Roof System Details
Our cabin roof is a lot different than a conventional roof. It consists of log rafters, a layer of tongue-and-groove cedar, roofing felt, more rafters, insulation, plywood sheathing, and concrete roof tiles. All of this required a lot of materials and labor.
Tongue and Groove Boards
The first layer to go on the roof was actually the ceiling of the cabin. Tongue-and-groove boards went over the log rafters face down, so from the inside you see a tongue-and-groove ceiling separated by log rafters.
The first board started at the bottom of the rafters so it actually formed the underside of the outside overhang. Then we continued up from there to form the inside ceiling.
We used 1x6 tongue-and-groove boards made from incense cedar. This is not the aromatic red cedar used to line closets; it's the kind used to make pencils! It's washable which also makes it good for paneling, siding and decking.
To hide the joints between the boards, we cut them so the joints landed right on the logs. The inside ends ran into a routed groove along the gable end which produced a tight fit.
We also put down insulation wherever the boards crossed the wall logs to prevent heat loss.
Roofing Felt and Rafters
Once the tongue-and-groove boards were in place, we needed to cover them as soon as possible with a layer of 15-pound roofing felt. This protects the boards from any rain or moisture you might get during the rest of the roof construction.
You roll it out the same as you would on a conventional roof. Overlap the edges and staple it down.
In a moderate climate we could've, at this point, nailed on some shingles and been done with the roof. But in our climate we need a roof with some insulating value. To create space for the insulation, we first built up the roof with 2x8 "sleepers." These actually look like rafters because we ran them 24-inches on center from the peak down to the overhang where they met a 2x4 sub-fascia board. The sleepers were notched to extend over the 2x4's. This created an open area where we installed soffit vents.
To insulate in between the sleepers we used a rigid insulation known as "blueboard." It's two inches thick and gives us an R-value of 10. It comes in 4x8 sheets but we cut them down to 22 1/2 inches to fit in between the sleepers. We used three layers of insulation for a total R-value of 30.
We left about an inch gap in between the insulation and the sheathing. We did this so in the winter the venting can bring cool air up under the sheathing and prevent ice dams.
Over the sleepers and insulation we put a layer of 1/2-inch plywood. This step was pretty similar to conventional roofing. Once the plywood was down, we put down another layer of roofing felt.
Concrete Roof Tiles
We chose a concrete tile for our roof to resemble the look of slate. Concrete is not typically used to cover roofs in the upper Midwest because of the frequent freezing and thawing cycles we get. These tiles have a very low rate of moisture absorption, which means water won't soak in, freeze and crack the tiles.
On our roof, one of the main challenges was keeping moisture from leaking in at the valleys between the gable dormer roofs and the main roof. Our subcontractors used 16 ounce copper flashing to protect each valley from moisture.
The installers first nailed down furring strips for the tiles. These strips are notched on the bottom to allow heat and moisture to escape without building up under the tiles.
The tiles came cast with knobs on the bottom to catch on the furring strips. They also had a hole to nail through onto the strips.
As with any shingles, they began with a starter course on the bottom of the roof and worked their way up. The grooves on the side locked together as they moved to the left, and they used angle pieces to finish the ends and hips of the roof.
Since the tiles are not glued or nailed to each other, they'll flex rather than crack as the roof expands and contracts from season to season.
Our tiles came in 5 different tones of brown and green, but each color shows traces of the others. The roofers mixed the colors as they installed them to get a random coloring like you see in a slate roof.