we are in the midst of planning a new deck and we are not sure how to span a 20' opening at the end of the deck to support a roof without adding any additional support in the center of the span length. can you recomend a timber size and type to solve our problem.

The required timber size would depend on how the rest of the deck is framed and the loads to be carried. I would suggest that you find an engineer in your area to look at the problem.

If you are comfortable with the calculations, there is also an article by ED Levin in the Guild Workbook that clearly covers sizing timbers. The workbook is available from this web site.