I've read a lot about cob and rammed earth, seen a lot of pictures detailing ways to attach the roof to the TOP of the wall, but none showing how to connect something like a porch roof to the SIDE of a wall. Here in central Missouri, that's an important detail in modern construction methods--I assume it's got to be even more important with earthen walls. How do you make that connection water resistant, particularly if it is a free-flowing (rounded) vertical wall? Modern construction techniques insist that stucco plaster is somehow supposed to overlap a metalic flashing...but how do you get a metal flashing to conform to and adhere to a cob or RE wall? Also:
1. Wouldn't a plaster finish simply crack horizontally where it flows over the flashing, and just fall off?
2. Will the plaster (and yes, I know it can't be portland cement based) just wick water back up into the wall?
A little background:
Somewhere I read 'look out your window, and build with what you see." Well, that would be silty loam, anywhere from 5" to 5' thick on 25% north-facing slope under a mixed oak and hickorycanopy, sliding down to a little creek with a moderate amount of coarse sandy gravel. I would love to build a two-story workshop: a lower level for grungy work, like a one-bay garage, and an upper level for fine woodworking and art studio (no dirty-handed mechanics allowed). Outside, I envision a shed roof (attached like a porch roof to the tall vertical exterior wall) for a little saw-mill operation.
The big difficulty would be protecting the wall above the shed roof from splashback. You need to make at least a foot, maybe more, waterproof, possibly by turning the roofing material and running it vertically, or with high flashing, or...
I would make as much overhang on the roof above as possible also. The steeper the shed roof pitch, the more rain will splash away from the wall. How far above the shed roof will the main roof be?
For the flashing to plaster transition, I would not try to run the plaster down over the flashing, but give the top of the flashing a 45 degree angle and run it into the plaster a couple of inches. Any water running down the wall would be trained outward. Recessing the flashing an inch at the top, with a sharp bottom edge to the plaster, might allow water to drip off and not meet the angled surface of flashing to possibly be wicked up.
The way you describe your soil, I would have doubts about trying to use it as is for rammed earth, and especially cob. You need a certain percentage of clay for binder. Do you have any source of clay or clayey soil?
From The Owner Built Home by Ken Kern. Like you, I did not find much on the edges or undersides where roof meets wall. I suppose it's one of those things where you get there, and figure it out.
I am also interested in rammed earth construction. My concern with your description is how you plan to fix the roof to the body of the house. This is serious shit, man. People could die.
In Puebla or "Santa Fe" type of adobe structures, the roof is essentially another floor resting on big beams (withcrosswise reinforcement at least two ways with reeds or ocotillo, or something thin and strong) that puncture the structure toward the upper third of the wall, all the way around the building. The roof is almost but not quite flat, with pourholes cut at intervals on the lower side to let the rain out. These holes are lined with another material to prevent erosion, and are often decorative.
If an apex style frame is what you're after, then there are basic measures that determine the slope and distance--like maximum amount of snowfall, prevalence of hurricanes, etc. Fixing this kind of roof to an earthen home is already a question mark--whatever you go with, please please pay the couple hundred bucks to have an engineer look over your plan before you do this.
Looks like you have some great, detailed answers up there.
I thought of a video that might also help you to visualize some methods and how they work. It's of a community center built by the Mexican Institute for Community Development (IMDEC), and designed to be highly earthquake resistant. Their website is in Spanish, but the video below is translated. Two building types are shown in the video, an adobe structure, and the community center made from wattle and daub panels - which they call "bahareque" or "bajhareque" in Spanish. The way they do wattle and daub is basically using a timber frame structure, with the wattle and daub/bahareque panels inserted within the framework. So the framework is the finishing on the sides, and is deliberately visible. It reminds me of some of the Japanese building styles, even though this is quite different. But both are communities concerned about earthquake survival, maybe that led to some similarities?
In the video below, they talk about some of the advantages of this method versus wattling in place. For one, you can have an inexperienced crew wattling the frames in one section, while others place them, and others mud them. Assembly line style. This may not be of use to you, but I thought you might find it interesting.
It's a fun video to watch. A ton of untrained volunteers helped build the community center. Very inspiring to see modern use of these sorts of traditional building styles. Plus looking at a building that gets finished substantially by untrained people; that's amazing. I have a hard time organizing friends at a dinner party, let alone a bunch of untrained strangers to build a house!
Be joyful, though you have considered all the facts. ~Wendell Berry
Glen, what is the right way to attach flashing to an earthen wall? I can't picture an adhesive being able to stick anything, whether EDPM or coil stock to a "dirt" wall. I've already learned from modern connstruction that nails rarely create a water-tight penetration (I've always wondered about the thousands of holes in house-wrap that are created when you staple or nail it on).
I'm still unsure what soil I have. The county soil survey calls it silty-loam, meaning the equal proportions of loam (sand/silt/clay) are a little lean in the sand/clay proportion and a little rich in silt. It's slick when wet, like clay, but drains well. A jar test shows a little less than 1/3 sand, but the remaining stuff settles out so evenly that I can't distinguish a line between the sand and silt. It doesn't "rope" or ribbon that well, but a dropped ball sticks together pretty well.
I pounded out three 9x18x3.5" RE blocks. The first wouldn't compress because the soil was too moist, the second was a lot harder, but 7 days later crumbled at the corners a bit easier than I think it should, and although I mixed lime into the third batch, it delaminated at the level where I'd added more soil to the form. I haven't added any straw or sand into the mix yet.
I've never tried to attach flashing to an earthen wall, but my opinion is that you can't do it. What you can do is trap the top of the flashing by angling it deep into the wall (a few inches, say), possibly with a crimp or roll at the top so it can't slip back out even if pulled a bit.
The notion of a separate veranda instead of attached shed roof has merit. It might work having a large roof overhang above (and not too many feet higher than the shed), with just some rafters or beams connecting shed roof to wall, and a foot or so gap between shed roof and vertical wall. This would not make the space totally rainproof, but nearly so, and you would not have to worry about flashing to any extent - just wrap/protect the beam tops and connections.
I haven't researched rammed earth so have no opinion on its composition.