shingle a 2/12 pitch roof???

Building codes and shingle manufacturers allow for shingles to be used on slopes as low as 2:12.

But when applying shingles on slopes between 2:12 and 4:12, the code requires that a double layer of felt paper be used that is lapped at 19" intervals.

regarding that 2/12 roof...

someone suggested putting a weather barrier under the shingles vs. felt paper (I assume he was talking about a single layer of felt paper). Would this have the same effect?

Also, do you know anything about the Liberty roofing system from GAF. Several people have suggested that too.

Either way, is there a substantial difference between the shingle and flat roofing systems in terms of warranty and the expected life of the roof? I am somewhat flexible with this roof as you do not see it from the street (though, you do from the back yard).

Thanks for all your help!