X2. I've had over 30 different breeds and once they know how tomatoes, strawberries and lettuce seedlings taste or how much fun it is to dig up all the newly planted asparagus roots they fly over. When even Jersey Giants discover the garden, once they finish their morning scratch, it's over the fence they go.If they want to get over the fence they will. My Aunt keeps her one EE Rooster and a couple of 5+yr old Red Sex Links in the kennel with the goat. The kennel is 12X12 and 8ft high chain link fence. They all sit on top of it all the time. As for the orpington not being able to make it that high, you'd be surprised how well those very heavy chickens can get themselves off the ground when the mood strikes them. I would definitely look into a covered run for where they are going to spend their unsupervised time. Just for their own safety.
I now raise a breed that can fly vertically at least 8 feet when startled and pressed, but otherwise they are content to stay inside a 2' fence. Short fences are SOO much cheaper.