Installing Rafters On Earthbag Building

Hi there everyone, my name's Jared and here's my first post. Sorry if I should have done an intro post elsewhere as forum etiquette, but I'm short on time and looking for some quick help.

I'm about to start building an EB shed for my mom, and need to hurry and submit plans to the local building code guy (who has already given approval to the general idea of EB).

The building will be, if all goes as planned, 13' 4" wide. We are going to put on some common rafters I'll be making at 4/12 slope. I'm hoping to use 2x6 lumber for the rafters.

My question is, since the wall is going to be approx 15" wide, how do I cut the birdsmouth/seat cut wide enough for the wall without cutting too deep into the rafter? If I cut too deep the rafter will be weak, but if I cut a shallow 15" birdsmouth without maintaining proper angle with the plumb cut I highly doubt things will line up right.

Anybody run into a similar situation? Ideas on how to overcome this? Do you need more info I might have forgotten to mention?

Thanks!

Hi Jared, a little sketch would help a lot.

Don't worry about the introductions. We all tumble around like socks in a dryer here-you'll meet pretty much everyone eventually.

My immediate impression is that if 2 by 6 threatens to be weak after cutting in, they also come in 2 by 8. The cost will increase but it might avoid a redesign which would cost more time and headache.

Hey Michael, thanks for reaching out!

I don't have the best quick drawing skills, and am not sure exactly what you'd like to see in more detail, but here's the drawing from the book I'm using as my main reference showing methods of attaching the roof to the wall. They say even 2x4 lumber will work, but don't talk anything about slant and seat cuts. The picture seems to show some kind of very shallow cut, but I don't know anything about proper method to determine the angle and whether such a cut would be code compliant anywhere.

Anyways, see attached photo for what the book vaguely explains to do.

ROPE!??

Whoah, dude.

That scares the shit out of me. In a hurricane with 100 mph winds, can you imagine being inside a structure held together with.....ROPE?

Don't build for the good days--build for the bad days. The last thing you want to think when you jerk on a roof beam is, "hmmm...ambiguous....."

If ever there was a time and place for 1/2" steel bolts (or even 5/8") with locknuts and washers on both sides, this woukd be it.

You want to anchor way down deep in the mass of a wall--say, six feet of steel pipe, or a large wooden beam, something with high tensile strength, anchored really really good beneath hundred and hundreds of pounds of something--then mount (at least the corners) to that.

Rope can be surprisingly strong and could quite possibly be stronger than a connection using nails or screws. The rope used for staw bales can take 200 - 400kg (440 - 880 pounds). As a loop has two sides it holds twice that.
So a few loops will easily exceed the strength of the beams.

You will install a wooden top plate on top of the earthbag structure.  Your birdsmouth will be cut to the wooden top plate, not the whole earthbag wall so no significant weakening.  You could use the rope method of attaching the top plate.  You could also get some 3/8 - 1/2" threaded rod and bend one end into an L and install it 2-3 courses from the top every 4 feet or so.  The two top courses will be impaled onto the threaded rod.  You will probably want to sharpen the end of the rod as well to allow easier impaling.  Then you can bolt on the top plate.  The top plate will also distribute your roof loads so you're not getting point pressure on the earthbag wall.  You will probably need to use a 4x4 or 4x6 as your top plate if you want to keep it in the middle or use a 2x and push it toward the outside so your rafter tails will clear the earthbags.  I would opt for a doubled 2x top plate as close to the center of the wall as possible so it is a little stronger and overlap the corners so the top is bound together more effectively.  I believe I also remember from the earthbag book that you can pour a cement bond beam on the top which seems way overkill for a shed.  Ask your inspector which method he would prefer.  The doubled 2x is what he is going to used to as that is the method used in stick framing.  But ask about the method of attaching it to the earthbags.  Good luck sounds but, careful with your back.

Oh.  Another method would be to put a 2x4 between the bags 2 courses below the top.  Then attach the top plate with long timberlocks through the top plate, 2 courses of bags, and into the buried 2x4.  That would probably be the strongest as there is no reliance on the tensile strength of the bags.  That would also be much easier than trying to impale bags into threaded rods while installing.

Hello Jared,

I was wondering the same question... I've also seen people doing it like in this youtube video:

https://www.youtube.com/watch?v=xFKVMX0p6Lc

It's basically a velcro plate embedded in earth bags a few layers below with 2 vertical pieces of plywood (Those are strengthened by the next 2 layers of earthbags pushing against it's sides). Once the rafters are installed, you continue the last 2 layers of earthbags to tighten everything together.

See this video and forward at 8:06
https://www.youtube.com/watch?v=Bh6N_WWGwkU&feature=share

I hope this helps

Jan

Jan Corriveau wrote:Hello Jared,

I was wondering the same question... I've also seen people doing it like in this youtube video:

https://www.youtube.com/watch?v=xFKVMX0p6Lc

It's basically a velcro plate embedded in earth bags a few layers below with 2 vertical pieces of plywood (Those are strengthened by the next 2 layers of earthbags pushing against it's sides). Once the rafters are installed, you continue the last 2 layers of earthbags to tighten everything together.

See this video and forward at 8:06
https://www.youtube.com/watch?v=Bh6N_WWGwkU&feature=share

I hope this helps

Jan

Howdy!
I read this whole thread and have also watched the video that is linked above to how the family at https://www.youtube.com/watch?v=Bh6N_WWGwkU&feature=share attached their rafters for building a reciprocal roof on a round earthbag structure. I think their method of installing the rafters on top of velcro plates with attached sidewalls for the rafters to drop in-between is a brilliant idea. The plates are anchored by the next row of earthbags. The extra height of the attached sidewalls allows the rafters to be aligned level with each other even when the earthbag wall is not. Brilliant!  

A reciprocal roof does not need a bond beam or a tension ring like a radiating yurt roof. A reciprocal roof twists under its own weight causing it to tighten upon itself rather than spread like a radiating roof.
If you are building a rectangular wall system the rafters can be supported by a ridge beam, or a truss in the shape of a gable or as a shed style roof.
Any of these roof designs exert very little pressure out to the side.

For building a small rectangular shed interlocking the bags at the corners and anchoring the rafters to the walls in the fashion shown in the video above should be plenty adequate for structural integrity. Ideally, earthbag walls have reinforced tensile strength in every row from the two-strands of 4-point barbed wire. Embedding steel bolts in the walls has shown to rust over time. Better to marry gravity and mass as an anchor as the Anasazi Natives have done for over 1200 years!

Kaki Hunter