K. Chugh asks:
Couple starter questions: what kind of cedar; how large a shed; how large ( length, width, thickness) will the joists be; how are your kids going to get under the shed to mess with PT joists?
I don't know of any span tables for Eastern red cedar, but it is stronger than MOST other American cedars (not all, but it is readily available, etc.). It is still quite weak.
Or are you writing of ceiling joists? Same story there, of course, except that there is no need for PT or cedar unless the ceiling is rather poor.
Charlie Self
"If a politician found he had cannibals among his constituents, he would promise them missionaries for dinner." H. L. Mencken