An external building surface formed by at least one of a roof and a wall, the external building surface comprising a plurality of battens spaced apart to define a plurality of transverse courses, a plurality of tiles supported by the battens in the courses defined thereby to provide an outer layer of said external building surface, which tiles are transparent to solar radiation, a plurality of strips arranged one for each course to form an inner layer substantially coextensive with the outer layer, which strips are absorbent to solar radiation, and a duct for a fluid heat transfer medium of a solar heating system extending lengthwise of the strips so that in use the fluid medium is heated when the strips absorb solar radiation incident through the tiles, wherein in each course there is a space wherein air is held over and along the strip in that course.