concrete. It just hastens rot due to trapped water. Gravel allows drainage and tight fit.
The spruce rails will rot (if untreated) faster than the posts or pickets.