Roofing advice

Hi, i’m going to have the walls built and finished up to wall plate on my extension by the beginning of next week, I will then be ready to put the roof on, but I need a bit of advice before I start. I don’t know the exact dimensions at present, I will take all measurements tomorrow.
From wall plate to wall plate is around 8.5m and the height from wall plate to ridge height is around 1.5/2m. The pitch is around 13 degrees. As I am having vaulted ceilings I am restricted on reinforcing the roof.
What I would like to know first is what size timbers would I need for the rafters.
Secondly can someone with some serious roofing knowledge talk me through the best way to strengthen the whole roof as I know that when I put the tiles on it will put a lot of strain on the walls.
Many thanks.

Structural engineer.

definatley not a question for a roofer. talk to an architect.