I built all three of my yurts using that basic plan. I wouldn't go with 21 on a 16' yurt the rafters won't line up on the lath crosses. Go with 36 like in the plan. Or, if you want to go light for camping, make the ring with 18 slots instead of 36, and just use 18 rafters on every other wall cross.
Forget the 1x4 spacers they need to be beveled to get 36 slots. Bad design. Use 4x4 that's ripped to rafter width in one dimension. If the rafters are 2.75" width, make the spacer blocks out of 4x4 ripped to 2.75, on one side. Use a miterbox/chopsaw to cut the 5 degree bevels each block. That allows plenty of clearance at the inside or the ring. It's GOTTA be beveled to work. 360/36 = 10. 10/ 2 = 5 degrees each side of each block. It the rafters are 1x or 3/4" wide, allow 13/16ths for slot width.
To lay out 30" dia. ring with 36 slots. 30 x 3.14 = 94.2. circumference. 94.2 /36 = [2.61] width per slot at the outside of the ring. subtract width of rafter, .75" plus 1/16th for clearance, that's .82.
So 2.61"- .82" =1.79 block width at outside ring. Cut 5 degree bevel on each side of the ripped 4x4, with the rafter width horizontal on the chopsaw table, nor vertical.
There's your spacer blocks. Lay out the plywood ring in 36 segments, 2.61 apart at the outside edge. The last gap will assuredly be slightly off so just fudge the layout back over ten slots or so. Tack and glue the blocks to one piece of plywood. Flip it over drill pilot holes and screw the ring and blocks together. Flip over glue and screw the second ring.
OR -what you REALLY need to do- just get the freekin plan to a carpenter. He can build the ring for you.
Pics of my 16' yurt are on google images.
Bob Rowlands homemade yurt
Mines the green one with rafter stashed either side of the green door. There's a pic or two there of the ring. You'll see there are no 1x4 blocks in the ring.
That is the plan you are using with three mods on the ring, door frame, and joining the wall sections. By God whatever you do, 'do not' use bolts to join the two wall sections! You'll be cussing like a carpenter trying to get the wall together I guarantee. Lap and lash like Mongolians do. You need to make one of the wall sections, one bay longer for the overlap. If you build three wall sections make two one bay wider. You need one bay extra per section overlap.
Good luck. Keep in mind it's a yurt not a rocket. You'll get it. Have a carpenter buddy help you.
