I put a metal roof on the extended rafters
Because I added the beds that came out 2 feet on either side, I extended the rafters by more than 2 feet. Then I put a metal roof on top of the rafters. The roof is about 14 feet wide by 12 feet long. There is a little overhang on the deck.
Rory the dog below sits in front of one of two escape hatches built into the old structure. This particular escape hatch was where the slide used to be. I put it off the deck and built a swinging door that latches shut inside the tree house to cover the hole. There's also a smaller hatch at the south end of the tree house where I built another swinging door.