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Asphalt shingles, clay and cement tiles, and wood shingles and shakes work well on steep slopes, where rain and melting snow drain rapidly. On shallow slopes, however, wind-driven rain and ice dams at the eaves can cause water to back up under the roofing and find its way into the house. That's why single-ply roofing materials, such as EPDM and modified bitumen, are a better choice for shallow slopes. But in many cases, putting a single-ply material on a shallow roof — a porch or a shed dormer, for example — clashes with the shingles used on steeper roof planes on the same house. Builders who want to use shingles on a shallow