I'm doing my term paper about wordbuilding. And I'm interested what is "mate" in the word "roommate"? Is it a second root so the word "roommate" is a compound, or is it a suffix?
All words ultimately lead to roots.
Both room and mate would trace the roots. Mate=*partner* from mette=*guest*. Room is a Germanic word from Raum.
In your example of roommate, it is a Noun+Noun compound word as in shipmate.