The Tile Roof Institute Installation Manual says that on 4:12 roof slope(33% Slope) or greater a single layer of ASTM D226 Type II (no. 30) (ASTM D4869 Type IV) felt paper or approved equal shall be applied shingle fashion. The industry standard for tile roofing, in short, is one layer of 30lb (ASTM) roofing felt paper. There are many many new underlayments on the market all claiming to be the newest greatest underlayment replacing 30lb felt paper. There are a plethora of different brands, too many to be compared here.
Tried and True: 30lb Felt paper as specified above in the TRI installation manual is the tried and true underlayment for shakes, composition shingles, and tile roofing. If you want some extra peace of mind Brazil Quality Roofing, Inc. has recommended double felting the roof with two (2) layers of 30lb (ASTM) roofing felt paper. If a a heavy wind blows rain and water up the roof slope on your roof tile then you will have two layers of felt paper sealing the nail holes to keep the water out, or if a branch falls and breaks some roof tiles during a rain storm, the two layers of felt paper will carry the load through the rain storm until it is safe to get on the roof and the tile be replaced.
(Picture above: 30lb Felt underlayment being installed to the roof deck.)
Double felt is also a requirement for certain roof deck conditions and applications of certain roof tiles.
Caution Needed: No underlayment such as 30lb Felt paper or other synthetic underlayments are a "stand alone roof system." They serve as a temporary weather barrier during the installation of the tile. Underlayments are a back up temporary weatherproofing system for instances discussed in the above paragraph. If water continues to penetrate under the roof tile and get onto any underlayment there is potential for the underlayment to rot out and leak in a matter of time. It has been found that if felt paper is properly installed under a tile roof and is not exposed to the elements that it will retain its weatherproofing properties. Many of the new synthetic underlayment products have not been tested over such a long period of time, and only time will tell if these products will hold together and retain weatherproofing properties for ten, twenty, thirty, forty, fifty years and beyond.
If you the homeowner are going to upgrade something on your roof, it is recommended that you first upgrade your metal work, because metal flashing's and the tile roof itself are what carry the heavy load of ensuring a tile roof does not leak. See FAQ page "How do I know my tile roof is installed correctly."
