Not sure if I've seen anyone systematically address the sag problem (other than Blu88 ). It must be a common issue with older shuttles. Occasionally in other forums I've seen posts that someone removed the inner layer and the honeycomb, but no pictures.
The sandwich of 1/8" fiberglass and cardboard honeycomb is structural, as you probably know. If it's sagging that means the integrity of that 'sandwich' is compromised, though honestly I'm not sure whether that matters other than visual appeal or water pooling.
Shuttles have high enough ceilings that you don't necessarily need to remove the inner layer and cardboard to fix the sag. That will be a tough, dirty, messy job; and besides, the honeycomb, or whatever is left of it, provides continuous support for the fiberglass--without it I'm not sure you could walk on the roof afterwards.
If I had to address it I'd probably see if I could get away with building plywood trusses, maybe 2-1/2" deep, that conform to the original slope and curve of the ceiling and support it where it's sagging.
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