How do you build a second floor?

[Qdude]

I wanted to make a second floor to my house, so I built stairs indoors and climbed up it. The roof looks like this at the top. https://steamcommunity.com/sharedfiles/filedetails/?id=2727299153
The house looks like this from the outside, and I'm wondering if this is a reason why.
https://steamcommunity.com/sharedfiles/filedetails/?id=2727305183
EDIT: I also tried building stairs outside that lead to the roof and the same exact thing happens, so I don't know how building second floors work.
I'm able to fall through SOME of those dark patches and into the floor below, but when you're in the floor below the roof is still there, no sunlight comes in and I can't hear the outside noises. So what exactly is happening here? Do I have to replace those dark patches with floor even though the roof tiles should be there in the first place?

[Qdude]

bump

[suki!]

Yes you have to put floor on top of the roof.