Raising the Rafters
There is little one can do that will show the final shape of a building like putting up the rafters. This is especially true of a round building. I don’t know how many people asked me if I was building a dome before I got the first rafters up. Not all round buildings are domes.
I had 3 specific traditional building styles in mind when I designed this building. These were the Mongolian Yurt, the African round hut and the Native American plains Tipi. The Yurt and the round hut are very similar in shape although the construction materials and methods are quite different, the Yurt being a mobile structure and the hut being fixed in place. The Tipi was also a mobile structure and featured a smoke opening that was able to make use of the frequent strong winds for ventilation. The dormer on the south side of the house reflects this influence.
I designed the roof to be supported by 12 large (4 inch x 12 inch) timber rafters with a 12 (up)/20(across) slope. These were to be joined near the top by custom built steel brackets. The idea was that if we wanted to, we could open the center of the roof for a skylight similar to that in the center of a yurt. The rafters are shown below with the brackets installed. I left the beams extending past the brackets out of paranoia. I also was a bit overloaded at the time and wasn’t able to locate an appropriate skylight. I may change this at some point in the future, but an expensive skylight in an attic seems out of place.
As can be seen from the photo, the rafters are not one solid piece. I was unable to purchase 4 inch x 12 inch timbers as long as I needed and had to use four 2 inch x 12 inch beams nailed and bolted together instead. These 2×12 boards were rough cut from beetle killed pine. They measured a true 2 inches and true 12 inches, which adds somewhat to their strength over dimension lumber.
Starting with the second floor (attic) deck, I established where the center point of the building was and erected a temporary tower built from scrap wood. This tower was needed to hold a pulley which held a rope and block and tackle I used to raise the first rafter pair. This was anchored to an old tractor. I needed 3 rafters attached at the center to have a stable structure to attach the other rafters to and I used this contraption to raise these 3 rafters. The photo below shows the first rafter pair after being raised. The brace holding them together was temporary to keep them from spreading.
Attaching the rafters to the deck is an important part of holding the building together. I cut 2 inch thick lumber for sills and secured these to the outer perimeter of the deck. I had found some heavy duty “L” brackets of galvanized steel at a scrap metal facility and used them to anchor the rafters where they contacted the sills. Lag bolts were used that were long enough to penetrate both rafter boards horizontally and deep into the blocking beneath the deck vertically. This should be considerably stronger than the traditional toe-nail attachment.
Once the first three rafters are up it is relatively easy to get the others up and attached. There is some fitting to be done at the top where they all meet snuggly. The next photo shows six of the nine rafters that attach directly to the sill installed. The other three get installed on posts later on to form the dormer. In addition, the first horizontal purlin is shown with an extension rafter attached to it. I refer to the horizontal roof framing members as purlins, and the vertical members as rafters.
To the right of the building you can see a trailer with a load of wood for the purlins. The temporary braces can be seen on the rafters as well. Since the rafters are built from four boards joined with nails and bolts, they tend to be more flexible than solid timbers would be. The braces keep them from moving while the purlins are installed. Below is a photo of more purlins in place. Once the purlins were in place, there is no place for the rafters to go as they are all tied in together. Since the entire structure forms a circle, there is no way for one piece to move without pushing against every other piece. This yeilds a very strong structure, similar to a dome.
As this was all rough cut lumber, I did not use the metal hangers that I had used on the ceiling joists for the purlins. That was too much work. This time I nailed ledger strips under the purlins and toe-nailed them to the rafters. The shape of the roof increases its strength, so I’m not worried about the joint strength here. The lowest purlin is a 4 inch x 10 inch timber. The upper purlins are 2 inch by 10 inch boards. All of them are set 1 ½ inches below the top of the rafters so ventilation channels can be built under the decking.
Above can be seen the vent channels installed awaiting the roof decking. 30 lb. Tar paper was laid down over the purlins and 2×4 (inch) “stringers” were laid over the tar paper and nailed firmly to the purlins. The decking is then placed over and nailed firmly to the rafters and stringers.
The roof decking is ¾ inch exterior plywood. I have a “thing” against chipboard aka OSB. They claim they don’t use formaldehyde in it any more, but then all those trailers that FEMA bought and are made of it have formaldehyde problems. Not sure how that jives with reality. I’ll just avoid OSB for all permanent uses and pay a little more.
So the previous photo shows the plywood roof deck going up. In this climate, with it’s strong winds, we decided against a steel roof. We’ve seen too many buildings out here with sheets of metal roofing peeled back or missing entirely. We decided to go with a sturdy deck with Malarkey “Alaskan” 3 tab shingles. I don’t normally reference brand names for materials. This is an exception. These are probably the most expensive 3 tab shingles around, but they are very flexible at cold temperatures and can withstand high winds before they seal without breaking. We had the shingles installed in November and went through a nasty winter with at least 8 major blizzards without loosing a tab even though many times it looked like all the tabs were standing straight up in the wind. Since that winter we have only had one tab lift and it was easily glued back down after the wind storm.
The dormer was a bit of a different beast to build. It consisted of three rafters lifted above the sill on posts built into the dormer walls. While to peak of the roof was at 12 feet above the attic floor, the dormer rafters are 5 feet and 9 feet above the attic floor. This means the peak of the dormer is 3 feet lower than the peak of the roof and about a foot below where it is held up by the brackets. The look this gives to the roof profile is very unique.
Here we can see the dormer side walls holding up the outer dormer rafters. The center dormer rafter is held up by a 4 inch x 4 inch post which will later be incorporated into the dormer wall.
Before the dormer wall was complete I hired an experienced roofer to shingle the roof. This was in late october and we were racing to beat the approaching winter. Sometimes winters here are mild, but this one was not destined to be so nice. The shingles were mostly finished by mid December and on December 20th the first of 8 blizzards hit. The shingles held up to the storms better than we did as we were living in the trailer in front of the house. It was a hard time and better to live in memory than the present.
Due to the weather I was unable to finish sheathing the dormer and the blizzards dropped quite a bit of snow on the attic floor. We learned that leaving the leeward side open meant that wind swirled around in it and dropped its frozen contents where we didn’t want it. While I was digging out of four foot deep drifts, I was having to also deal with two feet of snow on the attic floor. As soon as I could I put sheathing on the dormer walls.
Above is a view of the dormer installed and with windows and sheathing attached. The windows and doors are installed all around the house. You can see the low venting cupola and stove pipes as well. Two by four mini-rafters extend from the dormer studs for eventual use on the shading apron which will limit the late spring, summer and early fall sunlight from heating the house by the southern windows.
The next step was hiring an experienced mason to put up the bricks.