two layers is max most places, and some cities like the one I am in require tearoff, inspection of the wood, and when needed laying 7/16" OSB over the old sheathing, or tear off and replace sheathing. I just had a new GAF roof installed a week ago to replace the one from 1996, and the one thing I see that is different, is that product lines like GAF etc., sell a complete package instead of only shingles. They sell the underlayment, the tiles, and the venting. To meet code here the tiles have to be nailed on and not stapled. This new roof is an energy efficient reflective tile, it was 29 squares and cost about $13,000. That includes everything including gutters. My next door neighbor did his roof about 2002, and just laid over. They have lots of dry rot that was never dealt with so the next roof they put on is going to be expensive. A couple of their trusses are drooping so that tells me that the rot is into the trusses in that area. I only had rot on a couple facia boards, but I had complete tear off last time. The tar paper / felt that was under the tiles was so brittle that you can't pick a piece up without it shattering in to 50 pieces. That was 23 year's old, and that is mostly what seals your roof. Can you imagine what the 40 year old paper/felt is like on the house next door that did not tear off? The economy of just laying a new layer over may seem like a savings but it really isn't if moisture gets into the wood and causes rot. It took 5 or 6 guys about 2 hours to tear off this roof, and a trip to the landfill. @ $50 per person that is like $300 per hour or about $650. In 1996 it cost $250 to tear off. I don't see the savings by not tearing off. One saves $650 now and pays $8,000 is dry rot repairs later. That is a negative 10% rate of return.
Yikes. Sounds like fun. BTW, International Existing Building Code only allows two layers of asphalt shingles. The added weight creates a structural issue.
$$$ to fix over insurance but we fixed it.