To do a flat roof, you first need to create a ceiling for the boxes underneath, then take the foundation box from the build section (the one with no walls on the thumbnail) then place and drag where you want your flat roof to be. Then this will allow you to place the cornices/architraves to this box to cover the tile edge.
Another bonus I have found is you can actually apply some, if not all, of the roof patterns as flooring for your roof or elsewhere. Select to roof style you want then apply it to the flooring you just placed. I recently used it in my latest creation and works a treat.