Editing Roofs of Public Lots

I just built a custom School House for my world. When I put a tower on the corner of the lot, I tried to put the school bell on top. I got the message, "Roof is too low." Oh, I need to delete the roof on the tower and did so with the sledgehammer tool. Now the school bell is on top of the tower. Ooops, the roof of the school is also missing and I can't put it back. Earlier, I tried to edit the slope of the roof, but I couldn't do that either. How do I edit roofs on public lots?