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.
