That trapped moisture thing is exactly what freaks me out about layering. I always picture those old shingles just stewing in dampness like a forgotten sandwich in the back of the fridge. Plus, if you’re ever thinking about going solar or adding a green roof down the line, you really want a clean slate. Full tear-off is a pain, but at least you know what you’re working with... and your roof won’t start sagging like a tired hammock.
I get where you’re coming from with the moisture worries, but I’ve seen a few roofs where layering actually held up better than expected—especially on steeper pitches. My uncle’s place had two layers for almost 15 years, and when we finally tore it off, the old shingles were dry as a bone. I think a lot depends on your attic ventilation and how well the first layer was installed. Not saying it’s always the way to go, but sometimes the horror stories don’t play out in real life... at least not every time.
