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Sister rafters in attic

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

Re: Sister rafters in attic

What is the unsupported span of the rafters from the eave to the peak?

Re: Sister rafters in attic

We've had good luck skipping the fiberglass and installing the aluminum faced foam panels then installing drywall. The sheet foam doesn't have much R value, but it acts as a radiant barrier instead. This also gives great air flow over the panels between the joists.

Be sure to stagger the drywall and foam boards as not to have overlapping seams. 2 thinner layers of foam will eliminate any draft / chimney effect of air passing through the roof.

And you'll always be able to run wiring.

Re: Sister rafters in attic

The unsupported span from eave to peak is roughly 13 1/2'. In the majority of areas, the rafters are 24" apart but this is not uniform. In some areas it is 23".

I do not want to use drywall, only wood paneling (pine).

Re: Sister rafters in attic

Check with your local building compliance officer. Round these parts we MUST have a layer of 1/2" drywall to meet the fire code. If we want we can cover that with wood boards, but the drywall (at least one layer tape and float) is mandatory.

Besides, those boards will be drafty by themselves.

Re: Sister rafters in attic

I do not understand what exactly you are trying to accomplish here. I'm not trying to be nosy but depending on your project, there may be different answers.

Are you opening up a flat ceiling to make a cathedral ceiling or are you converting attic space into living space? If you are converting the attic to living space, will you be building knee walls and if so, is that 13.5' span to the knee wall or all the way to the eave?

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