Build walls without that edge/ foundation ?

Hello all, 

Im so new im embarrassed to ask, but im so stuck.

When i build walls it keeps making it with that edge/foundation at the bottom. Its like a trim? See the picture.

I've watched tutorials and been through this forum, but i cant get rid of it. 

There is no slider in the game.

If i select a room and use the arrow down this edge doesn't change.

I know it's maybe small, but it frustrates me and i just want to play.

Can someone help? Thank you