Hi Tony,
one (of the many) mistakes I made when constructing my roll-off roof observatory I embedded the exteriors supports for the exterior rails (where the roof rides over onto when open) within the building, so when the eventually rotted, it was a particularly difficult job to replace them.
So, I’d suggest thinking about how you might replace them when that needs to be done. Unlike the main building the exterior rails will be subject to all the weather…
Look forward to hearing how the project goes.
Clear skies
Callum