Roof slope is key in determining underlayment type and installation:
4&12 and above:
- #30 felt or comparable ((evaluation report required). Lap ends 6” (IRC 4”), 2” horizontal.
3&12 to less than 4&12:
- Type 90 mineral surfaced underlayment;
- Two layers #30 felt (19” overlap); or
- Built up roof
2½ to less than 3&12:
- Approved built-up roof, 3-ply minimum; or
- Approved single-ply roof membrane assembly
- Tile installed at less than 3&12 is considered decorative only.
- Requirements may differ by jurisdiction, especially in areas designated “high-wind”.
- In areas subject to the development of ice dams, some jurisdictions may require installation of an ice barrier (typically waterproof underlayment).
Typical headlap is 3 inches but can vary with roof pitch.