Many concrete men have already heard about the incredible concrete shell roof now nearing completion in Paris. The roof has by far the longest clear span in the world- approximately 715 feet- and is perhaps the clearest demonstration yet of the almost unlimited capabilities of concrete shells. The roof is composed of two parallel shells, one above the other, each only 2 and one half inches thick and joined by precast webs and diaphragm ranging from 6 to 9 feet high. It covers a column-free area of some 225,000 square feet, more than 5 acres. The unique engineering and construction problems created by a roof of this unprecedented size, and the solutions found for them, are of considerable importance to the designers and builders of large shell roofs in this country. Because of the size of the roof, the critical factors in its design were weight and safety. Two single shell designs were investigated, but both were rejected on the basis of an insufficient safety factor. In a roof of this size, buckling was also a critical factor. And it would have been sensitive to small construction inaccuracies and differential loads. By building two parallel shells, the buckling problem was overcome. The shells also were corrugated to provide resistance to local buckling without the use of numerous internal braces.