Is there a way how to put flooring on half wall?

Yesterday, I was trying to create a house, but the only way for me to build this roof was for me to use half walls without the flooring. I wasn't hoping to do the house like that, but it was the only way. I wanted to create a roof that was flat.

I don't know if I explained this good enough, but I tried to explain the best as I could. 

This was the house I was trying to build - https://www.pinterest.ca/pin/502573639665546278/