Quick rule-of-thumb guide I read :-
For joist thickness - divide span in feet by 2,then add 2
i.e. 3 m = 10 ft
10 / 2 = 5
5 + 2 = 7,so use 7" x 2" joists
However,as you are raising an existing roof,I assume it has joists now,so what size are they ?
And the same goes for the...