Foundation and Loft

Building a small 'guest house' as a practice run for our forever home down the road. Heres what I am looking to do:

1)under 200 sqft
2)simple rectangle
3)lean-to style roof
4)sleeping loft
5)load bearing


Two questions that I cant find answers on and am hoping you all might be able to help me out.

My first is concerning the foundation: I am comfortable with using a slab on grade with toe-ups, but do not want to invest the dollars into having that amount of concrete delivered and troweled. So, I am thinking a simple earth bag foundation with an earthen plaster floor. My concerns come in with attaching the bales to the bags and creating a moisture barrier. From what I have read, with toe ups you just use some 16 penny nails to velcro the bales to the lumber, cant really do that with bags. So, should I pin the bales to the bags with some rebar for the first course or two? And, how do I create a moisture barrier between the bags and the bales? Bags filled with stone, I got that, but do I put some plastic down between the layers somehow?


My second concern is the sleeping loft. I would like to just build this on top of the box beam, but i cannot find any information anywhere about calculating load capacity, or if this is even a good idea. (keep in mind this is under 200 sqft and does not need to meet any code requirements) I am thinking it may be a better bet to just build the loft like a free standing deck on the inside of the structure. Not a big deal to do it this way, I would just like to build it all as one, and save a few dollars and not have to buy a bunch of posts.

You can do nail boards, like toe-ups with nails both directions to "velcro" both directions. Build it like a wooden bond beam. That should be enough barrier but tar paper is so cheap you should use it as well.

R Scott wrote:You can do nail boards, like toe-ups with nails both directions to "velcro" both directions. Build it like a wooden bond beam. That should be enough barrier but tar paper is so cheap you should use it as well.

So your saying, build a second 'box beam' as a foundation plate for the top of the bags with nails on both sides and wrapped in tar paper? Should I just use timber, or should I wrap it with plywood and fill it with some insulation, or no plywood and fill it with stone?

Not that complicated. I thought I had a picture, but can't find it or remember which blog I pulled it from.

They used pressure treated plywood, cut into strips a little narrower than the bales--I don't remember if it was 9 or 12 inches (either 4 or 5 strips per sheet). Nails both directions. Simple continuous nailer/porcupine board.

R Scott wrote:Not that complicated. I thought I had a picture, but can't find it or remember which blog I pulled it from.

They used pressure treated plywood, cut into strips a little narrower than the bales--I don't remember if it was 9 or 12 inches (either 4 or 5 strips per sheet). Nails both directions. Simple continuous nailer/porcupine board.

That makes more sense, and a heck of a lot easier. Should I run the straps under the plywood, or should I include a course of bags as well?

Whats your opinion on the loft?

I would include a couple layers of bags.

For the loft, I would build it on top of/part of the box beam. You can span a shockingly large distance if you box it--simply skinning both top and bottom of the joists with ply makes it like 5 times stronger (don't quote the number). It doesn't have to be that thick of ply, either. But you need to glue it in place. You aren't talking a large building, but what is the free-span distance you are looking to do? I think I would do a sleeping loft with 2x6's, 3/4 ply on top and bead board or T-11 underneath (to make a pretty ceiling).