If underlayments could talk: which one would outlast the others?

That’s rough about having to redo the section—seems like roofing always finds a way to teach us patience. I’ve noticed the same thing with synthetic underlayment on my last job. It just seemed to “settle in” better through the hot-cold swings, and didn’t wrinkle like felt sometimes does after a cold snap. Not sure if it’s just the brand we used or what, but it stayed pretty flat even when the temps dropped overnight.

One thing I’m still figuring out—does synthetic actually breathe better, or is that just marketing? On a humid day, I felt like the attic was less stuffy, but maybe that’s just wishful thinking. Anyone else ever get weird static shocks working with synthetic too? Not a dealbreaker, but it definitely surprised me the first time.

Good on you for sharing the sanding story. Makes me double-check my prep every time, even when it feels like overkill.

I’ve noticed the static too—caught me off guard the first time, especially when I was working in dry weather. As for breathability, I’m a bit skeptical. Most synthetics are actually less permeable than felt, at least according to the specs I’ve seen. Maybe the attic felt less stuffy because of better venting or just less moisture trapped under the underlayment? Either way, I do like how flat it stays, especially after a cold snap. Wrinkled felt always made me nervous about shingle adhesion.

Static’s definitely a weird one—first time I rolled out synthetic, I thought I was getting zapped by the house. I’m with you on the breathability thing. I keep seeing marketing about “breathable synthetics,” but when you actually check the perm ratings, most are tighter than old-school felt. Maybe it’s just that synthetic doesn’t soak up moisture and wrinkle like felt does, so things feel drier overall? Hard to say.

I’ve always been a bit paranoid about wrinkles under shingles too. Last time I used felt, it bubbled up after a cold night and I had to go back and re-tack a bunch of spots. Not fun, especially on a steep slope. The synthetics do stay flat, which is a relief—less fussing around with staples and trying to smooth things out before the shingles go down.

One thing I noticed, though, is that synthetic can get slick if there’s even a hint of frost or dew. Nearly slid right off my shed roof last fall. Felt’s not perfect, but at least it gives a bit more grip underfoot (for better or worse). Trade-offs everywhere.

I guess at the end of the day, it comes down to what headaches you want to avoid. For me, dealing with less wrinkling and not having to rush before rain hits is worth the static and slipperiness. But if you’re in a spot where breathability matters—like an old house with questionable attic venting—I’d probably stick with felt or at least double-check what’s going on up there.

You’re not alone in being skeptical. Specs are one thing, but real-world experience always tells a different story. If your attic feels better now, maybe you lucked out with the venting or just got a good install. Either way, sounds like you’re paying attention to the right details.

That static is no joke—first time I unrolled synthetic, my hair stood up like I’d licked a battery. I get what you mean about the perm ratings too. Marketing makes it sound like synthetics breathe, but when you dig into the numbers, most are way tighter than felt. For my place (1950s ranch, not much attic venting), I stuck with #30 felt just to be safe. It wrinkled a bit after a cold snap, but once the shingles went down, it flattened out okay. If I had better venting or was in a drier climate, I’d probably go synthetic for the flatness and rain resistance. But yeah, that slick surface is sketchy—almost lost my footing more than once. Trade-offs everywhere...

That slick surface on synthetic gets me every time—especially if there's a little frost or dust. I hear you on the perm ratings too.

Marketing makes it sound like synthetics breathe, but when you dig into the numbers, most are way tighter than felt.