Yurt Platform Plans
Here 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).
Here is a selection of platforms built by yurt owners. They all worked fine. You want to make sure to end up with a circle the same diameter as your yurt, plus or minus an inch is fine. For a deck and/or stairs, build them an inch or so separate, and one step (about 7") down. This will allow snow and water to shed off the yurt without getting in, and creates a slot for the outer wall fabric to go down and cinch up tight beneath the floor. At the very end are four sets of plans you can use. If you build a platform that you like and want to send us a picture of it, please do. Have fun!
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.
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. Use platform 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.
--I'm not actually a fan of batt insulation. If you get rain between building the platform and raising the yurt, batts will saturate badly. Use 2" styro, if you use anything. Or foil-faced bubble wrap
--Locally-milled wood hasn't gone up in price like commodity-lumber. Support your local sawyer! Or go cut a tree yourself.