Some time ago, one of our long-time members, Bruce Perens, found himself with a roof sag. To restore the roof to its correct profile, he started looking at possible methods, developing plans and designs in the process. Bruce is an avid radio amateur, and at the same time he fixed the sag, he also wanted to add some support for a bunch of ham radio antennas and such. He ended up with an external framework that was quite successful, if my memory serves. You can read some of his "how I did it" posts here - there may be more.
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Bill