A method and corresponding structure using phase change materials to control the heat flux into and out of a building through the roof To control the heat flux through the roof, phase change materials are arranged in two locations inside the roof structure. A first phase change material is used in the upper part of the roof above the insulation. A second phase change material is used in the lower part of the roof below the insulation. The melting point of the first phase change material is higher than the crystallization point of the second phase change material.