First, i add some roof tiles into customizable house, everything's fine. Then i switch to other floor, or hit "synch" button, or exit customization mode, or backup/restore design, or just terminate client.exe process, or do whatever i can to refresh the design, and it turns out that server deletes walls and roof tiles on other floors! This is very annoying, because i have to restore all of the walls after every little fix to the roof, and also because of this bug i can't have two roof tiles at the same place on different floors!
I think the bug is server side, because the client does not act suspiciously until the time it synchronizes with server, which i beleive is what i'm experiencing here.