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