I don't remember ever needing to extend the roof rack on my D1 when I had it, and I will have transported a lot of moderately heavy and long things up there.
Examples would be 8x4 sheets of ply, a 25' boat mast and several lengths of 4x2 etc. I think I may have put something (rubber, cloth) over the rear roof.
What I do remember is the difficulty of getting stuff up there without serious strain and potential rupture, and the running board collapsing under me while lifting on one occasion. Not at all good, but gaffer tape got me home.