Spend the 20 bucks or so for a copy of "Aircraft Structures" by David J. Peery. Start working through it. You don't have to go all the way through to get yourself to a point where you can calculate stresses on a roof truss.

Get a copy of your local building code so you know how much load it needs to carry.

Once you're there, do the calculations and see if you're comfortable with the result, make up a test article in the back yard or garage and put loads on it (load it to whatever code requires and see how much it deflects, then load it until it breaks to know how much margin you have) to be sure, and then modify away.