I am currently building out my attic in my 1912 home. I want to use R19 insulation with baffling on the cathedral ceiling. I want to then install 1x6 tongue & groove paneling. My rafters are 2x6's and are 24" apart. The pitch of my roof is 8/12.

I have been told that I should "sister" my rafters because the load of the insulation and paneling will eventually make the roof sag and I need more load-bearing support.

I would like to know if this is sound advice and if there is a better alternative, or if anything more is even necessary.

thx for any help