I went back and forth for a while as well. My location, the ground is slightly sloped... back of the coop is downhill. If I were to allow the water to run off the front, it would likely move downhill, into the run. (I already have the garden there, that's enough moisture to contend with.) Also, I read other ppl's experiences with regards to egg collection... in the rain, if the roof (or portion of) is slanted towards the nest box, you get drenched trying to get eggs. Plus, I was hoping to eventually install some kind of rain barrel system on the back side. It's also a rather sunny location, and the hot evening sun would be shaded more effectively (the coop will face east). Lean-to roofs are also easier and cheaper to build. I decided the lean-to best suited my needs.