My question relates to the difference in shape (not thickness) between cedar shingles and cedar tapersawn shakes, both of which are sawn (unlike standard shakes which are split), and therefore have a smoother surface than a split shake.
I think the shingle is a right-triangle and the tapersawn is an isosceles triangle. Is that right?
I am trying to understand which specific difference between a shingle and a tapersawn shake makes it unacceptable to interlay felt with cedar shingles but acceptable to interlay felt with tapersawn shakes.
Best Answer
I found this PDF document which addresses the issue, an unnumbered technical bulletin issued by the Cedar Shake and Shingle Bureau (the industry standards body) in March 2007. Apparently It's a matter of some controversy: people are installing tapersawns in three-ply applications (e.g. 24 inch tapersawn at 7.5" exposure) without felt interlay because it is aethetically pleasing, less expensive than using three-ply with felt, and they fear the felt slows down evaporation and possibly leads to trapped moisture. But the method is not recognized by the building code at the national level, and so the CSSB does not sanction it. Some jurisdictions are allowing it.
I cannot find the original at the CSSB website, so the link above is to a third-party website.