That was exactly what I was doing but I thought it wasnt working because I didnt change the elevation of the 3D view and from level 0, the floor of level 1 is not showed.

Thank you for your quick reply

Hi,

I'm trying to build a house with false ceiling on some rooms but I can't find the way to do it properly. This is exactly what I'm trying to accomplished:
https://dl.dropbox.com/u/7441532/Falso%20Techo%20Salon.png
As you could see, on top of the inner door there is an air conditioning grille ...