Problem
What is the best shape for a roof?
Solution
Build the roof vault either as a cylindrical barrel vault, or like a pitched roof with a slight convex curve in each of the two sloping sides. Put in undulations along the vault, to make the shell more effective. The curvature of the main shell, and of the undulations, can vary with the span; the bigger the span, the deeper the curvature and undulations need to be.
Related Patterns
… if the roof is a flat Roof Garden (118), it can be built just like any Floor-Ceiling Vaults (219). But when it is a sloping roof, according to the character of Sheltering Roof (117), it needs a new construction, specifically adapted to the shape which can enclose a volume.
Leave space for dormers at intervals along the vault - Dormer Windows (231), and build them integral with it. Finish the roof with Roof Caps (232). And once the vault is complete, it needs a waterproof paint or skin applied to its outer surface - Lapped Outside Walls (234). It can be painted white to protect it against the sun; the undulations will carry the rainwater …
Alexander, Christopher. A Pattern Language: Towns, Buildings, Construction. Oxford University Press, 1977, p. 1036.