The ceilings are a part of the floor tiles.  If there are no floor tiles above the room you're wanting a ceiling in, you can't haz ceiling tiles.       Just in case you overlooked that.   

The cheat you want to use is "modifyNeighborhoodTerrain on", with no quotations, obviously.  But in order to place a lot over your newly created gorge, you'll need to go to C:\Program Files\EA GAMES\The Sims 2\TSData\Sys\Neighborhood.ini, open that file, scroll down near the bottom, find the line that says "MaxLotSlope=20.0" and change the "20.0" to 80.0, or any other number less than 90.  Do this for every expansion you have, ie, C:\Program Files\EA GAMES\The Sims 2 University (or Nightlife, Pets, Seasons, Apartment Life)\TSData\Sys\Neighborhood.ini. 

That cheat only came out with Free Time.  If you don't have that expansion, you'll have to get a premade neighborhood terrain with a built-in gorge. 

Oh, and back up your Neighborhood.ini file just in case you screw it up, or it turns out I'm talking completely out of my arse.