And let me share my mistake so you don't make it. I have a single roof vent as well as 2 windows. It worked fantastic all summer long and just fine in the fall and spring. But when my dumb butt is up on a ladder scraping snow off the coop roof in the winter, I curse myself for not thinking this through. Soffit ventilation would have been an awesome option but the way my coop is built, adding it now is going to be a challenge.
One other thing, after you build but before you add anything (including chickens) get some Flex Spray and hit the creases, corners and window frames so you aren't wishing you did at the first heavy rain. It's a lot harder to do after you have bedding and birds in there.