I was hoping to get some feedback on a foundation I am planning to start in the next few days. We have a 30ft yurt coming in about a month, we hope to use the yurt to get ourselves going on the property, but eventually repurpose the fabrics and transition to home to be straw bale/cob. The yurt will have an earthen floor inside.
While yurts are typically build on wood decks over piers, or on a concrete slab, financially a wood deck is not an option for us and I want to avoid concrete as much as I reasonably can. The place we purchased the yurt from provides foundation diagrams, one of which is for a concrete slab which I added as an attachment.
...but I had a few questions/concerns; Blue Ridge Yurts concrete diagram shows an overhand of concrete on their version of a concrete slab where the yurt attaches to the foundation via tapcon screws, my sketch also has this overhang but I am concerned about the weightbearing capacity of an overhang like this. If a 3" layer of concrete overhangs the wall by 2 inches, where the yurt wall itself is made of lattice and 2x4's, nearly half the wall will rest on this overhang. I would think if the yurt place thinks an overhang is safe, then it must be safe? But they also use a complete slab so I'm not sure..
My version of this foundation includes rigid foam sitting under the overhang to try to create a frost protected version of the slab. I am in Maine and our frost line is 4ft down, however I don't believe we could get 4ft down without having to break through a lot of ledge. We are sitting on solid rock that goes hundreds of feet below us, the soil on top is fairly shallow. This was my motivation to frost protect. To decrease the likelihood of water being trapped under our foundation and freezing/thawing over and over, I thought we should add a rubble trench like region underneath the walls.
I suppose my main concerns are; 1. Will the overhang be strong enough to bear the weight of the walls? Or should I chuck the overhang idea, have the walls sit directly on the stem wall, and just stick rigid foam around it more awkwardly?, and 2. Should I have some form of rebar, J hooked into the bottom plate of concrete, going up through the mortared stone, and J hooked again through the top plate of concrete, in order to better anchor the foundation? We are on the peak of a large hill, in a relatively flat old farm field. We have yet to be there for a winter but the neighbors have told us about some crazy strong wind. I do not want our house flying away during a storm.
As a Civil Engineer who has experience with these issues, I have a few ideas;
- Concrete if used where its best is a good product, despite energy issues, replacing a water damaged floor a few times may not be worth it.
- the overhand on the drawings is designed to prevent water creeping in under the walls
- maybe design the foundation to suit strawbale
- instead of removing rocks to make a foundation, think about anchoring the concrete slab and insulation to that rock.
- I am not sure what the lift ioff speed for a flying yurt is, but maybe come over the brow of the hill for some protection.
- Such action will help with holding the heat inside as well I think.
Much progress has been made! With some alternations to my original diagram.
Our yurt has been delivered and we are working away at the foundation. I have zero masonry experience, but inspired by an image of a stone yurt foundation I decided to go for it. I'll add the picture I'm talking about below. it seemed many people were interested in this foundation option, but every time I saw someone hoping to replicate it, someone would ask how the yurt would be anchored. Interested myself I searched for anyone who had been able to safely attach the yurt to a foundation like this but came up short. I wanted to share my efforts to do this for anyone else searching for the same thing. Again, I do not have masonry or much construction experience. I do not know how well this will work but I am just going to try my best and keep you guys updated.
After removing the sod from the area where our yurt will be going, we started digging a trench for the stem wall. We live in Maine where the frost line is 4ft down. Having already done a lot of digging for planting this past spring, having to hand dig our well line and leach field, and sitting over a ledge that comes and goes, we opted for a less deep solution for the trench. This trench is 28 inches deep where the original grade was highest. This is at the front of the yurt, in the back the grade is much lower. The trench was filled in first as a rubble trench on a 1-2% slope; for this we laid out heavy duty landscape fabric to prevent silt from clogging the drainage, we put down 2 inches of clean crushed rock, put a 4 inch corrugated pipe in, and coved that with a minimum of 2 inches of crushed rock over the top. The fabric was folded over the rock. In the midst of this a big storm passed through and water filled our trench. we had not yet dug the outlet area of the last piece of pipe, so we had to do our best to drain the trench. I was pleased to see that the rain water collected well at the bottom of our trench, the slope seemed perfect.
We then spread granite stone dust over drainage and tamped it down. We have a lot of stone dust on the ground after having a well put in so it was convenient to use and packed incredibly well. I used as much as I needed to create a level base to build on. We are required to use a minimum of 16 inches of below grade insulation for this to be considered a 'frost protected foundation'. Our codes here are kind of wonky, we have so few people in our town we aren't required to follow much, but heat loss is a rule that's a must. So that's how I ended up with the rigid foam board. All 16 inches of the foam will be back filled around outside the foundation for the foundation to be considered frost protected, as the foam has to be below grade. We put the foam up and starting laying the base layer of rock.
We have two large field stone piles available on this land, so stone was a semi convenient and free option for us. Having no masonry experience my methods were basically to use big boy rocks in the bottom of the trench. I try to place stones so the slope into the wall rather than away, I do my best to not allow joints in the rocks to overlap, the further a rock reaches into the wall the better. I use a lot of small rocks stuck into the mortar as hearting stones. I use a 5 gallon bucket for my mortar mixture. The mix is made up of a heaping bucket of stone dust (in place of mason sand, our stone dust is fairly gritty), half a bucket of Portland cement, a quarter bucket of hydrated lime, and about two gallons of water.
I am finally getting to the top of the foam board on one half of the stem wall. The yurt is 30' across and will sit on top of the rock wall, it will not overhang the insulation as I first sketched out. Right now I am placing rebar into the stem wall. We have bent the rebar into a soft 'V' shape (trying to not hurt the rebars strength the best we could) with an L on each leg. At the bottom of the 'V' shape we will be hooking 1/4" galvanized steel wire, tying the wire with cable clamps. This wire will run up the yurt wall, hidden in the layers of canvas and insulation, up to the top of the yurt wall where it will connect probably to our 2x6 studs that sit beneath each rafter. I felt this wire was fitting for the yurt as the top of the wall is already held together by a 1/4" wire that comes with the yurt kit. This is my solution to anchoring the yurt to this foundation. I am also going to set up an anchoring system with the dome, that I'll explain and show later.
The inside of this stem wall is going to be filled with road base that we will tamp down. This will build the floor up and make a level base to build our earthen floor on.
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