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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.

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    I have to leave now, but will put together a diagram tonight. You can think about it in the meantime. Since the tapersawn doesn't really come to a point, but to a very narrow edge, it's really a long slender trapezoid. 24" inch high trapezoid with a 7/8" wide base and a 1/16" wide top. When you lay them on top of one another (on a plane) with a 7.5" exposure, there's a bowing that occurs, leaving an air gap between the layers.
    – TRomano
    Dec 9, 2016 at 15:38
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    The restriction against using felt with shingles is against using felt interlay between the shingle courses, not felt house-wrap. cedarbureau.org/installation-and-maintenance/roof-manual
    – TRomano
    Dec 9, 2016 at 15:47
  • The restriction is mentioned on page 2 in a sidebar.
    – TRomano
    Dec 9, 2016 at 15:54

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It's all about the headlap. The reason for the felt interlay on Tapersawn Shakes is so you can maintain the weather exposure at 10". 10" weather exposure on a 24" long shake only creates a 2-ply system. Once the felt interlay is added, it creates a 3-ply system, essentially adding 14" to the top of your course of shakes. Shingles are installed with a much shorter weather exposure, 7 1/2" max. on 24" long shingles. The felt interlay between shake courses allows you to increase the weather exposure, but maintain the appropriate amount of headlap needed for the shake roof installation. By doing this, you also decrease the amount of shakes needed to cover the roof area, as well as, lower your material cost.

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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.

Applying a tapersawn shake in a 3-ply fashion and using a felt interlay has raised concerns regarding the trapping of moisture and the potential to shorten the life of the roof. The key discussion point is that a tapersawn shake is sawn on both sides just as a shingle, but a shingle does not use a felt interlayment system regardless of edge (vertical) or flat grain content. Building officials MAY approve eliminating felt interlayment between tapersawn shake courses when tapersawn shakes are applied at weather exposures of less than 1/3 the total length (3-ply roof). ALWAYS check with the local building official for project approval PRIOR to deviating from standard application guidelines. Check with the manufacturer for product warranty requirements.

The 3-ply application of tapersawn shakes without felt interlay is NOT an official method approved by the CSSB or building codes.

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We specify shakes over "spaced" sheathing (1x4 with about 1" between boards) with felt interlaced (laid in each course). We get a lot of wind driven rain here and it keeps it from leaking and the spaced sheathing helps the shakes dry-out. (They lasted 30+ years on my house.)

We specify sawn shingles on continuous layer of building paper on solid roof sheathing (plywood). We install felt interlaced every 3rd course. This keeps the moisture from accumulating, but still allows the shingles to dry out.

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  • Thanks for the info. Your reply doesn't specify the type of shake. Are these handsplit or tapersawn shakes? My question relates to tapersawn shakes, which have some of the qualities of a shake (thickness) and some of the qualities of a shingle (flatness). The CSSB says not to use felt with shingles, so your every-third-course would be considered non-standard or a locally approved variation.
    – TRomano
    Mar 1, 2017 at 11:54
  • Yes, we use hand-split cedar shakes (not shingles).
    – Lee Sam
    Mar 10, 2017 at 4:26

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