Roof slope is key in determining underlayment type and installation:

 

Tile Roof Institute Guidelines:

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

Notes:

  • 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.