My partner and I are thinking about creating an attic bedroom, bath and study in her Mansard roofed duplex. The house is approximately 100 years old. We're wondering if we'll have to redeck the roof before insulating and drywalling the walls/ceilings. The roof is currently decked with 8-10"x1" boards. There are gaps between many of the boards, but the roof is watertight. I'm thinking that we probably will have to but I'd like to avoid the expense, if possible. What do you think?