Layering shingles always sounds like a shortcut, but man, I’ve seen it backfire more than once. Last spring, I helped a neighbor after a hailstorm—he’d gone the “just add a layer” route a few years back. When we finally peeled everything off, the decking was so soft you could poke a screwdriver through it. Ended up costing him way more than if he’d just done the tear-off in the first place.
On the underlayment front, I’ve used synthetic a couple times now. It’s lighter and doesn’t wrinkle up like felt when it gets wet, which is nice if you get caught mid-job by rain. It also holds up better if you’re dealing with ice dams or wind-driven rain. Only downside is it’s pricier, but I figure that’s offset by not having to redo things as often.
As for attic temps, swapping out old insulation made a noticeable difference for me—less ice damming and the upstairs didn’t feel like a sauna in July. But I’ve heard mixed results from others, so maybe it depends on how bad your old stuff was to begin with.
I’ve torn off a lot of double-layer jobs where the bottom layer was hiding all sorts of rot—usually folks think they’re saving money, but it’s like putting a bandaid on a broken leg. Curious if anyone’s actually had a second layer last as long as the first? I’ve never seen it hold up, especially in places with big temp swings.
That’s what I’m worried about—my place has a low slope and we get wild temp swings here. If you do a second layer, does it make ventilation worse? I keep reading mixed things about trapped heat and moisture... anyone actually see that play out?
If you do a second layer, does it make ventilation worse? I keep reading mixed things about trapped heat and moisture... anyone actually see that play out?
I'm right there with you on the confusion. When I bought my place, the inspector warned me about double layers—said it can definitely trap heat, especially with low pitch. Haven’t seen real damage yet, but my neighbor had to deal with some weird attic moisture last winter after going that route. Not sure if it was totally the shingles or just our wild weather swings making things worse. It’s a headache for sure.
That’s a fair point about attic moisture—stacking shingles can definitely complicate things, especially if your roof’s pitch is on the lower side. I’ve seen a few cases where that second layer really did a number on ventilation, mostly because it creates more mass for heat to build up and less opportunity for the roof deck to “breathe.” It’s not just theory—thermal imaging on double-layered roofs often shows higher surface temps, and that heat can radiate down into the attic. If your venting isn’t dialed in, you’re basically asking for condensation issues in winter and higher cooling costs in summer.
There’s also the matter of what’s already under those old shingles. If you’ve got any warped decking or minor leaks hiding out, putting another layer over top just seals in the problem. I get why people do it (cheaper, less mess, quicker turnaround), but it’s kind of like sweeping dirt under the rug. You might not notice anything off for a while, but when you do, it’s usually a bigger headache than if you’d just started fresh.
I’m always surprised how many contractors still push double-layering as a “standard” option. In climates with big temperature swings—like here in the Midwest—it can be a recipe for ice damming and mold. One neighbor went this route to save money, but after two winters, he had to rip everything off anyway because of rot around the eaves. Ended up costing him more in the long run.
If you’re thinking about ventilation, it’s not just about ridge vents or soffits either. The shingle layers themselves can block airflow between the roof deck and the exterior, trapping moisture that would otherwise escape. That’s why most of the newer green roofing guidelines push for full tear-offs before installing anything new, even if it means a bit more upfront work and expense.
It’s not always black and white—some older homes with steep pitches and great venting seem to get away with it. But if you’re already seeing moisture issues nearby, I’d say err on the side of caution. The “out of sight, out of mind” approach rarely pays off with roofs...