Hi sboydturner, thanx for your reply.
Yes, the elements are placed +410 m above the ground.
I was told by my former colleague this was because, in order to make a terrain, he had to manually set altitude values for all isohypses point in AC, which he got in a 2D *.dwg file. Now that I am thinking about it, all altitudes are around 415 m asl, so I suppose he started lifting in AC giving the starting value of 0 instead of eg. 410, which is the lowest altitude, grrr.
This resulted in a huge slab height:
https://www.dropbox.com/s/1jx4k47w77tz0y9/ground.png
I lifted the ground level because when switching on the "infinite ground" option at 0 level, all I saw around my terrain was black and it was not realistic to render bordering trees. Also, Heliodon sky wasn't visible in many angles. Lifting the ground to 402 m helped a lot with this, but I can't say if that was the cause for a bad referencing to the 'backup file'.
I will do what you suggest and based on your suggestion I got one extra question:
is there a way to check if both of my files have the origin center at the same place? If they are different, I could set the origin of the 'roofless file' to match the origin of the 'backup file' and then the referencing should work. Hopefully.
The question of why the whole roof disappeared still remains a mystery to me.
Thanx,
Vesna