I was trying to make a roof-top beach (the roof is flat by the way), I managed to put sand and palm trees on the roof but I'm struggling to get the pond tool to work on the roof, it comes with an error message "must be placed on exposed terrain" or something like that. The swimming pool tool didn't work on the roof either which is strange because I'm sure I've put swimming pools inside my Sims houses before. I had activated the "moveObjects on" cheat at the time but that made no difference. Tried searching for other cheats but couldn't find anything.

Is it possible to put ponds on the roof? I know this is a weird question lol.