No offense meant, but that is a difficult design you are working with there. Picturesque, but what works for horse- and tobacco-barns does NOT scale down well to miniature chicken coops (except in always-warm always-dry climates).
If it were me I think I would do the following:
1) If at ALL possible, change the roost (I assume it is lengthwise?) to be two parallel roosts as far towards one end of the coop as possible. I say two because I'm guessing your coop is maybe 2 1/2 feet wide(?) which is not enough roost length for five chickens unless they are *very* friendly. Then
2) Make hinged plexiglass vent covers for both sides' vents at that end of the coop, so that in cold weather I can close that half of the coop off entirely (maybe leaving the gable-end vent open, depending on wind and temperature). Also
3) Make separate hinged plexiglass vent covers for both sides' vents on the OTHER half of the coop, so you can operate them independantly of the roost half of the coop. As wind and weather require, close one side or the other partly or entirely, whatever it takes to keep rain out. Having those hinged vent covers maybe 1/3 open won't interfere too much with your airflow while preventing most rain/snow from blowing in. You will need some kind of chocks or external frame or whatever, to brace them firmly in that part-open position so they don't blow open or shut, but this is not difficult to achieve.
Alternatively, IF you are in an area that doesn't get awful cold in the winter (like, maybe it gets down into the mid to upper teens occasionally, but mostly not) you could just build extensions of the upper little roof so that the extensions essentially cover the two main roofs... a great big roof overhang over the vents, you know? You still might well need the capacity to close off the upwind long vents on windy nights and it would be a bit drafty/cool on the roost but at least precipitation would mostly stay out.
Good luck, have fun,
Pat