On researching my coop design, I noted that some exterior nest boxes had access doors that opened on top (the roof) and others the backside. I had a hard time deciding which design was better, as I could see advantages to each. So I broke down the variables as follows: access, weather, cleaning.
Access: Since the top of the boxes are up off the ground about three feet, access seemed easiest if the roof hinged so it could be opened. Lift the roof, look inside for eggs. The backside access requires me to stoop a bit, especially to see to the front of the nest.
Weather: Seems that the backside door favors keeping a heavy rain out. The roof hinge makes the nests more vulnerable to a hard, driving rain, with water coming through the hinge crack on top. The backside door requires the rain to be pretty-well blown sideways to enter, although backside seems like it could take a hit from a cold wind. However, you can run a small strip of wood to cover the backside hinge crack from the inside of the nest to further reduce wind and water from entering.
Cleaning: If you put a lip on the front of the nesting box to keep the bedding in the nest, how do you readily clean the nesting boxes? With roof access, you essentially have a pocket that you have to clean out and that can't be easy. With backdoor access, you just open the door and sweep out the waste. However, with the backdoor access, you also have nesting material that could fall out each time you open the door.