If I want common rafters to sit against ridge beam but extend 1inch above top of ridge beam for ventilation purposes, does this impact the calculation process for determining rafter length, plumb cut, birds mouth, and if so, how and where do I adjust calculation for it?
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I presume this is in regard to a cathedral ceiling to allow airflow across the entire roof span between the rafters from the soffit venting. Allowing for the desired 1 inch increase height over the crest of the ridge beam causes the upper rafter end degree to increase from 90 and the lower rafter end degree to decrease toward 90.
There are a couple of engineering factors to merely consider. The force of wind load on a roof structure is upward and outward. Cathedral ceiling require LSTA straps on each rafter that projects across the ridge beam to the opposite rafter. How your elevation may or may not affect this issue is a question for the engineer. The second factor is that in a cathedral ceiling the ridge load is very specific in relation to the rafter supports and their endpoint attachments. Again, a question for the engineer.
Lastly, if this is indeed regarding a cathedral ceiling then I have most often observed the cross-ventilation on either side of the ridge beam resolved from below the beam rather than above it by using small collar ties capable of leaving the same amount of ventilation to occur on opposite sides of the ridge beam. This is, of course, unless you're particularly intent upon achieving a perfect crest where the drywall or other ceiling material meets at the bottom of the ridge beam.