[Cob] roof support, earth bags w/cob plaster, lime wash

[Jilly]

Why is it that if you build a cob wall, you can place windows anywhere you like, but if you use earth in bags (i.e. Cal Earth style) you can not place any windows in the long sides of the vault? How is the roof support different in cob vs. the bags? 


As copied from Cal Earth email to me:
"Hi, vaults must have solid walls on both sides, because the entire load of 
the roof is carried by the walls. If openings are necessary then buttressing 
must be created on both sides of the walls and have a very strong arched lintel. 
But the end walls are almost unlimited in the amount of windows, unless you 
need to stiffen them for the earthquake area.  Regards,  Cal-Earth"

So, if I "bump out the wall" ...to make a "built in couch" with windows at the back of the couch and solid sides that form a buttress, can I sufficiently support the roof?

Also, books I read suggest cob to cover the earth-filled bags. But if there is no overhanging roof (they are domes and vaults) the cob isn't protected - aside from the lime weatherproofing wash they put on. Why couldn't you use this same lime wash on a solid cob wall? In the book, "Building with Earth" there is a shower shown, that is all earthen with a painted lime on top. So, can you use the same method outside on the cob walls? Can you use this to make the kitchen sink as well? 

We have made several things (benches, walls, sculptures, etc) with various building methods to see how they come out. The earth bag building seems attractive to me, as the mix is a "no-brainer", with no worries about getting a cob mix just right. This is especially relieving in putting on roofs, or lofts... 
Thoughts? 

Thanks, 
jilly