Is the maximum recommended exposure the same or different for 18" perfections and 18" tapersawn shakes? Does the butt thickness of the tapersawn shake have an effect on its max exposure? Does interleaving felt paper permit a greater exposure?
For 3 ply coverage 5 1/2" would be the max exposure, butt thickness has nothing to do with exposure.
I am not disagreeing with you, since I don’t know. But butt thickness affects the total thickness of the ply, whether it’s two or three ply.
By my calculations, with a 7" exposure on 18" tapersawns @ 7/8" butt, the thinnest part of the two-ply is about 3/4" thick. With a 7" exposure on 24" tapersawns @ 5/8" butt, the thinnest part of the three-ply is about the same thickness. What makes three-ply at 3/4" minimum thickness better than 2-ply at minimum 3/4" thickness?
Lets make it easy. Here is the bible on application: http://www.cedarbureau.org/manuals/imperial/2015/RFI/RoofManual-0407-i.pdf
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Where does the document explain the rationale for the exposures? I’m not a roofer, but a customer trying to understand why one roofer is recommending 24 inch royals with 3/8" butt and another roofer is recommending 18" tapersawns at 7/8" butt and telling me that with interleaved felt paper these 18-inchers can be taken out to 7" exposure.
You are not taking headlap into consideration.
You need to decide what you want your roof to look like than choose shakes/shingles accordingly.
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Use the Royals. Quit trying to engineer the shingles. The advantage of using shingles over, say, comp is that you don’t have to use felt on each course. Singles are tight and very waterproof. Royals are longer and even more tight. Have the contractor use a smaller exposure with the Royals and they will last forever. If you ever visit Deer Valley next to Park City, Utah you will see all the buildings done in Royals. Absolutely beautiful.