Terracotta roofs in morning fog

Seen from above, the pattern of a tile roof comes from how the tiles are laid. The Portuguese capa e canal tile is concave and laid in rows with alternating, inverted positions, as Portuguese Wikipedia puts it, so that one row of tiles carries water and the next covers the joint. English Wikipedia describes the same two-piece logic in the Roman imbrex and tegula: the tegula is a flat tile laid on the roof surface and the imbrex a semi-cylindrical tile laid over the joint between tegulae. Later tegulae had raised edges to channel water away from the gaps.

The English article says Roman roofs generally avoided nails and relied on gravity to hold the tiles, which may explain why Roman tile appears on low-pitched roofs, and that fired tiles began to replace thatch on Corinthian temples between roughly 700 and 650 BC, largely for fire resistance. In the mission or monk-and-nun style, both pieces are arched, and early examples were shaped by bending clay over a worker's thigh, which gave the curve and could taper one end. The article does not say this is how Portuguese tiles were made.