On a tile roof, the tile is the armor and the underlayment is the actual waterproofing. Most tile roof leaks in Florida are underlayment failures, not tile failures — which is why lift-and-relay work is so common on 20-plus year old tile roofs.
Because the underlayment beneath it has aged out. That is the waterproof layer, and it has a shorter life than the tile.
Often yes, if it is sound. Reusing tile lowers cost and preserves the original look of the home.
O'Brien Contracting — Florida roofing contractor, license CCC1335947. Call (239) 919-4485.