We don't have much money, but at least we 'have a roof over our heads'. [=at least we have somewhere to live]
You'll do what I say as long as you're living 'under my roof'. [=as long as you are living in my house]
We hope to have sales and service 'under one roof' [=at the same location] in the near future.
(..) ...that this goodly frame the earth seems to me a sterile promontory This most excellent canopy the air, look you, this brave overhanging firmament... This majestical roof fretted with golden fire... Why, it appears no other thing to me but a foul and pestilent congregation of vapours (..)2015 Hamlet