You're trying to make a roof as part of the wall, when in fact it is it's own wall section that just happens to sit above a wall. What you've discovered is the part that fills in the middle of the roof, but you haven't actually made a roof.
To make a roof:
1) make your wall
2) copy your wall and move your new wall/roof over your old wall and then up a little
3) change the root template to something with roof in the title
4) flip/switch your new haphazard mess of roof tiles until it doesn't look like shit
5) If your roof is large enough, there will now be an empty portion in the middle. This is the part that you've discovered and it's where you need to use the roof options of the sidebar to determine what material fills that in. Small roofs won't even need to ever check 'roof enabled' since they won't have an empty space.
