Cheap cob roof?

We will have about 1420 sq ft of roofing to do when the building is complete. We're trying to figure out the most affordable yet durable way to add a roof. We are doing an earthbag cob hybrid. We like the quickness of bags and the character of cob. One idea we had was this: build everything as domes, cover it in cob (so we can add sculpting designs onto the houses), then apply a lime plaster. Would that protect the houses enough so that we could build as domes and not have to add a roof to anything? But we also want to catch rainwater, so would it be safe for the rain to roll down the ebag house with the lime plaster on it and into a gutter and downspout into our barrels?

Unless you live in an extreme desert environment where rain is rare, the lime coating may still allow enough water in to build up in the cob. I don't think Cleveland qualifies as extreme desert, if that's where you live. If it ever cracks, there is a guaranteed failure point. You need to shed the water with a covering that lets air circulate between cob and top surface so the cob can stay dry.

Check out the latex cement roofs at Velacreations web site

Glenn is right, I posted on some CA thread I can't remember where dome cob here in CA I am at now and will visit the building site soon that decided to use NHL 2.5 to manage any vapor, rain, climate erosion, s/b NHL 5 or Type S. Once it cracks more than likely in the curing state if not wet cured all is lost since domes see alot of stress from it's own weight/loads, on the outer surfaces. Their thinking NHL 2.5 self heals small cracks on the finished product over time by adsorbing CO2 I don't agree with. Big cracks need to be prevented and they are load driven, NHL 5 is stronger better common to the environmental & structural loads. Hire a PE one that also understands breathable assemblies. Add a vapor barrier like latex/plastics, makes matters worse unless the interior humidly is controlled to less that 35% RH to allow drying in, may do nothing to stop internal cracks that are a problem or not completely homogeneous by the lime binder. Earth domes have span limitations ~12-15' or they can collapse and cause bodily harm, especially in high wind or seismic or humid zones, or if they have internal or exterior cracks. Its tough analyzing dome loads, find a good conservative PE that uses high safety factor like 3. They are efficient no doubt especially to resist internal wind pressures like in tornadoes/hurricanes, but the apex of the upper curve can see alot of stand alone span loads and collapse needing additional structure to the foundation. Some try to make them thin to increase the weight to strength ratio but, the clay binders for the most part do not have enough strength to do that. OPC or another stronger binder, or fiber reinforcements, are needed.

You'll see alot of hype from lime manufactures that claim they can handle big loads I think these natural builders fell prey too, and/or are trying to mark up and sell out here on permies, trying to sell European raw material based limes in the US like here: http://www.limes.us/

An affordable US manufactured Type S lime mortar with a liquid water shedding surface sealer siloxane/silane (natural silicone) 100% + breathable would work great here, or, a magnesium phosphate (POS) stucco you talk to Premier about.....It will dry fast needs no wet curing, you have to be careful of the salt content leaching and the drying speeds. Limes have the same leaching/cracking issues, so does portland cement, but the right ad mixes can mitigate. The difference is the structural and vapor chemical properties of well designed mags are 2-3 times that of lime and/or portland cements, for the same or less CO2 kiln temps as lime ~ 1700 F.

Glenn Herbert wrote:Unless you live in an extreme desert environment where rain is rare, the lime coating may still allow enough water in to build up in the cob. I don't think Cleveland qualifies as extreme desert, if that's where you live. If it ever cracks, there is a guaranteed failure point. You need to shed the water with a covering that lets air circulate between cob and top surface so the cob can stay dry.

What if we just do cob on the inside of house and the outside is lime plaster over the earthbags? Maybe with a coat of linseed oil over it? Would that protect it more? We've decided to build a stand alone roof over the courtyard we're going to build with fiberglass as the roofing and then leave the domes hatless like how I mentioned above if you think that would work. We get about 53" of rain each year.