OK, here's my solution - and no, mine won't go out in the rain either! I have a covered porch, concrete, but neither wanted to "go" on the concrete, so I bought some cheap outdoor carpet samples at home depot. I put them down at the end of the porch and they will "go" on them. Of course, I have to put an old chair down there so that Rowdy will have something to "pee" on

Rex, on the other hand is not a marker and will "go" in one spot till he is empty. Rowdy has to pee on every chair leg. Anyway, after a few rains, I just through out the piece of carpet and put down a new one.
Now, tell me this, why won't they go outside on the porch (no carpet) when they (really just Rowdy) will "go" inside sometimes??? (sigh) They have me wrapped around their paws!