An overlay puts new shingles over the old ones. It is cheaper, and in Florida it is usually the wrong choice.
You cannot inspect or repair the deck under an overlay, you cannot install a proper secondary water barrier, and you carry the old roof's weight and defects forward.
Florida limits how many layers can remain, and the condition of the existing covering affects whether an overlay is even permitted.
Layered roofs complicate claims and inspections, and buyers and their inspectors discount them.
O'Brien Contracting — Florida roofing contractor, license CCC1335947. Call (239) 919-4485.