I’ve had both felt and synthetic underlayment on different roofs over the years (asphalt shingles, northeast climate—lots of freeze/thaw cycles, heavy snow, and the occasional sideways rain). From what I’ve seen, synthetic definitely shines when it comes to exposure during installation. I had a project delayed by almost two weeks due to a surprise nor’easter, and the synthetic held up without any leaks or wrinkling. Felt would’ve been a mess in that situation.
Once the shingles are on, though, I’m not convinced there’s a huge difference in day-to-day performance. The underlayment is really just a secondary barrier at that point. For ice dams, the bigger factor seems to be whether you have proper ice & water shield along the eaves and valleys, plus good attic ventilation. I haven’t noticed synthetic underlayment making a noticeable difference with ice dam issues—if warm air’s leaking into the attic and melting snow, you’ll get ice dams no matter what’s underneath the shingles.
Long-term moisture? Maybe synthetic resists rot and mold a bit better if water does get under the shingles, but if you’re seeing that kind of chronic moisture, there’s probably a bigger problem with the roof design or ventilation. I will say synthetic is lighter and easier to handle, especially on steeper pitches, but the cost is definitely higher. I guess it comes down to how much peace of mind you want during installation versus after everything’s buttoned up.
Curious if anyone’s actually had a roof leak years down the line where the underlayment made a difference? Or maybe someone’s seen felt break down faster under shingles in our climate? I’ve only replaced roofs after 20+ years, so hard to say if the underlayment was ever the weak link...
Curious if anyone’s actually had a roof leak years down the line where the underlayment made a difference? Or maybe someone’s seen felt break down faster under shingles in our climate?
I’ve torn off a couple old roofs around here (central MA, so similar weather) and the felt underlayment always looked like it’d been through a war zone. Brittle, torn in spots, sometimes even stuck to the decking like old gum. But honestly, the leaks I found were always traced back to bad flashing or ice damming, not the felt itself giving up. One time I did see mold between the felt and the wood, but that attic was basically a sauna—zero ventilation.
Synthetic definitely feels more “space age” when you’re rolling it out. Last summer I helped on a roof where we left synthetic exposed for almost a month (thanks, rain delays), and it was totally fine. Can’t imagine felt surviving that.
But yeah, once the shingles are on, I don’t know if it really matters unless you’ve got bigger issues. If you’re seeing chronic leaks, probably time to look at the attic or the gutters before blaming the underlayment.
That’s interesting about the mold—makes me wonder how much attic ventilation really factors in compared to underlayment type. I’ve always heard that synthetic is supposed to last longer, but I’m not sure if that’s just marketing or if anyone’s actually seen it hold up better after, say, 20 years under shingles. Has anyone done a tear-off where synthetic was used and could see the difference firsthand?
I’m in a similar climate (southern NH), and my last roof had 15# felt. After 18 years, the felt was basically dust in some spots, but the decking was still solid. No leaks except where ice dams backed up, which I guess is more about insulation and ventilation than what’s under the shingles. I’m leaning toward synthetic for my next roof just for the peace of mind if there’s a delay, but I keep wondering if it’s worth the extra cost long-term, or if it’s just insurance for the install phase.
Anyone ever had warranty issues where the underlayment type actually mattered? Or is it really just about what happens before the shingles go on?
my last roof had 15# felt. After 18 years, the felt was basically dust in some spots, but the decking was still solid.
That’s almost exactly what happened with mine (central MA). The old felt just disintegrated, but the plywood underneath was fine except where I had ice dam leaks. I went with synthetic this time mostly for the install—weather delays are a pain and it just seemed tougher. Haven’t seen a warranty actually denied over underlayment type, at least not from neighbors or my own experience. If you’ve got decent ventilation and keep up with insulation, I honestly think either works, but synthetic does feel like cheap insurance when the weather turns mid-project.
- Felt turning to dust after 15+ years seems pretty normal from what I've seen. Decking holding up is a good sign—means the main job got done.
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Couldn’t agree more. I’ve watched guys scramble to tarp over felt that ripped up in a storm... synthetic just sits there and shrugs it off.“synthetic does feel like cheap insurance when the weather turns mid-project.”
- Haven’t heard of anyone getting denied on warranty for underlayment either, unless it was installed totally wrong.
- Ventilation and insulation are huge. If you’ve got those dialed in, underlayment is almost a backup plan anyway.
- Only thing I’d add: synthetic can get slippery if you’re working on a steep pitch or early morning dew. Not a dealbreaker, but worth keeping in mind.
- All in all, sounds like you made the right call for your situation. I’d probably do the same if I was re-roofing today.