Every roof has a layer you’ll never see from the ground. It sits between your shingles and your roof decking, and it’s the reason a missing shingle in a rainstorm doesn't automatically mean water on your ceiling.
That layer is underlayment. It’s a water-resistant (or in some cases waterproof) barrier that covers the entire roof deck before shingles go on top. Think of it as the backup plan. Shingles do the heavy lifting, but when wind-driven rain gets under a shingle edge, when a nail backs out, when ice dams push water uphill, the underlayment is what keeps that water from hitting plywood.
There are three types. One of them is the right choice for most of your roof. Another belongs at specific vulnerable spots. The third one should probably retire.
Synthetic underlayment: what we use on every job
Synthetic underlayment is woven polypropylene. It’s light, strong, and doesn’t absorb water. When you unroll it across a roof deck, it lies flat and stays put. Crews can walk on it without tearing through, and it holds up in sun exposure for weeks if the shingle installation gets delayed by weather.
That last point matters more than you’d think. On any re-roofing job, there’s a window between stripping the old roof and getting new shingles down. Sometimes that window is hours. Sometimes, if you get an unexpected storm or a material delivery gets pushed, it’s days. Synthetic underlayment handles that. It’s engineered to resist UV degradation for 30-90 days depending on the product (GAF FeltBuster specs 90 days of UV exposure). Felt paper starts deteriorating within hours of sun exposure. Literally hours.
Installation speed is the other advantage crews notice. Synthetic comes in wider rolls and lays faster, roughly 30% faster than felt by most estimates. On a full roof replacement, that’s not trivial. It means less time with the deck exposed and a tighter, more efficient installation day.
We use GAF FeltBuster or Tiger Paw on our projects. Both integrate with the GAF warranty system for Golden Pledge installations. Tiger Paw is the heavier-duty option with a higher tear strength, which we’ll sometimes use on steeper roofs where foot traffic during installation is harder on the material.
Cost
Synthetic underlayment runs about $0.15-$0.25 per square foot for material. On a 2,000-square-foot roof (roughly 2,300 square feet of roof area), that’s $345-$575 in material. The labor to install it is essentially the same whether you’re using synthetic or felt, so the real cost comparison is material only. Synthetic costs more per roll than felt. It also covers more area per roll and goes down faster. The net difference on a full job is maybe $150-$250.
For that difference, you get a product that won’t turn to mush if it rains before shingles are on.
Felt paper: the one we stopped using
Felt underlayment (also called tar paper, roofing felt, or #15/#30 felt) is what your roofer’s grandfather used. It’s an organic or fiberglass mat saturated with asphalt. It works. It’s been working for decades. And it’s been surpassed.
I’ll keep this brief because the argument is straightforward. Felt tears when you walk on it. It wrinkles when it gets wet and doesn’t lay flat again. UV destroys it fast, so if your shingle install gets pushed a few days, you’re potentially re-doing the underlayment. It absorbs water. In cold weather, it gets brittle and cracks.
Is it cheaper? Yes. A roll of #30 felt costs less than synthetic. But the speed difference, the durability difference, and the risk of having to replace damaged underlayment before shingling make it a false economy on most jobs. We’ve watched crews on other job sites spend an hour patching torn felt that wouldn’t have torn if it were synthetic.
Some contractors still use it because it’s what they’ve always used. Some homeowners request it because they’ve heard felt “breathes” better (and it does have slightly higher vapor permeability, which is a legitimate technical point on certain roof assemblies). But for standard residential roofing in our climate, synthetic is the better product. We switched years ago and haven’t looked back.
If a roofer quotes you felt underlayment in 2026, ask why. There might be a valid reason. There usually isn’t.
Ice and water shield: the targeted specialist
Ice and water shield isn’t field underlayment. It doesn’t go across your whole roof. It’s a self-adhering rubberized membrane that goes at specific locations where water intrusion risk is highest: eaves, valleys, around chimneys and skylights, in dead valleys, and on low-slope sections.
We wrote a full article on ice and water shield that covers brands, installation details, and where it should go on your roof. I’m not going to repeat all of that here.
The short version: your roof should have synthetic underlayment covering the full deck, with ice and water shield underneath it at the vulnerable areas. That combination is what most building codes in our area require (or should require), and it’s what gives you a properly layered water management system.
Some contractors run ice and water shield across the entire roof deck. We generally don’t recommend this unless the roof is very low-slope. The membrane has extremely low vapor permeability (under 0.5 perms), and wrapping an entire roof deck in a vapor-impermeable layer can trap moisture in the attic if ventilation isn't perfect. Targeted installation at vulnerable spots with synthetic over the rest is the approach that balances water protection and moisture management.
How underlayment stacks on your roof
People ask about this, so here’s the layering order from the deck up:
On a standard residential roof, ice and water shield goes directly on the clean, dry decking at the eaves, valleys, and penetrations. Then synthetic underlayment goes over the remaining field areas, overlapping the edges of the ice and water shield. Then starter strips at the eaves, then shingles.
The overlaps matter. Synthetic underlayment should overlap at least 4 inches at horizontal seams, with upper courses laying over lower courses so water sheds downhill over the seam, not into it. Sounds obvious. You’d be surprised how often we see it backwards on tear-offs.
One thing I’ve noticed on tear-offs of older roofs, maybe 1990s installs: some crews back then just ran felt paper right over the eaves with no ice and water shield at all. Not every time, but enough that I mention it. Building codes have tightened since then. If your roof is from that era and you’re not sure what’s under there, a roof inspection before it becomes urgent is worth the money.
FAQ
What type of roofing underlayment is best?
Synthetic for the field, ice and water shield at eaves, valleys, and penetrations. That’s the standard for good reason.
Do I need underlayment if I have ice and water shield?
Yes. Ice and water shield belongs at specific vulnerable spots, not across the whole roof. Synthetic underlayment covers the rest of the deck. They work together but serve different purposes. Think of ice and water shield as the waterproof barrier at high-risk zones and synthetic underlayment as the water-resistant layer everywhere else.
Can you install underlayment yourself?
Technically, sure. The material isn’t complicated. But you’re on a roof, and that’s the part people underestimate.
How much does roofing underlayment cost?
The material itself is one of the cheaper components of a roof. Felt: $0.05-$0.15/sq ft. Synthetic: $0.15-$0.25/sq ft. Ice and water shield: $0.90-$1.75/sq ft. On a full roof replacement, underlayment is maybe 3-5% of the total project cost. Not the place to cut corners.
Does underlayment go on before or after ice and water shield?
After.
How long can synthetic underlayment be exposed before shingling?
Most products are rated for 30-90 days of UV exposure. GAF FeltBuster specs 90 days. Felt, by comparison, starts degrading within hours of direct sun exposure. This is one of those practical advantages that doesn’t show up on spec sheets but changes how a job runs in real weather.
Questions about your roof’s underlayment?
If you’re getting quotes and want to know what’s being spec’d, or if you’re curious what’s under your current roof, we’re happy to take a look. Free inspections, straight answers.
