Answer:
1.) A roof over our head - This idiom means to have a place to live or shelter.
Example sentence: After being homeless for months, I finally found a small apartment with a roof over my head.
2.) Hanging over my head - This idiom means a problem or a worry that someone has that they can't stop thinking about.
Example sentence: I hate having this project hanging over my head all weekend, I wish I could just finish it now.