I guess you guys can always tell when I'm playing. Constant newbie questions - today: roofs

So I decided to enlarge my greenhouse to accommodate all the extra plans and trees I'm growing. I made the walls large enough to hold two gardening stations. I used the glass windows to make it look like a greenhouse. When tried to place the flat roof, it's making me leave a huge hole in the middle. I seem to be able to cover two squares out from the walls. The add-on is the exact same size as my previous greenhouse (which is still there. I did take out the wall in between them to give me a nice large open space.

What would cause this?

EDIT: wanted to tell my solution in case anyone else had this happen. The problem was that I needed to add support walls halfway in my room. I did that, added the roof and then removed the walls. Problem solved!