I am going to install Landmark lifetime shingles over an existing roof. The old shingles are flat, and do not leak. The roof is 3/4" tongue and groove over 2x8s on 16" centers 4/12 & 6/12 pitches. I have been advised to install synthetic felt between the existing and new shingles. Another source says not to use felt between the layers. Which is correct, and why? I will install on Halloween (this Sat}
I have done only a handful of roof overs in my time and I have never used felt between the layers or known of anyone who does. I can not really see the purpose of it considering you already have a layer down. Be aware that if you plan on living in the house a long time, the life expectancy of the shingles will decrease with the second layer.
I would advise laying over your old shingles. It will cut the life of your new shingles dramatically. Also, the next time the roof needs to be done, it will take much greater effort to remove two layers of shingles versus one (and considerable cost if you hire it out). Also realize that if your roof is 30 squares, you will be adding about 9000 pounds to your roof. That's a lot of weight that most roof systems are NOT designed to handle.