Framing the Yurt
Exploring the Joys of working in the round
After removing the concrete form for the last time, the next step was to drill holes in the treated sills so that they would fit down over the anchor bolts that rise out of the concrete foundation -- the bolts that will connect the walls to the foundation.
Each wall section will be eight feet in circumference, and once the treated sills were in place, we could see that the last section would need to be four inches longer in order to close the circle.
With that worked out, it's time to take the treated sills down to the workshop so that they can serve as patterns for cutting the non-treated sill components.
Laying out the treated sill arches
A "stick built" wall is assembled with a top and a bottom, sort of like the letter H laid on its side. With a straight wall, you simply line up the vertical "studs" and nail another "two by" to the top and bottom of the studs, and then stand the wall in place. In our case, it's not that simple since both the top and bottom have to be cut to conform to the curve of the building, which is now literally "set in concrete."
Also, we have to deal with another key point, that being that the wall's header not only has to support the weight of the roof, but it also has to function as a torsion ring insuring that the weight of the roof can't push out the walls. In order to insure that sort of strength, we're making the upper header out of two arcs of 3/4" plywood that will be glued together with the joints staggered by at least two feet all around.
The upshot is that the next task to deal with involved cutting a dozen plus eight foot long arches out of three sheets of 3/4 ply. As you can imagine, that works out to more than two hundred feet of cut -- no small task using the Sawsall (a sort of over-grown jig saw) that was used to cut the original arches.
Cutting curves with a straight-line saw and a bit of beeswax
The problem was solved by using the power of the worm-drive saw and some bees wax. With the depth of cut set to just barely cut through the plywood, the kerf of the saw provides just enough leeway to enable the saw to track the arc, and the bees wax serves to cut friction enough so that the blade wouldn't bind up or over heat in the process.
The reader might want to note that having a hunk of beeswax handy is good for a number of reasons, one of which being safety. A waxed tool moves and cuts easier, and since the operator doesn't have to exert as much force, it's easier to maintain control over the tool -- which is why there's an old truism that observes that a sharp tool is a safe tool.
Once the dozen eight foot arches were cut, it was time to cut the three short arches, remembering to add in the four inches so that the short arch matched the actual footing size.
We're going with six inch thick walls so that the yurt will stay cool in the summer, and warm in the winter, so the next step was to cut some twelve foot 2x6's in half. Usually a "framing stud" is cut so that when it's framed into a wall using a "two-by" on the top and bottom, the total heighth of the wall adds up to eight feet -- and thereby ready for the installation of paneling or sheetrock without having to make any cuts.
Note: a "two-by" isn't actually two inches thick - a number which refers instead to it's rough cut dimension when it's cut green. After it's had a chance to dry -- and shrink a bit -- it's run through a planer to give it a smooth finish, but that removes some wood and so the final dimension is actually closer to one and five-eighth's inch thick.
The next step was to go ahead and actually assemble one of the wall sections. We started with section number two since the first section is where the door will go, and it's always wise to practice on an easier section first. The treated sill was used to pre-drill the holes for the anchor bolts, and once assembled, it rode up to the yurt site on the flat bed trailer. Backed into place, it was an easy matter to tip it up, swing it around, and drop it into place.
a dozen eight foot arches ready for assembly
the first wall section set on the foundation
A pleasant afternoon's work saw to the construction of three additional eight-foot wall sections plus the final short section. The original intention was to install the door in the short section, but we decided to put it in the first section instead because that gave a more southern exposure to the entrance into the yurt.
The frames were put together using heavy construction screws so that adjustments could be made as necessary, but this time everything just fell right into place -- sometimes it just works out right :-)
The design concept is that this is intended to be the sort of yurt that a hobbit would build, and so naturally we have to install a round-top door, something which will take a bit of time and care to fabricate once I get into town and pick up a radiusing attachment for our big router.
All sections in place but the last
It's standard for there to be three or four projects underway at any given time here. As things proceed each project inevitably gets to the point where we need more supplies in order to continue, and therefore that project goes on hold until the next run into town.
These days we're dealing with the increase in the cost of gas by making fewer runs, so it will be later in the week before this part of the project moves ahead again, but no matter as there are plenty of other things to attend to. It's spring and there's no shortage of worthwhile things to do.
the last frame with the door opening in place
the footers blocked apart to make room
for the construction adhesive to be applied
We lifted up the framed sections and set blocks of wood between then to provide space for injecting the construction adhesive.
the footers glued and bolted down to the foundation
With the adhesive in place, we pulled the blocks and lowered the framed wall down onto the treated footer. We then used sections of 2x4 as shims, added the over-sized galvanized washers and bolted the frame snuggly to the concrete foundation.
a shop- made radius attachment for the plunge router
Since the local Home Depot didn't carry a radius attachment for our heavy router, I looked around the shop for what was handy and made one out of two lengths of all-thread rod and a bit of box tubing.
The box tubing was made with three holes; two 3/8" holes to pass the all thread rod, and a smaller hole that will just pass a heavy construction screw. A nut in front and back of the square tubing hold it in place, and then it's just a matter of measuring from the far side of the router bit to the box tubing.
a shot of the cut underway
Two details to note: the plywood being cut is raised up off the wooden deck to give the router bit clearance, and if you look close you can see the construction screw that acts at the pivot point.
the finished arch cut to match the yurt's round top door
The door space is thirty-six inches wide, so the radius would be set to eighteen inches, but this first arch was cut at a radius of seventeen and a quarter inches in order to provide a surface for the door to close against. The remaining inner arch segments will be cut with the full eighteen inch radius as the header is built up layer by layer.
The header is built up using scraps of 3/4 ply glued and held in place with brads. Then a trim bit is used to cut the upper ply to the same shape as the ply under it. The bit has a ball bearing that rides on the lower profile as the blade cuts the upper profile to match.
building up the next layer with plywood left over from making the header arcs
That step was repeated until the header was four and a half inches thick, at which point it was ready to be mounted in the final yurt section and hauled up to the site for a final check to make sure that the last section still fit, which I'm pleased to relate, it did :-)
the round-top "Hobbit door" header in place
The pins used to form up the concrete foundation stick out of the concrete a few inches, and the inside pins needed to be removed since they would be in the way of the insulation which will be installed along the inside of the concrete foundation. So here's a pic of Todd using a pipe wrench to twist them off. They come pre-scored, so it only takes a half-turn or so and the head of the forming pin snaps off. Very slick.
Todd twisting off the form pins
steel plates spanning the header joints add to torsion strength
The header in a yurt is different from your usual header in that it not only has to support the weight of the roof, it's also under a torsion load in that the weight of the roof will press outward, a force which will be resisted by circular header. It's ability to do that is based on it's integrity, and so there are a number of things we'll be doing to enable the header to function as a single circular piece.
The first step is to span each of joint in the lower half of the header with a steel plate that's nailed into the vertical 2x6 studs.
Next, the top of the header is given a liberal coating of exterior wood glue
The upper header is formed in place out of two layers of 3/4" plywood that are glued and nailed together with steel-reinforced, overlapping joints in order to form a single, strong circular unit that will support the yurt's center-less roof.
driving sinker nails down through both header layers
and into the underlying 2x6 stud
Once the upper layer of plywood is in place, hot-melt coated sinker nails are driven through both layers of plywood and on down into the 2x6 studs.
Well, that pretty much completes the task of framing the yurt; now it's time to start creating the yurt's roof -- from the top down :-)
a final touch -- bolting the wall sections together
It probably wasn't necessary, but just to add an extra bit of strength to the torsion header, we went back and bolted the wall sections together with 3/8" carriage bolts. That's probably overkill, but we'd rather err on the side of too strong than risk the alternative.
Notes From Windward - Index - Vol. 66