When I was a kid I used to catch a lot of black rat snakes. If you ever saw a juvenile, it would look nothing like the adult. As a black rat snakes grows it loses the juvenile coloration and turns mostly black on it's top side. Adults sometimes maintain faint markings from their juvenile colorations. Looking at the picture, and hearing your discription, it sounds like you have found a black rat snake.

Living in the country, I have seen in them in trees, in barn rafters ect. I once had one fall out of tree by the front poarch. Do a google image search and you might be able to get a better idea, as you can fully few the snake in question. The black rat snake leaves a huge geographical footprint. What state are you in? I would guess the black rat snake is found in your area judging by the picture.