S Benji - From reading some more in John Hait's book, it looks like he recommends 24 inches of dirt above the waterproof umbrella but because of the weight that that causes, he has vey little dirt between the house and the umbrella. He did say that the increased soil depth on the plastic kept you from having your roof turn brown from lack of soil moisture and that it also created a sort of transition zone between the outside and the home.
I'm still only in the 3rd chapter though so my understanding may be incomplete yet.
It seems like if I used 24 inches above the waterproof layer, then I would need significant insulation between the house and that since the freezing depth itself would be two feet in the winter in the area I'm planning to build.
You're right though in that I was asking how strong the roofs were designed for on the existing WOFATIs just to get an idea of what kinds of structural strengths might be needed.
I like the longer rectangle design of the Earthship homes though. It seems you would have more natural lighting that way. Where the house is basically only one room deep and all the rooms have one side that opens to the windows to allow light penetration.