I use the term "frat" very rarely - usually only when quoting something someone has said to me. By and large, I stick to "fraternity". I even trained my non-greek husband to say "fraternity". "Frat" does have negative connotations.
I do use "house" from time to time, usually when I'm talking about fraternities and sororities collectively. I know that's strange, coming from someone from an unhoused chapter, but there you go
