Why do sloped roofs help protect buildings from rainwater?

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construction of apartment buildings — construction Apartment buildings under construction in Cambridge, England.

Sloped roofs are designed to protect buildings from rain by utilizing gravity to facilitate drainage. By setting a roof at a pitch or slope, water is directed off the surface rather than pooling, which prevents moisture from penetrating the structure.1

Historically, this design was essential for early thatched roofs, where the angle ensured that rainfall would drain away effectively. In modern construction, while flat roofs rely on waterproof membranes like tar and felt to repel water, sloped roofs typically utilize overlapping materials—such as shingles, tiles, or metal sheets—arranged to prevent water from passing through the joints. By shedding water quickly, the sloped design minimizes the risk of leaks and water damage to the building.1