Tongue and groove is a technique used to secure lumber, on floors, outside walls and paneling. It employs a tongue or protrusion on one edge of a board that fits into a groove or slot on an adjoining board. Each board has a tongue side and a groove side. Boards are nailed or fastened through the tongue, to conceal the nails. It gives an even, smooth finish without any nail heads showing. But removing it can be a bit of a challenge, since there are no visible nails to pull.
Locate a starting place. The first step in removing tongue and groove material, whether it is on a floor or a wall, is to find a starting place. Basically, this involves destroying one board. If the porch or other floor or the siding is rotted, that may give you a starting point. But you have to remove one whole board first to gain access to other boards. On a floor, start at a wall or porch end. On a wall, start at the bottom for horizontal siding. Sometimes you can use a circular saw---make two cuts the length of the board, parallel to the edges and in the center of the board. Remove that cut-out strip to get access to both tongue and groove edges of that board. You also can use a hammer and chisel to cut out that first board.
Remove the cut strip, if you used a saw. If you used a chisel, pull out all broken wood. Gain access to both edges of that first piece. Pull out the groove side, which should come away easily. Then, pull away the tongue side any way you can---sometimes you can grab the wood with locking pliers and just pull the tongue away, or you may want to use your pry bar to pry it away. Then, pull out any remaining nails.
Move to the next board. Use your pry bar to lift it up, then pull the groove away from the adjoining board. It should come away easily, since it is not nailed to that board. Then, use your pry bar and nail puller to loosen the nails in the next tongue edge. Pull the nails, then remove the board. Continue this way until you have removed all damaged boards or all that you want to replace.