Cedar shingle roof underlayment system
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?