The flags are laid to create a perimeter that the grennhouse base sits on. The flags under lap the edge of the house by about 5″, so roughly 95% of the floor space inside the house is soil. I then covered this with about 4-5″ of well rotted manure and green waste compost to create a no-dig undercover growing area.
I understand your dilemma re: lifting; I went for 2×2′ flags because 3×2′ are too heavy for me! Perhaps a willing friend could help? If you go with wood, I would treat it heavily with something like osmo oil as Charles recommends – otherwise it will rot within a few short years.
Also, the 6:1 mortar mix will not set like concrete – so if I leave the site and the next tennant wants to remove the house, the base will crumble away with a pick axe quite easily.