Which holds up better over time: felt or synthetic underlayment?

Had a similar experience with synthetic under metal, actually.

“I’ve seen some of these newer synthetics get brittle or curl up after a few years under metal - especially if there’s a lot of sun exposure.”

- Had a similar issue with synthetic underlayment on a low-slope metal roof, southeast Texas.
- After about 5 years, the stuff got brittle and started tearing near the fasteners - especially on the sunniest side.
- Felt isn’t perfect, but I’ve noticed it tends to stay flexible longer, even when it gets damp.
- Synthetics are lighter and easier to roll out, but in hot, humid climates, I’m not sure they’re worth the trade-off.
- If I had to do it again, I’d probably stick with a heavier felt or look into some of the newer “breathable” options, though those are pricier.

That’s interesting - especially the part about the synthetic tearing up after just five years. I’m in central Louisiana, so not too far off climate-wise, and I’ve been going back and forth on this exact thing. Here’s how I broke it down for myself:

Step 1: Figure out what you’re up against. For me, it’s heat, humidity, and the occasional hurricane. I kept reading that synthetics are “tougher,” but then I saw stuff like this:

After about 5 years, the stuff got brittle and started tearing near the fasteners - especially on the sunniest side.

Step 2: Compare the install. Synthetics are definitely easier to handle solo (I’m not exactly a pro up there), but if it’s going to crack in the sun, that’s a problem.

Step 3: Think about longevity. I’ve seen old felt underlayment still hanging on after 10+ years, even if it’s a bit soggy. Not perfect, but at least it doesn’t just disintegrate.

Step 4: Budget check. The “breathable” options look cool, but my wallet says nope for now.

I’m leaning toward a heavier felt too, maybe with some extra fasteners on the windward side. If it gets me through a decade, I’ll call it a win.

I kept reading that synthetics are “tougher,” but then I saw stuff like this:

Step 2: Compare the install.

I get where you’re coming from, but I’ve actually had the opposite experience with synthetics. My neighbor used a mid-grade synthetic on his shed, and after two hurricanes and a ton of sun, it’s still holding up - no brittleness yet. Maybe it depends on the brand or thickness? Felt always seemed to get mushy for me after a few years, especially when we had those long stretches of rain. Just my two cents...

Maybe it depends on the brand or thickness? Felt always seemed to get mushy for me after a few years, especially when we had those long stretches of rain.

That’s interesting - I’ve noticed the same thing with felt getting soggy over time, especially in wet climates. I wonder if some of the issues folks see with synthetics are from using thinner rolls or maybe not fastening them right? I’ve seen some brands that are paper-thin and others that feel almost tarp-like. It’s tough to compare unless you know exactly what was installed. Either way, your neighbor’s shed sounds like a good test case.

That’s a good point about not knowing exactly what was installed. I’ve had the same struggle trying to compare - one house I owned had a thicker felt, and it held up okay for a while, but after a few years of heavy rain, it started to break down and curl at the edges. The next time around, I went with synthetic because it was supposed to last longer and be more tear-resistant. It did feel a bit flimsy when we rolled it out, but so far, no leaks or mushy spots, even after some nasty storms.

Honestly, I think a lot comes down to how careful the install was and the quality of the material. Sometimes saving a few bucks on thinner rolls just isn’t worth it in the long run. Your neighbor’s shed is probably going to be a great way to see how things hold up side by side. Always nice to have a real-world example right next door... makes the decision a little easier when you can actually see how it weathers.

You nailed it - install quality really does make or break the whole thing. I’ve seen “premium” felt go south in a couple years just because someone rushed the job or left seams exposed. On the flip side, my cousin’s place has the cheapest synthetic you can buy, but he spent a weekend fussing over every overlap and fastener, and it’s still bone dry after five years of Midwest storms.

I get what you mean about synthetic feeling flimsy. The first time I unrolled it, I thought, “This stuff’s basically a glorified tarp.” But it’s weirdly tough once it’s down. I do miss the heft of old-school felt, though - there’s something satisfying about it, even if it does eventually curl up like a potato chip.

Having your neighbor’s shed as a test case is pretty ideal. Nothing like a little real-world science experiment right next door. If only all home improvement decisions came with a free preview...

That “glorified tarp” feeling with synthetic is real - I had the same reaction the first time I handled it. But honestly, after seeing how it holds up to wind-driven rain and ice dams, I’m convinced it’s more than just marketing hype. The thing that gets overlooked is breathability and how some synthetics actually let moisture escape better than felt, which can matter a lot if you’re in a humid spot. Still, I get the nostalgia for felt - there’s something about that weight and smell that just feels right... until you’re patching leaks two years later.

I get where you’re coming from with the breathability argument, but I’ve actually had the opposite experience in a couple of my buildings. We’re in a spot that gets wild temperature swings - hot, muggy summers and then freezing winters. The synthetics we tried on one property did fine for the first few years, but then we started seeing condensation issues in the attic. Not leaks, but enough moisture buildup that it messed with the insulation and we had to go back in and add more venting.

Felt’s not perfect, but I’ve noticed it sort of “forgives” minor mistakes in ventilation or flashing. It’ll absorb a bit of moisture and dry out, whereas some synthetics just trap it. Maybe it’s just our climate or the way the roof’s built, but I’m not totally sold on synthetics being better across the board. They’re lighter and easier to install, sure, but I wouldn’t call felt obsolete just yet.

I’m in a similar boat - first roof replacement, and I went with synthetic because my contractor swore by it for our area (humid summers, cold winters). I’ve been wondering about the breathability thing too. Have you noticed if the felt gets brittle after a while? My neighbor said his tore pretty easily after a few years, but he’s got a low-slope roof. Just curious if that lines up with what you’ve seen, since I keep hearing mixed stuff.

I’ve actually had both on different houses - felt on my old place, synthetic on the current one. The felt definitely got brittle after a few years, especially where it was exposed at the edges or around vents. I remember patching a spot near the chimney and the stuff just crumbled in my hands. That roof was a pretty standard pitch, nothing super steep, but we get a lot of freeze/thaw cycles here and I think that sped up the wear.

With synthetic, I haven’t noticed any of that cracking or tearing, even after a couple of rough winters. It feels a lot tougher, almost like a tarp. Breathability is still a bit of a mystery to me, though. Some folks say synthetic doesn’t “breathe” as well, but I haven’t seen any moisture issues in the attic so far. Maybe it’s more of a problem on really low-slope roofs, like your neighbor’s? I do wonder if the slope makes a bigger difference than the material sometimes.

Anyway, if you’re worried about longevity, I’d lean synthetic, but I get the mixed reviews. Every roof seems to have its own personality...

- Totally get what you mean about every roof having its own “personality.”
- I went with synthetic on my place last year. Just seemed like the safer bet after hearing stories about felt falling apart, especially in wet climates (I’m in the PNW, so rain is a given).
- Haven’t noticed any moisture issues either, and my attic’s actually stayed drier than I expected.
- Slope probably does matter more than people think - my neighbor’s got a really low pitch and fights leaks no matter what underlayment he uses.
- It’s kind of reassuring to hear someone else say synthetic holds up better. Makes me feel like I didn’t overthink it for nothing...

Funny you mention the PNW - I've seen more than a few roofs up here where felt just didn’t stand a chance. There was this one house I inspected last winter, maybe 15 years old, and the felt underlayment had basically turned to mush in spots. The shingles looked fine from the street, but once you got up there, you could see where water had crept in along the low-slope sections. The attic was musty, insulation was clumping... not a great scene.

On the flip side, I’ve also come across some older synthetics that held up surprisingly well, even after a decade or so. One homeowner told me he’d picked synthetic mostly because his installer said it was “the new thing,” but he ended up being glad for it after a couple of those classic sideways rainstorms we get. His attic was bone dry, and the plywood underneath looked almost new.

I do wonder sometimes if folks underestimate how much roof pitch changes the game. Like you said, your neighbor’s low slope is just asking for trouble, no matter what’s underneath. I’ve seen some people try to compensate with extra layers or ice-and-water shield, but it’s never quite as bulletproof as a steeper roof.

Curious if anyone’s had issues with synthetic getting slippery during install? I’ve heard a few roofers grumble about that, especially on steeper pitches. Also, has anyone noticed any difference in noise during heavy rain? Some folks swear synthetic makes things louder, but I haven’t really noticed it myself.

It’s wild how much these little details - underlayment type, slope, climate - can add up over time. Makes me think there’s no one-size-fits-all answer, but it’s always interesting to hear what’s actually working (or not) for people in different spots.

I do wonder sometimes if folks underestimate how much roof pitch changes the game.

Yeah, I totally underestimated this when I bought my place. The inspector mentioned the low slope but I figured “how bad could it be?” Turns out, pretty bad. My felt underlayment was toast after just 8 years. Haven’t noticed any noise difference with synthetic, but I’m still paranoid every time it rains sideways.

Pitch really does make a bigger difference than most people realize. Low slope is just brutal on underlayment longevity, especially if you’re in an area that gets a lot of wind-driven rain. I’ve seen felt underlayment fail in less than a decade on low-slope roofs, even when everything else looked fine from the outside. It’s not always obvious until you get a heavy storm and suddenly there’s water where it shouldn’t be.

Switching to synthetic was probably the right call. The newer synthetics are definitely more resilient against moisture and don’t break down as fast as felt, especially when they’re exposed to standing water or repeated wet/dry cycles. I get the paranoia, though - once you’ve had one failure, it’s hard not to worry every time the weather turns ugly.

Noise-wise, I haven’t noticed much difference either, unless you’re dealing with metal roofing where underlayment can sometimes muffle or amplify sound depending on how it’s installed. With shingles, it’s usually a non-issue.

If it helps, you’re not alone in missing the warning signs early on. Inspectors will mention low slope but rarely spell out just how much faster things wear out. It’s one of those lessons you only learn by living through it (or fixing it). At least with synthetic, you should get a longer run before needing to worry again - assuming the install was solid and your flashing is tight.

One thing I’d keep an eye on: make sure your attic ventilation is up to snuff. Poor airflow can cook both felt and synthetic from underneath, especially on low slopes where heat and moisture get trapped more easily.

It’s frustrating, but you caught it before major damage set in. That’s more than a lot of folks manage.

I had no idea low slope would be such a pain until I bought this place. My inspector just kind of shrugged and said “keep an eye on it.” Fast forward to last spring - surprise, water in the attic. I swapped to synthetic too, but now I’m paranoid about ventilation. Anyone ever actually fix attic airflow themselves, or is that a pro job?

I get the paranoia - low slope is just a whole different beast. About attic airflow, I wouldn’t say it’s always a pro job. Depends how handy you are and what you’re working with. I’ve seen folks add baffles and extra vents themselves, but cutting into the roof or soffit can get sketchy if you’re not comfortable. Just be careful not to overdo it and mess up the balance. Sometimes too much venting can cause its own problems, especially with synthetic underlayment since it’s less breathable than felt. It’s a tradeoff, honestly.

Yeah, I hear you on the venting - too much of a good thing can backfire, especially with synthetic underlayment. I swapped out old felt for synthetic a few years back thinking it’d be a “set it and forget it” deal, but my attic got stuffier than expected. Had to tweak the vents after the fact. Felt breathes better, but man, it tears so easily if you’re not careful. Synthetic’s tougher, but you’ve gotta watch that airflow or you’ll cook your insulation. Always some tradeoff, huh?

Had a similar situation last summer - swapped out old 30# felt for synthetic on a steep asphalt shingle job. The durability was great, but I underestimated how much less vapor passed through. Ended up adding more soffit vents to balance things out. Synthetic’s definitely tougher, but it’s not always “install and forget” like the marketing says. Always seems like you’re trading breathability for strength.

That’s a really important point about vapor permeability. Synthetic underlayments are marketed as a cure-all, but there’s always a tradeoff, especially in older homes or roofs that weren’t designed with low-perm materials in mind. I’ve seen a few jobs where folks switched to synthetic for the durability - especially on steeper slopes or in high-wind areas - and then had to go back and rethink attic ventilation. It’s easy to overlook how much less moisture can escape through those synthetics compared to #30 felt.

Adding soffit vents was a smart move. I’ve had to recommend ridge vent upgrades in a couple of cases for similar reasons. The upside is, once the airflow’s balanced, the synthetic does hold up better against UV and foot traffic during install. But yeah, “install and forget” isn’t always realistic, especially if you’re dealing with older framing or insulation that can trap moisture.

It’s always a balancing act - durability versus breathability. I lean toward synthetics for most new builds, but on retrofits, I’m a lot more cautious. Good to hear you caught it before it became a bigger issue.