I'd like to install a screen door for accessing my deck. Currently I have French doors with one that is stationary and one that opens to the inside. Looking at the existing setup, it appears that there might(?) be a rail for attaching a screen door to? Is the ridge in this picture for a sliding screen door, and if not, what is it? It seems like it would impede me installing any sort of swinging or sliding screen if that's the case.
I’d be careful screwing anything down through the threshold.
The “lip” (ridge as you call it) is for keeping wind blown rain from backing up to your interior swinging door.
An exterior swinging screen door would be difficult at best. You’ll need a screen door frame that is “L” shaped that will lap over the existing door frame. I doubt if you’ll find one that is the exact correct depth, so you’ll need to install furring stripes on the head and jambs. Sealing the head is usually with flashing, but maybe you could use glazing tape.
Likewise for sliding screen doors you’ll have similar installation problems.
My daughter just installed magnetic screens on her deck door and she (and her dogs) love it.
You (or a pro) can easily install a pair of swinging screen doors for your exact situation. You need to specify the purpose and the doors will require an astragal and a special latch assembly. Go to a pro door shop.