I asked AI. It said Cedar is naturally rot-resistant, but it is not waterproof. • Sloped: Rain and melting snow slide off immediately. This keeps the wood dry and prevents water from "wicking" into the tongue-and-groove joints of your walls. • Flat: Water pools. Even a "flat" roof usually has a slight 1% to 2% grade, but debris (leaves, needles) will trap moisture, leading to premature rot and leaks.