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Community and Q&A

Cedar shingle roof underlayment system

Randall Thomas | Posted in Energy Efficiency and Durability on

I am writing a specification for replacement roofs on historic Forest Service buildings in eastern Idaho, ASHRAE Climate Zone 6B.

The Sate Historic Preservation Office is insistent on maintaining the wood shingle roofs. The original roofs are 16-inch shingles with a 6-inch exposure, and that must be replaced in kind. So, I plan to specify 16-inch, No. 1 Red Cedar, Premium Grade, 100 percent vertical Grain, 100 percent clear 5/2 perfect, shingles. The older buildings, (pre WW-II), are skip sheathed with pine or fir 1X material, and newer ones, (post WW-II) are solid sheathed typically with 5/8 plywood.

The Forest Service traditionally “oiled” the roofs every five years, with a mixture of 5-gallons of boiled linseed oil, and 1-gallon of green oil based paint. (had to argue with SHPO about whether they were painted). I plan to specify a heavily tinted “log oil” product. We have used this for two roofs in the last five years, and it is holding up well.

Under the shingles we have been specifying Cedar Breather by Benjamin Obdike, over 30-lb felt. Several Contractors have inquired about using synthetic under-layments, so I am investigating them. From my readings thus far, I am tending to think a perm of 10 or greater is appropriate, because it will allow vapor to escape from the surface of the sheathing up into the cedar breather, and then to the ridge.

Most of the buildings are only used seasonally from two weeks before Memorial Day, to three or four weeks after Labor Day, so I have been installing an impermeable membrane from the eave to 2-ft inside the exterior wall line.

What are your thoughts?

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Randall,
    I'm not aware of any studies of the issue, but I'm all in favor of asphalt felt, for the reason you mentioned. Asphalt felt is a "smart" vapor retarder.

    I'm suspicious of the regular applications of "log oil" on cedar shingles. You are specifying a high-quality (and expensive) type of red cedar shingle. I advise you to leave the shingles alone.

    I'll bet that the Forest Service workers who climb up on the roof to apply log oil every few years do more damage to the roof with their boots than can be justified by the unlikely theory that the oil has any positive benefits.

  2. Jon R | | #2
  3. User avatar GBA Editor
    Martin Holladay | | #3

    Jon,
    OK, I learned something. I guess there are experimental results that justify the use of certain preservatives for red cedar roof shingles. Thanks for the link.

  4. Steve Knapp CZ 3A Georgia | | #4

    Randall. I'm sure the historic preservation folks might object, but did you consider cement shingles? I imagine that many of the Forest Service structures are in fire-prone areas where linseed-coated wood shingles is maybe not the best choice.

  5. Randall Thomas | | #5

    Steve Knapp, I agree a cedar shingle roof impregnated with oil is a fire issue. I would much rather install metal roofing, and update the venting systems to comply with fire smart technology guidelines, that the Forest Service developed at their San Dimas Technology Center, (and which have become the backbone of the International Code Council Wildland-Urban Interface Code). It would be a big benefit to demonstrate "Fire Smart" to the public, and local AHJ. But the SHPO folks are adamant about the new roof closely matching the old one. I kind of look at it that if it burns to the ground, because of their historic roof, I would have one less historic structure to deal with. We have over 500 buildings on this forest, and the newest is from 1962. The SHPO folks won't let me replace the siding on buildings built in the 1930's by the CCCs that is old and splitting, with new same species siding, custom milled to match the original. I keep trying to explain that if they force us to try and use, old inefficient buildings, we will eventually accept the adverse action, and demolish them to construct new efficient buildings.

    Martin, the SHPO folks are the ones who drive the green sealant. In the days old days, when the green colorant was a copper compound, it probably functioned to keep the moss off the roofs. Now, I don't think it does anything but make the archeologists, and historians happy.

    My biggest concern is my idea, that I want the roof to be able to dry to the inside, and whether anyone sees problems with that approach.

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