By Bill Olsen (reprinted from 2005 newsletter)
Some years ago, when I worked at Frontier Builders in Land O’ Lakes, Wisconsin, company founder Jerry Wait and myself were out looking at buying stumpage when we stopped by a log home under construction.
A professional log crafter and former member of the GLLCA were building the home. The builder had just finished the roof
system and, at the time, I was contemplating the type of roof to put on a small log home I was completing for myself. The roof design he used was simple: first he covered the ridge and purlins with 2” x 6” T&G decking; rigid insulation was laid on top of the T&G; and 2” by 2” sleepers were laid on top of that. Long screws that penetrated through the insulation anchored the sleepers to the T&G decking and hold the insulation in place. Sheathing was then fastened to the sleepers. Tarpaper and shingles completed the job. During the drive back to the log yard, I asked Jerry what he thought of this building method.
“There’s no structure,” he replied.
What do you mean?
“There ain’t nothin’ holding it together in a big wind except prayer,” he said. That was as much as I would be getting from Jerry on the subject in light of his economic vocabulary
that was developed when words used to cost a nickel each. And come to think of it, there isn’t much more to say on the subject.
When it came time to do my own roof a few weeks later, I figured Jerry was right and I opted for Structurally Insulated Panel or SIPs. Although there’s plenty of strength with
SIPs, after the first winter in the home, I had something called “shingle ridging.” That’s caused when moisture migrates through panel joints and causes the OSB to expand and push up the shingles. Although the situation usually reverses itself after summer heat, the ridging will repeat every year—not a healthy situation in the long term. I’ve since sold that small log home, but the subject of roofs was a concern again last year on another home for me. I remembered Jerry’s concerns about sleepers and combined that with what I knew about SIPs and came up with something I call a “modified rafter” roof system. The following method was used on a cathedral ceiling that covers a 20’ by 30’ wing. It’s strong, can be vented or un-vented, and has an extremely high R-value (mine is approximately R-55). Although it’s a labor intensive roof to build, I believe it has high economical value and high structural integrity. Living in the snow country in Michigan’s Upper Peninsula, we receive an average of 250 inches of white stuff per year. So the roof had to be stronger than your average one, and since I have a penchant for working alone, the process had to be solo friendly.
You’ll notice from the pictures that the rafters running down the rake of the roof were spliced over the purlins. This was because I had a limited supply of rough-cut dimensional lumber that was at least 20 feet long. And also because I wanted 3” x 6” rafters supporting the four-foot eave overhangs but didn’t need rafter material that thick for the upper rake of the roof. (Incidentally, this past winter the roof easily withstood three feet of heavy snow piled on the eaves.)
If you’re interested in building a similar style roof here are a few pointers:
-Logs used for purlins and top plates should be straight—it is easier to build the roof system on top of them and quicker to install insulation if the logs have little sweep. A 6-inch space won’t necessarily accept three 2-inch sheets of foam. (Sometimes it did, sometimes it didn’t. In either case, have on hand plenty of canned spray-in foam insulation to fill cracks and voids that you’ll inevitably have. (I went through 40 cans.)
-The key to this style of roof is the 16-inch Oly Log Fasteners purchased through Schroeder’s Log Home Supply. They hold the rafters directly to the ridge and purlins. Don’t skimp on screws (GRK Fasteners also work) or you’ll be the one who gets screwed in the end.
-On top of the T&G decking before the insulation was installed, I put down a layer of snow and ice shield—expensive but worth it. (A second layer of snow and ice shield was used over the sheathing before shingling.)
-Stagger the joints of the sheets of insulation so heat or moisture does not have direct passages out. (Available now is a special tape to seal the seams.)
-This same roof design would be excellent for professional spray-in foam insulation applications such as Icynene. Although I considered it, the nearest contractor is 150 miles away and it was easer and perhaps a bit less expensive to use sheets of insulation.
Join GLLCA for more support and tips from a network of experienced log builders!