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

I’ve lost count of how many times I’ve been called out to “fix” a leaky roof, only to find the underlayment’s still in decent shape but the attic’s basically a sauna. Folks get sold on synthetic like it’s some kind of force field, but if your vents are choked with blown-in insulation or bird nests (yep, seen that more than once), you’re just asking for trouble. Doesn’t matter if you spent extra on the fancy stuff - trapped moisture will win every time.

I’ll give synthetic credit where it’s due: after Hurricane Irma, I saw plenty of roofs where the shingles were halfway to Georgia but the synthetic was still hanging on. Felt would’ve been shredded. But I’ve also seen jobs where someone slapped down synthetic and skipped proper flashing or didn’t overlap it right, and water found its way in anyway. It’s not idiot-proof.

Felt’s fine for a quick patch or if you’re on a tight budget, but down here in Florida, it just doesn’t hold up long-term. The sun bakes it, storms tear at it, and before you know it, you’re patching again. I get why some folks still use it - cheaper upfront, easier to work with - but you’re rolling the dice.

One thing I wish more people would do is actually poke their head up in the attic once in a while. You’d be amazed what you find - blocked soffits, disconnected bathroom vents dumping steam right into the insulation... Had a guy swear his “roof was leaking,” but turns out his AC drain line was clogged and dripping onto the ceiling. Not even a roofing problem.

Long story short: synthetic’s tougher, but it won’t save you from bad airflow or lazy installs. If you’re paying for a new roof, make sure whoever’s doing it actually checks the venting and flashing before they start rolling anything out. Otherwise, you’ll be calling someone like me sooner than you’d like.

“You’d be amazed what you find - blocked soffits, disconnected bathroom vents dumping steam right into the insulation...”

This is so true. I’ve had tenants call about “roof leaks” and it turned out to be condensation from a bathroom fan venting into the attic. Do you think most roofers actually check attic airflow, or is that something we should always double-check ourselves? I’ve noticed even with synthetic, if the attic’s cooking, you still end up with issues over time - especially in humid spots like ours.

“I’ve noticed even with synthetic, if the attic’s cooking, you still end up with issues over time - especially in humid spots like ours.”

I get what you’re saying, but I’m not totally convinced synthetic is the problem here. I’ve had both felt and synthetic on different houses, and honestly, when my attic was a sauna (thanks to a blocked soffit), both types started showing wear. I think airflow is way more important than underlayment type. Most roofers I’ve dealt with barely peek in the attic unless you ask... so yeah, I always double-check myself. If you don’t fix the venting, even the fanciest underlayment won’t save you long-term.

Totally agree with this:

“If you don’t fix the venting, even the fanciest underlayment won’t save you long-term.”

I’m starting to wonder if underlayment type even matters as much as everyone says. My place is in the Midwest, and last summer I found a couple spots where the synthetic was curling up - turns out the attic vents were half blocked with insulation. Fixed that, and things seem better now. But for those who’ve had both, does felt hold up any better if your venting isn’t perfect, or does it fail just as fast?