ひとつ屋根の下を英語でなんと言うか？

under the same roof, under one roof のどちらかです。 0:14 I cannot stay under the same roof as your brother and his wife, not one more night.

Treme Season 3: Recap #2

https://www.youtube.com/watch?v=ftCCQjIH8fA  9:26 on this farm in Vermont, three generations are living under one roof.
The Death Of The American Middle Class

https://www.youtube.com/watch?v=k84vPdgg_sM  1:41 now both families are under one roof until Romanski finds a new place to live.
Communities pull together during flood relief

https://www.youtube.com/watch?v=41TyycZHMWw  2:57 bringing the family back under one roof. 　
Trump redecorates White House with gold walls, chandelier

参考リンク
https://www.youtube.com/watch?v=l_iRdgGZ6Xc  1:33 And when the night wind starts to sing a lonesome lullaby. It helps to think we're sleeping underneath the same big sky.
Somewhere Out There - Film Version - An American Tail