Below are three photos of us building the most recent 20' platform at our place. We followed the plans in the pdf at the bottom of this page. We used pressure treated because the mill couldn't get us any green hemlock, which I would have preferred. Notice no KD anywhere: it rots, even undercover. In the first picture, see how we laid beams on top of beams to get the correct angles traced out. No math involved. The second picture shows how one beam can be attached to another with long screws. Tie plates work too. Then we attached 2x6 tongue and groove directly to the beams without using any joists. Would not recommend. Run the joists over top the beams, every 16" or 24", cantilevering all around to go from an octagon shape to a circle. Toe-screw the joists down into the beams. Lastly, attach a 1/4" strip of plywood around the outer edge extending 2" above the finished floor. This will hold the lattice to the platform and keep it from walking off the edge. Leave a 47" gap for the door (see picture near bottom of page).

Should I insulate under the floor??   You're right to be dubious about floor insulation. The roof/wall insul is R4? R8? Depends how you calculate in the foil layers. Since R is merely resistance to Conductive heat transfer, it does not account for Reflective, or Evaporative, or Convective heat transfer (which are a proportionally larger percentage of the equation in a small yurt with center stove). So there are competing mathematical claims. Whatever. . .  warm air does rise, of course; so I've had good luck with no insul underneath at all (provided you stop the drafts!).  But 2" polystyrene between the joists is by far the most common choice of yurt-dwellers. That way you'll know that if you're cold, it's because the fire went out, not because you didn't insulate the floor.

Every platform insulating material has some pros and cons. I often don't insulate at all as a result.

-Most common: 2" polystyrene between the joists. On 1x2" ledgers. Often spray-foamed in the gaps
-Most green: Rockwool batts or rockwool boards between the joists

-Cleverest: 1/4" - 1" of anything between the subfloor and finished floor (requires visible fasteners in the floor) or atop the joists.
-Fast and dirty: 1 layer of foil-faced bubblewrap atop the joists
-Bare bones: 6mil poly atop the joists
-Worst: fiberglass batts.

Below: Showing skirt board and 47" gap left for door. (It's installed a bit high here) The skirt board should extend a couple inches above finished floor and 4-6" below it, for a total skirt board width of 6-8" (shown=6"). Use 1/4" or 3/8" plywood. Give it a coat of urethane or oil to prevent mildew. 

A note about mold: If your yurt is going to mold anywhere, it's on the skirt board, what with being tucked away in the corner of the floor.  Or the bottom few inches of the lattice. If you have a mold problem it will most likely be the first year, because your floor or lattice/rafter wood is not fully dry. It typically happens in July or August with yurts that go up in the spring and are then sealed up for the summer. The moisture has nowhere to go. For the finish flooring, use wood as dry as you can. Keep the platform well tarped before raising day--this is best accomplished by placing something in the center (eg. a garbage can or step ladder) to give the tarp slope. Your first summer in the yurt, keep the windows open (when it's not raining) and ventilate as much as possible. Running the wood stove during the first winter will dry everything--floor and framing--completely.        

(Note: these stairs are dangerous! Also, the tyvek/batt insulation method used in this picture isn't my favorite)

Notes on the four sets of plans below:

--The carry beams rest atop the piers: no attaching needed. 

--Start by laying out your blocks so their outside edges are roughly even with the circumference of the circle. The piers shouldn't stick out beyond the edge of the yurt. Then lay out your carry beams to land roughly centered on them. 

--To cut the carry beams, lay them atop each other and scribe a line on their intersection (see the second picture at the top of this page)

--The carry beam and joist lengths shown are for sawmill ordering purposes. You will cut them after laying them on the cinderblocks. Numbers shown may not be final cut lengths. Your job is to make sure they don't stick out beyond the circumference.

--The 2x6 joists run OVER the carry beams. Toe-screw them in. Then they cantilever out over the carry beams. Leave them long till after the flooring is on. Then cut them off flush as you go around cutting the flooring round. The picture at the top of this page shows this best. 

--Use wire instead of string to mark the floor round--string stretches. To cut the floor round, you can use a chainsaw, but a circular saw is much cleaner. Save the chainsaw for chopping off the ends of the joists last.

--Locally-milled wood hasn't gone up in price like commodity-lumber. Support your local sawyer! Or go cut a tree yourself.

Platform Q & A:

Q: I'm on sloped ground. My uphill block stack is a foot tall and my low-side stack is about 18 inches taller. The door is located midway, and when I add up the beam, joists & flooring, I’m getting 2-1/2 feet off the ground at the front door. I feel like I’m missing something, it feels too high. I keep thinking the beams need a foot of air under them to keep them dry, but I’m starting to wonder.

A: I agree, sounds a little high. If possible, it's nice to have the door about 14" from ground to finished floor. This is 2 steps up. Three steps (~21") up is alright too.  Keeping the entrance low helps with the physical (and therefore psychological) ease of going in and out all day. Most people living in yurts don't spend their whole day just in that one space. The space outside the yurt is very much a part of the home. Keeping floor/ground heights close is one (of many) ways of creating positive outdoor space.

The point of space under the yurt is ventilation, so stuff doesn't get dank under there in summer. Since you'll have plenty of space at the low-ground point, you can cheat the high-ground side down a bit, but not so much that you have wood-ground contact. Assuming your chimney blocks are 8" high, what if you remove a block all around? That would put the bottom of the lowest beam on the high-ground side at 4 or 5 inches from the ground? If so, that's seems perfect. Another option is to rotate the yurt so that the door is closer to the high-ground point, though that may not work with the direction you want the door to be.