How to make overlap effect with buildings

So basically I want so when I go behind a building the roof and body of building become transparent and the floor doesn't. I have that done, but now it stays transparent even after I am no longer behind the building, how do I make it detect that I am no longer overlapping the roof of the building? I do not want to have to add collisions for each building I make around the roof. (I am making a 2.5D game)

Are you doing something like -

add all roofs to a family

if player sprite over roof family

..choose that family member and make opacity less ?

Any chance you can post the capx ?

edit - sorry re-read post

Have you looked at grouping the walls and roof of each building together using an instance variable ? So you can quickly choose which walls/roof need to be hidden ?

How are you deciding when the player is behind the building ? Z position and overlapping or just working on the y position ?