How to apply snow load to a roof by using a "group" of the roof members?

[Cherokeedesign]

STAAD V8i, When I try to use the "Load Case Details" to generate a Snow Load on the roof and click on "ADD", the window "Add New Load Intes" opens. I select Snow Load and the comment "At least one floor Group needs to be present" ahow in the "Snow Load" window. I have previouly made a "Roof" group but it not reconized by the Snow load window.

Thanks for your reply.