Hey, I had this same problem and I fixed it by re-making the roof walls.
I found that some of the ways I was editing the roof after copying the walls beneath would cause that "Type" field to become uneditable, so perhaps you can try to set that to roof or floor before making any necessary changes to the walls. For me, even after fixing it, I still had to set that field to floor rather than roof to get it to work, but I haven't found any issue doing in that way.
Hope this works for you!
