Abstract: A building (1) has a roof structure (2) clad with a plurality of ambient heat collection tiles in the form of panels (3), mounted on parallel battens (4) carried by roof rafters (10), and disposed in edge-overlapping relationship. The panels (3) are of identical form. Each panel comprises a plank-like aluminum extrusion of substantial length and of rectangular plan form, having edge-coupling portions (5, 6). The interior of each panel (3) is formed with pair of substantially parallel ducts (7, 8) disposed side by side, which define flow and return paths respectively, for a heat transfer fluid, such as water. The flow and return ducts (7, 8) are divided from each other by an integral barrier wall (9). A panel (3) is not of uniform lateral cross section throughout its length. The cross section is uniform except at the ends where the dividing wall (9) has been cut away at one end so as to allow water to pass from the flow path (7) to the return path (8) in a re-entrant manner.