I am expecting that we could put wire mesh over the straw insulation on the dome and just shot-crete the roof with an inch or two, as would be done for the outside of the bale walls. We could get playful and treat it as kind of a decorative seam around the dome. So, here's a challenge to the rest of you. Your mother is a wise women in search of the simple life with low maintenance. Listen to her.
Water usually runs downhill.
I learned my trade in repair and remodel first. I can't count how many flat roof and deck systems I've seen fail. If I have to do a flat deck now I have at least 2 redundant fail-safe systems. Opening a can of worms to all who care to comment. Here is another Low Cost Roof Suggestion.