Bangin' for roof

After getting to a new town where you don´t have any place to stay, instead of going to a hotel, you go to a bar to pick up a local stranger and have sex with them in order to spend the night for free at their place.

This term was made famous in the episode "The naked man" of the CBS comedy "How I met your mother"

John wants to backpack through Europe but he is really down on budget so he will be bangin' for roof all the way