Roof Underlayment Types: Felt vs Synthetic in Wisconsin

What roof underlayment does (and what it does not)

Underlayment is the sheet that goes on your roof deck before the shingles. Its job is secondary water protection. Wind-driven rain that sneaks past a shingle, a little snowmelt backup, water running down a nail shank: underlayment gives that water a path off the roof instead of into your sheathing. It also keeps the deck dry during construction.

What it does not do:

• It is not waterproof on its own. Field underlayment is water resistant and mechanically fastened, which means it has holes in it.
• It will not stop an ice dam. That is the job of self-adhering ice-and-water shield.
• It will not fix bad attic ventilation, rotten decking, or sloppy flashing.

So the honest framing is this: underlayment is your backup, not your primary defense. And in south-central Wisconsin your backup gets tested constantly. Freeze-thaw swings, a February thaw that refreezes overnight at the eave, straight-line winds that lift shingle edges for a few seconds at a time. That is exactly when the difference between paper and polypropylene shows up.

What Wisconsin code requires, and the permit nobody mentions

One- and two-family homes in Wisconsin are governed by the Uniform Dwelling Code (SPS 320-325, with roof coverings in SPS 321). It is based on the International Residential Code but it is not word-for-word identical, so do not quote IRC numbers at an inspector and expect a nod.

In practice, three numbers matter when you read a bid:

• Field underlayment: on slopes of 4/12 and steeper, one layer of #15 felt (or an approved equivalent) is the minimum. So yes, a 15 lb felt bid is legal. It is the floor, not a standard of care.
• Ice barrier: required from the lowest edge of the roof to a point at least 24 inches inside the exterior wall line. On a house with deep overhangs, that often means two or three courses of membrane, not one.
• Low slope: below 4/12, you need a double-layer underlayment application or an approved self-adhering low-slope membrane. Down there, underlayment stops being backup and becomes the actual waterproofing.

Madison and some Dane County municipalities can be stricter than the state minimum, so confirm the details on your permit.

Speaking of which: the City of Madison and most Dane County municipalities require a permit for a roof replacement. We pull it. And depending on the municipality and scope, the inspector may want to see the roof once it is dried in, with decking, ice barrier and field underlayment visible before shingles cover them. That is a good thing for you. It is also a reason a contractor who plans to cut corners under the shingles would rather nobody looked.

The three roof underlayment types you will see on a quote

15 lb asphalt-saturated felt

The classic black paper. Cheapest option, meets minimum code on standard slopes. That is the whole argument for it.

What we see in the field:

• Tears out at fasteners, especially in wind before shingles go on. Once the sheet tears loose at the cap or staple, a gust can peel a whole run off a dried-in deck.
• Absorbs water. Get it wet before shingling and it cockles into ridges.
• Those ridges telegraph through the shingles. We pulled shingles off a 1920s bungalow in Nakoma a couple of seasons back and the south plane had visible waves running the full width, straight lines of wrinkled felt printing through a 20-year-old install. The homeowner had looked at those waves from the driveway for two decades. Ask us for the tear-off photos, or browse our recent projects and you will see the same story on other houses.
• Goes brittle in UV within days.

30 lb felt

Heavier, more body, better tear resistance, holds foot traffic better. There are places we will genuinely spec it: a detached garage, a shed, a porch roof, a budget repair patch, or a plane that is going to be re-covered again inside a few years anyway.

What it will not do is stop being asphalt-saturated paper. It still drinks water, still wrinkles, still degrades in sun, and it is heavy to handle, which means more seams up on the roof.

Synthetic underlayment

Woven polypropylene or polyethylene, usually gray or tan, printed with walk lines and nail patterns. This is the modern standard, and it is what most manufacturers build their system warranties around.

Why it wins:

• Tear strength. Manufacturers publish trapezoidal tear values (ASTM D4533) that typically run several times #15 felt. The practical version: you cannot rip a good synthetic by hand, and it does not tear out around a fastener in a gust.
• No wrinkling. It does not absorb water, so your shingle planes stay flat.
• Exposure tolerance. Weeks, not days. More on that below.
• Fewer laps. One roll covers far more deck at a fraction of the weight.

Fasteners: this is where synthetic installs go wrong

Synthetic gets fastened with 1 inch plastic cap nails, and that is not optional. Most manufacturers require plastic cap fasteners for their enhanced coverage to apply. Staples and bare roofing nails pull straight through a thin sheet under wind load, which is how you get a dried-in roof that unzips on a Tuesday night before the shingles go on.

We run cap nails on every synthetic install, at the manufacturer's spacing, tighter in the field on windy exposures. If a bid does not say how the underlayment is fastened, ask. It takes ten seconds and it tells you a lot.

What we actually install: GAF Tiger Paw is our standard synthetic on shingle replacements, FeltBuster on smaller and accessory work, and GAF Deck-Armor when we need a vapor-permeable sheet. For ice-and-water we use GAF WeatherWatch or StormGuard depending on the detail. Ask the crew to hand you the roll wrapper on your job. If a contractor will not do that, that tells you something too.

The honest downsides of synthetic

An "honest comparison" with zero drawbacks is a sales pitch. Here are the three real ones.

Vapor permeability. Most standard synthetics are close to vapor-closed. On a normal vented Wisconsin attic with working soffit and ridge vents, that is a non-issue. Attic moisture comes from warm indoor air leaking through ceiling penetrations, and it leaves through ventilation, not through the sheathing. But if you have a cathedral ceiling, a finished attic, a hot-roof assembly, or soffit vents that were stuffed with insulation in 1974, we spec a breathable synthetic (Deck-Armor) and we talk about ventilation before we talk about shingles. Putting a vapor-closed sheet over a roof with no drying path is how you get damp sheathing and mold complaints two winters later.

Traction in frost and dew. Some synthetic sheets are genuinely slick when frosted, walk surface or not. In practice a September or October crew starts a little later in the morning instead of shingling over frost. Schedule wrinkle, not a quality problem.

Metal roofs need a different sheet. Standing seam and metal panels run hot, and standard underlayment can soften or degrade under them. Those jobs get a high-temperature-rated synthetic or self-adhered membrane. If you are pricing metal roofing, make sure the underlayment line says high-temp.

Why exposure tolerance matters so much in Dane County

Roofing here is a race against weather, so the published exposure limit is a real spec, not a footnote. GAF's own numbers illustrate the spread: FeltBuster sits at the short end (roughly a month), Tiger Paw in the middle (around 90 days), and Deck-Armor at the long end (up to about 180 days). Manufacturers revise these, so ask us to show you the current spec sheet for the product going on your house. Felt, by comparison, gives you a day or two before it wrinkles and starts breaking down.

We tore off a two-story in Sun Prairie one September, dried it in with synthetic on a Thursday, then it rained on and off for three days before we could shingle. Deck stayed dry, underlayment stayed flat, nobody lost anything but a weekend. With felt, that same delay means wet, wrinkled paper that should come off and be redone. Plenty of crews will not bother, and the homeowner lives with the waves for 25 years.

The same math applies after a hail event. During a busy storm damage restoration season in Dane County, your roof may sit dried in longer than anyone wants.

One question homeowners always ask: how long is my house actually vulnerable? We only strip what we can dry in the same day, usually one plane at a time, and the deck gets ice barrier and underlayment before the crew leaves. If weather rolls in mid-tear-off, that exposure is the contractor's responsibility, not your homeowners policy. Ask any roofer for a certificate of insurance before you sign. We hand ours over without being asked.

Underlayment and ice-and-water shield are not the same thing

They work together, and a correct Wisconsin roof uses both.

Ice-and-water shield is a self-adhering rubberized membrane that bonds to the deck and seals around nails. It goes where water sits or backs up:

• The eaves, from the edge to at least 24 inches inside the exterior wall line
• All valleys
• Around chimneys, skylights, and pipe penetrations
• Low-slope sections and most wall-to-roof transitions

Synthetic underlayment then covers the rest of the field, lapped over the top edge of the membrane so water always sheds downhill.

If your overhangs are deep, one 3-foot course will not reach 24 inches past the wall line, and a second or third course is not an upsell, it is the code line. Cost check, and this moves with supplier pricing: membrane runs roughly $50-$90 per square installed, so one extra 3-foot course along 100 lineal feet of eave lands around $150-$270. Ask for the coverage in feet, on paper.

If anyone tells you synthetic across the whole deck lets you skip the eave membrane, walk away. Ice dams push water uphill. Only a self-sealing membrane stops that. Our asphalt shingle roof replacement page for Madison walks through the whole assembly, deck to ridge cap.

Not sure what is on your quote? Call Buckshot Exteriors and we will read it with you line by line, no charge and no pressure. Want a second set of eyes on the roof itself? That inspection is free too.

Does insurance pay for underlayment and ice-and-water shield?

Usually, for what was already there. A hail or wind claim is written to restore your roof to pre-loss condition, and "remove and replace underlayment" is a standard line item in an adjuster's estimate. Ice-and-water shield gets included when it existed or when code requires it, and code-required items are often covered under ordinance-or-law coverage if your policy carries it.

Two honest caveats. A carrier pays like-kind, so if your 1998 roof had 15 lb felt, the base estimate reflects felt, and upgrading to premium synthetic is generally a modest difference you cover. And ACV policies release depreciation only after the work is done, so the first check is not the whole approval.

This is exactly why we meet your adjuster on the roof. Underlayment, ice barrier footage, and decking are the line items most often missed on a first estimate.

The real cost difference

Ballpark, and it moves with supplier pricing: underlayment runs roughly $10-$20 per square (100 square feet) for 15 lb felt installed, and roughly $20-$35 per square for a quality synthetic. Call it a $10-$20 per square swing.

Worked example on a 30-square Madison roof: about a $300-$600 difference on a replacement that will run in the five figures.

That is why felt is not our standard on full replacements. Saving a few hundred dollars for wrinkled shingle lines and weaker wind performance (fastener tear-out first, then whole runs blowing off before the shingles are down) is a bad trade, and we are the ones who have to look at that roof from the street for the next 20 years.

Warranties matter here too. Enhanced manufacturer coverage, including what we can offer as a GAF Master Elite contractor, generally requires approved underlayment, cap fasteners and accessories from the same system. Substituting cheap felt limits what can be stood behind.

What your quote should specify

Underlayment by brand and product name, not just "synthetic" or "felt paper"

• Fastening method: plastic cap nails, at what spacing
• Ice-and-water shield locations and coverage in feet, including the 24-inch-past-the-wall-line detail at eaves
• Decking: is damaged sheathing replacement included, and what is the per-sheet price if more rot shows up
• Tear-off: how many existing layers come off (Wisconsin homes often have two), and confirmation that it is a full tear-off, not a layover
• Permit: who pulls it and whether it is in the price
• Drip edge, starter, ridge and hip cap, and ventilation called out separately
• Low-slope and metal areas handled with the right membrane, spelled out

Red flags: a one-line bid, no product names, "whatever's on the truck," or a contractor who cannot tell you the exposure rating of the underlayment they carry.

Talk to somebody who has been doing this since 1979

Buckshot Exteriors is family-owned, running out of Madison, WI and Centerville, IA since 1979. We have pulled off enough wrinkled felt and soft decking to have strong opinions about what belongs under your shingles, and we will show you photos from local tear-offs so you can see it yourself.

Here is what happens when you call: we come out, get on the roof, take photos, check the decking and the ventilation, and show you what we found on your phone or at your kitchen table. If your roof has years left, we will say so. If there is hail or wind damage worth a claim, we document it and meet your adjuster on-site so nothing gets missed.

No cost, no pressure, no obligation to hire anybody.

Call Buckshot Exteriors for a free roof inspection and written estimate in Madison, Dane County, and across south-central Wisconsin, or reach out through our contact page and we will get right back to you. We also run 24-hour emergency storm response, because weather does not wait for business hours.