Underground Tinyhouse Roof Shape, Mimicking a Yurt?

We are planning on building an 'apartment' of sorts on our family property, earth-sheltered. It will be round, roughly 150 square feet, so the size of a large bedroom. We plan on the rough structure to be the 'crown and pole' basic shape, but with a single large center post to give extra support to the roof (since it will be under a foot of soil), and 1in thick wood boards circled around (see the roof in the photo for example of the boards). We are thinking, a solid crown, with 2x4s as the poles (possibly metal bracketing connected to the crown). Still working out the exact way to connect the poles to the crown, maybe like the second photo. The base of the poles will rest inside cob walls, we are thinking of putting a wooden 'ring' inside the cob along the top of the walls to screw the poles to before covering it with cob. (After it's built, we will put down roofing and waterproof sheeting, etc.)

Is this viable? We obviously don't want a roof collapse. Is adding the 4x4 center post enough to hold the structure? Does anyone have suggestions or tips?

Thanks!

I am no carpenter, but your dimensions on the ,lumber seem rather small if you actually want a full foot of soil on top.
I would think more in terms of using 2" x 12" lumber for the poles and as many as 8 sandwiched together as the center column. 2"X 4"s would make decent boards to replace the 1" boards.

I didn't mean one inch by one inch; morelike 1 x 8 or 10. Same as that photo. How would I use the 2 x 12 boards as poles? That would make for thicker coverage by the crown, sure, but with only 8 boards that would leave a lot of space between them down by the walls... which would mean i would need heavier slat boards all around it.

A foot of dry topsoil is about 75 lb/sqft load on the roof. The number goes up from there for sand, gravel, or wet. Then you need to add any snowload on top of that.

A safe estimate is 150 lb/sqft higher than code requires for your area. Run load calcs and timber sizes from there. Mike Oehlers' $50 house book has load calcs for roundwood.

You will definitely need more than 8 rafters.

No snow load here; too hot to snow, and doesn't rain often. I'm not using roundwood, I'm using whatever's available at home depot. Plus there will be the 4x4 center post; from that post, in any direction, the load bearing walls are 5ft away. So whatever I use needs only to hold up a 5ft length. And I figured that numerous smaller poles (2x4s, or 4x4s if needed) would be a better bet than 8 wide pieces. And I don't have the $50 home book.

OR - would a reciprocal roof be a better bet? With 12 4x4s?

http://bct.eco.umass.edu/publications/by-title/understanding-loads-and-using-span-tables/

Here is a start of a load table. Most load tables stop at 100psf live load, which would just barely cover your needs if it is a light well draining mix on the roof.

You need to base it on the distance the beams are apart at the base of the walls, so if you only want 12 you have a really wide span at the bottom. This is why people put sub-rafters in a reciprical roof.

What you are trying to do takes some serious engineering to be safe, I don't feel safe giving an answer. I know what safety margin I would want to include.

Do you think a gambrel roof will work instead? Will it be able to take the load? I wouldn't be as happy with it because it would stick out on the corners of my round room-house, so I'd have to make the (at least front, showing) wall strait to accommodate that. And it'd be harder to cover with dirt, being so much taller on the front and back, than a round roof.

I don't want to do gable roof because it would prevent me from putting in the loft bed (the framing being too low).

Here - I found a link to a reciprocal roof used for an underground earthbag building. The same can be applied to an underground cob/stone building, can it not?

http://www.naturalbuildingblog.com/emergency-earthbag-shelter-with-reciprocal-roof/

Here also seems to be a plethora of houses with the same roof style; one is buried, the other have sod roofs.

http://www.beingsomewhere.net/index.htm

I think you can make any of them work as long as you size the poles, rafters and decking appropriately. You are in the realm of moderate to heavy snow load, not full-on earth-sheltered house (four FEET of dirt). It will take much bigger lumber than you think, though, if you want a big open span.

Look at the wofati threads, there is a lot of talk about spans and strength requirements.

Another example.... if he can put all those haybales up there, and then the sod and plants (and water those plants!).... I think I can do it, with 6in of dirt, right?

http://www.thatroundhouse.info/how.htm

Oh, I don't care about the size of the span, actually, since I will be covering it.

I would orient the 12'' side of the 12x2 vertically, not horizontally. That way any force would be resisted by 12" of material instead of 2"s or 4"s of material.
I would make a massive center column out of 12 x 2s stacked together, or perhaps mortared stone.
Load calculations are currently beyond my ken, so I would just massivly overbuild it

Sahara

> I don't care about the size of the span...

Please go very slowly here. If you don't have a "feel" for construction (most people don't until they have worked with the materials for some time) it's almost impossible to get good results trying to half copy, half modify somebody else's work. It's a concern because the size and loads you talk about can hurt people - badly.

Notice the roundhouse "rafters" are very close together. And the photo of the "yurt" connectors - those connectors were never intended for that type of installation with the wood members on the flat; that builder may have made his design safe using other (unseen) methods - or he may not. IOW remember that just because you see it on the internet...

I also am not comfortable with saying anything more on building methods Construction is a wonderful trade, very broad, complex and rewarding if you're the type. It needs to be approached and practiced with respect because failures can be severe. There is a learning curve.

Think safe. Those SAFETY signs in every big construction site aren't there for decoration - the bean counters wouldn't put them up w/out a _very_ good reason...

Best luck.

Rufus

My comment was in reply to the central open span of the reciprocal roof, not the yurt style one. As in, I don't mind if it's a large span due to using larger sturdier lumber, because I will be closing in the central hole.