The pitch of a roof makes a big difference. Even without snow on the roof you have the weight of the rafters, purlins, decking and steel or shingles. It will be the forces perpendicular to the beams that count. Gravity wants to pull everything straight down so if a beam is horizontal then you have 100 percent of the force acting against the beam. If a beam is straight up like a wall then there is (for this exercise)no force acting on the beams. The roof in the picture looks like a 12/12 pitch roof which will have 50 percent of the load acting against the beams.
You will have to brush up on your Trig. & Geometry to calculate the actual forces acting against your roof members.
There are likely some tables kicking around as well.

Fun with math wink

Cheers


Life is short so put your heart into something that will last a long time.