Nobody stands in their driveway admiring the underlayment. It's the one layer of a roof you'll never see after installation day — and it's the layer that decides what happens when wind drives rain sideways under a shingle, when an ice dam ponds meltwater at your eaves, or when a nor'easter arrives two days into your mid-project roof. Every replacement estimate in New Jersey has an underlayment line, most homeowners skim past it, and contractors count on that. Here's the layer explained properly: felt vs synthetic, where the fully waterproof ice-and-water membrane is required by code, and the two sentences to look for on your estimate before you sign anything.
| Felt (15/30 lb) | Synthetic | Ice & water shield | |
|---|---|---|---|
| Water behavior | Water-resistant; absorbs & wrinkles | Water-resistant; lies flat | Fully waterproof; self-seals nails |
| Tear strength | Low | Several times felt | High (adhered) |
| UV exposure rating | Days | 30 days – 6 months | Weeks (varies) |
| Relative cost | $ | $$ (couple hundred more per project) | $$$ (used in zones) |
| Where it belongs | Budget re-roofs | Field of the roof (our default) | Eaves to 24" inside wall, valleys, penetrations |
What underlayment actually does
Underlayment is the membrane between the wood deck and the shingles — the roof's backup goalkeeper. Shingles shed the overwhelming majority of water, but they're a gravity system: wind-driven rain, ice-dam backwater, and any cracked or lifted shingle can put water on the deck's doorstep, and the underlayment is what turns that moment into nothing instead of a ceiling stain. It also serves as the temporary roof during installation — the "dry-in" layer that keeps your house watertight between tear-off and shingles — and it's required both by the residential code NJ towns enforce (built on the model codes published by the International Code Council) and by shingle manufacturers as a warranty condition. Three products cover the territory: traditional felt, modern synthetics, and self-adhering ice-and-water membrane. They are not interchangeable, and the mix on your roof matters more than most line items twice its price.
Felt: a century of service, showing its age
Asphalt-saturated felt — "tar paper," in 15 lb and 30 lb weights — dried in America's roofs for a hundred years, and it still works the way it always did: adequately, with caveats. In its favor: it's the cheapest option, every crew alive has installed it, and 30 lb felt in particular offers respectable protection. Against it, the list has grown as alternatives arrived: felt tears easily around fasteners and underfoot; it absorbs moisture and wrinkles, telegraphing bumps through thinner shingles and complicating flat installation; its UV exposure tolerance is measured in days, so weather delays mid-project become genuine problems; and the rolls are heavy and narrow, meaning more seams — and every seam is a future question mark. Felt isn't wrong; it's simply been outrun. On re-roofs where every other component has improved since 1995, the underlayment shouldn't be the one nostalgic holdout.
Synthetic: what we install, and why
Synthetic underlayments — woven polypropylene or polyethylene sheets — took over the professional market for reasons you can feel through your boots: several times felt's tear strength, so fasteners hold and foot traffic doesn't wound it; zero water absorption, so it lies flat through rain instead of wrinkling; UV exposure ratings from 30 days to 6 months depending on product, which is precisely what lets an NJ project ride out an unsettled week fully protected; lighter, wider rolls with fewer seams; and slip-resistant surfaces that make crews faster and safer. The honest cons: it costs more than felt — though underlayment is such a small share of a total replacement budget that the whole-project difference is typically a couple hundred dollars, the cheapest meaningful upgrade on the estimate — and the word "synthetic" spans everything from premium engineered membranes to bargain-bin film, so the specific product name belongs in your contract. One technical note that connects to another decision on your roof: most synthetics are vapor-impermeable, which is a non-issue on a properly ventilated attic and one more reason the ventilation system gets engineered in the same project. And if metal is going on instead of shingles — including over existing shingles — the spec shifts to high-temperature-rated synthetic, because metal cooks standard products.
Ice & water shield: the waterproof specialist NJ code demands
Felt and synthetic are water-resistant — they shed water moving over them. Self-adhering ice-and-water membrane is the fully waterproof member of the family: a rubberized asphalt sheet that bonds to the deck and seals itself around every nail driven through it. That distinction decides its territory — the places water doesn't flow but sits: the eaves, where ice dams pond meltwater against gravity every NJ February (code requires the membrane from the roof edge to at least 24 inches inside the exterior wall line — the ice-barrier rule our climate zone triggers); valleys, where two slopes concentrate their runoff; and penetrations — chimneys, skylights, pipes — where flashing does the primary work and the membrane backstops it. Whole-roof ice-and-water coverage exists and has its niches (low slopes, coastal exposure), but for most NJ homes the standard build is the right build: synthetic across the field, ice-and-water at eaves, valleys, and penetrations. That sentence, roughly, is what you want to find on your estimate. Its absence is what February finds instead.
Reading your estimate like a roofer
Two lines separate a specified roof from a shrugged one. First: "Underlayment: [named product], synthetic, installed with cap nails per manufacturer spec." A brand and product name means accountability and a known exposure rating; "felt or equivalent" means whatever's stacked in the shop. Second: "Ice & water membrane: eaves to 24" inside warm wall, all valleys, all penetrations." Locations spelled out means the crew is building to NJ's climate; a bare "ice and water included" is a quantity mystery. These details also feed the warranty math — manufacturer system warranties generally require their qualifying underlayment and accessories as a package (GAF's tiered warranties are the familiar example), so the underlayment choice can decide which coverage level your roof even qualifies for, as our warranty guide lays out. If any of this is missing from a quote you're comparing, our estimate-reading guide covers the rest of the fine print worth demanding.
The bottom line
The underlayment question has a short honest answer: synthetic in the field, ice-and-water where NJ code and NJ winters demand it, and the product names in writing. The upgrade from felt costs less than a nice dinner, buys tear strength, flat installation, and weeks of weather protection mid-project, and quietly raises the floor under everything else you're paying for. It's the least glamorous decision on your roof and one of the highest-leverage ones — which, after eleven articles about everything that can go wrong up there, is about the most New Jersey conclusion a roof can offer: the boring layer, done right, is what keeps the interesting problems away.
Want an estimate where every layer is named, specced, and in writing? Call 973-355-0890 — free quote with the underlayment and ice-and-water plan spelled out, down to the product.
Frequently asked questions
What is roofing underlayment?
It's the water-resistant membrane installed between the roof deck and the shingles — the backup barrier that catches whatever wind-driven rain, ice-dam backup, or a damaged shingle lets through, and the temporary weather protection that keeps your house dry mid-project. Code requires it, manufacturers' warranties assume it, and it's the layer you'll never see doing a job you'll definitely notice if it fails.
Which is better: felt or synthetic underlayment?
Synthetic wins on nearly every measure that matters in New Jersey: several times the tear strength, no water absorption or wrinkling, weeks-to-months of rated UV exposure instead of days, lighter rolls with fewer seams, and better footing for the crew. Felt's remaining advantages are price and tradition — and the price gap on a whole project is a few hundred dollars. We install synthetic as the default.
Is synthetic underlayment worth the extra cost?
It's the cheapest meaningful upgrade on a roofing estimate: underlayment is a small fraction of project cost, and stepping from felt to a quality synthetic typically adds only a couple hundred dollars on an average NJ home — for a membrane that's dramatically tougher and stays watertight if weather hits mid-install. One caveat: "synthetic" spans a wide quality range, so the product name belongs in your contract.
Is underlayment waterproof?
Standard felt and synthetic underlayments are water-resistant — they shed water running over them but aren't sealed at every nail. Self-adhering ice-and-water membrane is the fully waterproof member of the family: it bonds to the deck and seals around fasteners, which is exactly why it goes where water sits or backs up — eaves, valleys, and penetrations — rather than relying on shedding alone.
Where does NJ require ice and water shield?
New Jersey's climate puts it in ice-barrier territory: code requires a self-adhering membrane at the eaves extending from the roof edge to at least 24 inches inside the exterior wall line — the zone where ice dams pond meltwater. Good practice adds valleys, skylights, chimneys, and pipe penetrations. If an estimate doesn't spell out ice-and-water locations, ask; it's the detail NJ winters test every February.
Can you use synthetic underlayment under a metal roof?
Yes — with the right spec: metal roofs run hotter at the underlayment layer, so panel manufacturers call for high-temperature-rated synthetics or high-temp ice-and-water membranes. Standard products can soften or degrade under metal's heat. It's one of the small spec details that separates a metal roof installed by the book from one improvised over whatever was on the truck.
How long can underlayment stay exposed before shingles go on?
Felt is rated for days — it wrinkles with moisture and degrades quickly in UV. Quality synthetics carry exposure ratings from 30 days to 6 months depending on the product, which is what lets a project ride out a rainy NJ stretch mid-install without risk. That exposure rating is doing real work in scheduling; it's worth knowing what your roofer is installing.
Does building code require underlayment?
Yes — underlayment is a code requirement under the residential code NJ towns enforce, with the ice-barrier requirement layered on top for our climate zone, and shingle manufacturers require it for warranty coverage as well. The code sets the floor; the felt-vs-synthetic and ice-and-water-coverage decisions determine how far above that floor your roof actually performs.