Timbrel vaults offer another low cost, sustainable roof building method. This method is suitable for do-it-yourself owner builders if you do the research. Suggestions include: use a simple design with a reinforced concrete bond beam and modest spans. Look into using low-fired, lightweight brick like the type used in Mexico and SE Asia.
“The ‘Sustainable Urban Dwelling Unit’ (SUDU) in Ethiopia demonstrates that it is possible to construct multi-story buildings using only soil and stone. By combining timbrel vaults and compressed earth blocks, there is no need for steel, reinforced concrete or even wood to support floors, ceilings and roofs. The SUDU could be a game-changer for African cities, where population grows fast and building materials are scarce.”
Source: No Tech Magazine (This site has hundreds of great articles on low cost living and building!)