This is the exact same size and shape (circular) abode that I intend to build. I've done a ton of research on this, built spreadsheet models, CAD drawings, etc. A two-story, twelve-foot radius roundhouse with a loft breaks the 1000 square foot barrier for house-versus-cabin. If you have a center post (one only), not only will you find it FAR easier to build a roof with something to pin to, but in addition you will reduce your horizontal clear spans to twelve feet, a very important consideration in putting in second stories or lofts.
People have lived in circular houses and teepees for ages and have yet to find themselves desperately seeking square angles, like they had a gene for that or something. You build-in everything on the walls . . . not like Americans, who stack their furniture in front of the walls. The walls are rock-hard, you use them, and you don't build sideways, you build up. You put planters around the central column. You build-in a rocket mass heater, heating a couch, run it halfway around the circumference, exit with a stove pipe and put your stove right there, using the same exit stovepipe. Stairs go the other way around. There is plenty of perimeter to work with. Use those walls, put shelves and pegs everywhere above eye level, for instance.
As for the roof, go with a 3-4-5 triangle (53 degrees, or rise of 4, for run of 3, for cant of 5) because that is the traditional roof pitch for all ancient roundhouses, for many and various excellent reasons. (Mainly, if you don't do it right in the first place, do you have the money and time to do it all over again?) DON'T be so stupid as to make a roof with a big hole in it.
Let the roof overhang the walls by at least a yard to protect against rain and give it a dry, high foundation. Slope the ground away from the building and dig ditches to carry away any water. Never use concrete finishes like stucco, or oils, or paint, or sealants on it. They trap water inside. That or any roof leak will ruin it.
What you use on your roof depends on your climate. Shingles or roof tiles are okay for temperate climes, to keep rain and the occasional snow out. Yurt roofs are fine providing you repair or replace the canvas and felt yearly -- and people also lift these roofs up, slide inside and steal your belongings. For Alaska, I would definitely consider two-foot thick thatch and an overbuilt roof to handle heavy, thick snow load. Don't plan on snow just sliding off. Does it just slide off your car windows?
You said you were going to build your roundhouse to house a water tank? Are you kidding? You can't tar it or plaster it?