Underlayments are very important to consider. Believe it or not, tile roofing systems will allow water to get below the tile and on to the underlayment. The flat tiles are the worse compared to the ville profiles with humps and pans. This is one reason the battens have a space every four feet so water can escape down the roof instead of being trapped behind the battens. Having said that, you can imagin the underlayment is more important than if you are using another roof product like asphalt shingles.
Tile roofs systems normally use a 36 inch felt. The popular type used is the #30 but the thinner #15 is available is an exceptable standard. I recommend the #30 at minimum but I ask you to consider double felting with the #30 or use synthedics.
These photos are of a tile roof with a shake profile that, after 11 years has leaking issues. Here the tiles are being removed. And a new synthedic underlayment is being installed. We continued with flow through battens and then reset the old but useful tiles..
I have gone as far as installed Dibetin torch down roofing systems proir to the tile application. The tile roofing system is all about the underlayments so use the best you can afford. The tile is there for looks and to protect the underlayment from the UV sun rays.
If this was helpful like us on Facebook or share it with someone with a leaking tile roof.