Florida now requires that any reroof job have either a self-adhering underlayment OR the roof deck seams taped with a self-adhering flashing that meets ASTM D 1970.
To be Florida Building Code approved, any self-adhering underlayment membrane MUST pass ASTM D 1970 which includes a nail sealability test. This means that the membrane will seal around all roofing nails and prevent the roof from leaking should the main roof material get blown off in a storm.
As mentioned previously, even though the membranes are FBC approved, they don’t all perform to the same levels. Some are stickier than others.
The least expensive method of doing a reroof to meet the new code is to tape the deck seams and then put on a layer of felt. When you reroof, the felt is removed, the tape left in place, and a new layer of felt added.
If you use a full-roof self-adhered underlayment, then it is best left in place during a reroof. You can either roof directly over the old underlayment or put a layer of felt down then reroof.
Granule surface underlayments are the least expensive, but they often don’t seal well at the edges and overlaps which can allow leakage unless the main roof is installed immediately or unless those overlaps are sealed.
A film surface underlayment will normally seal to itself at the overlaps and can easily be patched with another layer of self-adhered membrane. Make sure there is a textured surface on the film so that you have some traction when working on the roof. Although the film surface underlayments are slightly more expensive, they are much easier to work with and provide a more watertight roofing surface. You can even put an additional layer of self-adhered underlayment on top of a film surface underlayment when doing a reroof.
There are also fabric surface underlayments used for metal and tile roofs. These perform similarly to the film surface products but offer better traction for stacking tiles and for installing with adhesive foam systems. Again, with a reroof, you can either put the new roof over this underlayment or add a new layer of self-adhere underlayment or felt if you feel it is needed.
As a disclaimer, I work for MFM Building Products and we manufacturer an entire line of self-adhering roofing products.