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.
(..) They're all whispering, and I just know it's about her. What's it matter what they say, Aunt Pittypat? But Scarlett is living under my roof so they all think I'm responsible for her... ...and for a widow to appear in public at a social gathering! Every time I think of it I feel faint! (..)1939 Gone with the Wind