Check my math...Earthbag Building

After coming up with this version of the houseplan (which I still really like), I've been working on the roof plan and doing some math to see what we need to order for bags and fill.

You might notice I moved the water tank and tweaked the kitchen a bit after rewatching the 1949 kitchen video.  Other than that, everything is the same.

Here's what I have so far.  Mind you, math is NOT my best subject, so please check my figures:

Roof:  The roof will be 30'x40' which gives a 2' overhang over the entire building.  4 beams across the wall sections give support for the mono pitch trusses we're planning on using.  My research says 1:12 pitch for turf roof.  Since the back is curved, I added posts to support the end of the beam.  At 1200 square feet, the roof is almost twice as big as the interior of the house.  I put the trusses 2' on center because the turf roof will be heavy and snow isn't going to slide off of it (not that we usually get bad snows here) The root cellar will have it's own roof of beams and will be completely covered in turf.

Math:

According to Owen Geiger, a filled 18"x30" poly bag (that's what we're ordering) gives you 2/3 of a square foot. (5"x15"x22")

The "U" part of the wall is 78.5 linear feet (I rounded to 79). The root cellar wall is 18 linear feet.  The dividing wall between the sunroom and kitchen area is 26 linear feet.
The wall between the living/bedroom area will be cobwood as will the sunporch wall.

79+18+26= 123 linear feet total.  Figuring 8' walls that give me 984 square feet of wall

984 /.7 = 1405 bags.  We're going to order 2000 because that's the best deal and we'll probably use them for other projects. $880/2000

Now for the fill.  We're using screenings from the limestone quarry just up the road.  This stuff is really inexpensive ($5 a ton) and packs SOLID!

Here's my Estimate - Could definitely be off...

1 filled bag @ 100#

1405 bags x 100# = 140,500# / 2000 (1 ton) = 70.25 tons / 10 (tons in a load) - 7.25 loads - We'll get 8.  $1600 @ $200 per load delivered.  We could definitely save money if we got larger loads, but I'm skeptical that a 20 ton truck could navigate our driveway.

Barbed wire: comes in 1320' rolls / 123' of linear wall = 10.73 runs per roll / 2 (doubled runs on courses) = 5.36 courses per roll.  20 courses @ 5" per course = 4 rolls - $160

1 roll of baler twine for tying stuff and woven hurricane straps $40

Grand total $2680

I'm sure there will be things I haven't thought of, but I think I have a decent handle on it.  I should probably be terrified at the though of moving 140,500 pounds of earth by hand, but I'm not.  After all, how does an ant eat an elephant?  One Bite at a Time!!

Of course this doesn't include the foundation stuff, but I haven't gotten that far.

Do these initial figures seem like they're in the ballpark?

Just bumping this up.  I realized my title was too similar to my last post....

Hi, yes I checked your numbers on the bags and barbed wire, estimates look good.

Thank you! Math was never my best subject....

Your price on barbed wire jumps out at me as questionable.

Locally the cheapest barbed wire is $75 a roll and the thick stuff is $120.

How are you sizing your trusses? (did you have someone engineer them)
And what are you using for the roof deck on top of the trusses?

I ask because I question if 24" on center is close enough.

How big are your cross beams?

The two beams towards the front of the house(bottom of page) are supported by the walls below so they don't concern me too much.

But the two beams towards the back of the house(top of page) have spans of 8 and 10 feet that aren't supported between the walls and posts.  With a turf roof  those need to be pretty big beams.  I  haven't done any math on your design but based on similar designs I have done the math for those back two beams (if made from a single spruce timber)should be about 12x10's.

In the last few years I have done a fair amount of sizing log for beams and joists and they always end up needing to be much closer together, and better supported and larger than I originally expected.

Screenings only pack tight when rammed or vibrated.

I recommend switching the bathroom with the bedroom, so that it has an emergency exit window?
The plumbing might also prefer being on the bermed side with temp that are more stable and above freezing.