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.
(..) Dialogue: Marked=..,Default,NTP,!Effect,No. I think l'll leave it somewhere\Nfor Longbottom to find. Dialogue: Marked=..,Default,NTP,!Effect,How about on the roof? Dialogue: Marked=..,Default,NTP,!Effect,What's the matter, Potter?\NBit beyond your reach? (..)2001 Harry Potter and the Sorcerer's Stone