you need to determine what the total loading of the joist will be. This includes such items as snow, wind, live load and dead load (weight of building materials). Check the manufacturer's span tables for maximum span for the size I-joist you want to use. This will also provide you with the required spacing.
You may want to contact your local building department to determine the building code requirements for some of the loads such as snow, wind and live.
Hope this helps. Good luck!
"You get what you pay for, and sometimes free costs more!"