A method for fabricating a grid-stiffened structure from fiber-reinforced composite materials. A skin is applied to a smooth, hard base tool. Ribs comprised of carbon-fiber tows are formed on the skin, and shallow cavities are formed between the ribs and the skin. An expansion block is placed in each of the cavities, and is held in place by an elastomeric contact adhesive having adhesive properties that are substantially diminished when the adhesive is heated to an elevated curing temperature. The assembly is then autoclave cured. After cooling, the formed structure is separated from the base tool and the expansion blocks are removed from the cavities. A grid-stiffened sandwich structure is formed by applying an outer skin over the ribs and expansion blocks, before curing the assembly. After cooling, the outer skin is removed to allow extraction of the blocks, and subsequently bonded to the ribs.