Did you have MOO on when you were trying to place the roof objects? And when you tried to place anything, did you see either red or green outlines around the tile where you were trying to place the items. Just trying to determine at which step in the process that the object disappears. If you get the red, it means the object can't go in that spot so it won't place and will disappear when you let go of it but if you got the green, then the object should place and be visible.
There is an issue that if you try to place roof objects with MOO on, they will jump up into the air and can't be placed back on the roof and just float above the roof. Sometimes they can jump quite high up and depending on how close you are zoomed in, you might not be able to see them at all until you zoom out.
Hope this helps.
