Trees can sometimes impede signals if there is a large stand between your location and the towers. I personally have about 10 very large, very old oak trees in my "line of sight" and while I get pretty great reception with only occasional issues which generally happen hand in hand with inclement weather. I do get nearly perfect reception in the winter when there are no leaves by the way.
The metal roof is more my concern. You would need to get the antenna up and over the metal on your home for the best chance at reception. Metal will bounce the signals away from your antenna so height and having the face of the antenna "pointed" in the direction of the towers will give the signals more surface to "land" on. As long as you are in a good location for reception and can get the antenna over the potential blockage you may be able to cut the cord!