A method for fabricating a grid-stiffened structure from fiber-reinforced composite materials. Ribs are formed on a smooth hard base tool. Expansion blocks are placed in the shallow cavities formed by the ribs and the base tool, and held in place by one of several means while a skin is placed over the ribs, expansion blocks, and base tool. The assembly is then placed in a vacuum bag and autoclave cured. After cooling, the formed structure is separated from the base tool and the expansion blocks are removed from the cavities. This abstract is provided to comply with the rules requiring an abstract, and is intended to allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.