A dedicated ceiling is lower than the next floor above, so you see the gap in between. You can create something to place in the gap, use a floor-ceiling combo segment (one with different texture on top and bottom), or raise the height of the ceiling in the segments .fps file by changing the "offy0=" value.
For instance, the typical value of model pack 3 ceilings is...
offy0 = -62
Change the value to something higher, perhaps -12 and adjust the height from there. Remember to delete the segments .dbo file to make the changes apply.
Brian.