For low slope shingle roofs like mine, the city requires 2 layers of felt. My 3:12 does not actually require it, but I was planning on using 2 layers of #15.
A roofer suggested that 2 layers of #15 had no benefit over 1 layer of #30.
In that case I would use 2 layers of #30. What do you think?
I use 30# on all my sloped roofs. I do not use 15#. Minimum code sucks. However I don't see any added benefit of using 2 layers of 30#. Are you going to use ice shield at your gutter lines and valley areas? (I don't think you don't get snow in San Diego, but the ice shield also helps against water infiltration, especially in the valleys.)
The information found in this post is not to be considered legal advice. The author of this post takes no responsibility for any losses that occur. Use at your own risk.