Hi Dave K, I recently had to consider the same problem when I replaced our upstairs front porch. I think you might want to shed water better instead of putting something in that won't rot. On mine, I figured water could come in either (1)between the tongue and groove boards or (2)where the wall and the floor meet. I took the bottom two siding boards off the wall and had my sheet metal man make a (squared off) "J" shaped flashing where the bottom siding board would be replaced inside the bottom of the "J" filled with some good quality (paintable)silicon calk he had. This flashing was made of a heavier meatal than most flashing is made of.....I'm sorry I don't know what gauge it is. The calk was also run under the bottom of the "J" (flashing)where it meets the floor. It is sitting right on the porch floor. When I put the floor in I pulled each board up tight against the other with a small sledge and a piece of scrap. I put it down with two and a half inch galvanized screws (but I had a full one inch in treated flooring) and then put two thick coats of the oil based paint on, making sure to get in the cracks extra heavy. I also trimmed the bottom lip of the groove on each board about a half a kerf to make sure the top fit tightly. I also have light fixtures and ceiling fans and wainscoting and a stairway below. So far (about a year) I'm OK. I had conferred with several of the guys working on my house and this is what we thought was the best we could think of.