You also need to consider if they are scared will they fly up and get out. I lost a very sweet hen that way, the dog never would have gotten through the fence but she didn't know that and panicked and flew out.
Our outdoor pen is uncovered, but the hens have access to going inside their coop as well as under their coop for protection. You might consider providing some kind of shelter within the pen that they can go to if frightened. ~G