Every homeowner knows what shingles are. Almost none can name the layer directly beneath them — the sheet that gets rolled across the bare deck before the first shingle goes down, disappears under the roof within an hour, and is never seen again until the next tear-off. That layer is the underlayment, and it is doing a job most people assume the shingles are doing: keeping water out of the house.
Shingles are not waterproof. They are water-shedding — overlapping tiles that guide water downhill fast enough that it never sits long enough to get through. That works beautifully as long as every shingle is intact, flat, and sealed. But a shingle blows off in a windstorm. A branch punches a hole. Wind-driven rain gets forced sideways and uphill under the tabs. Ice backs water up the slope. In every one of those moments — and they will happen across a 25-year roof — the only thing standing between the weather and your decking, your insulation, and your ceiling is the underlayment.
It is the roof’s second line of defense, and it is cheap enough that cutting a corner on it saves a contractor almost nothing while costing you a deck. This is the guide to the layer nobody sees: what the choices actually are, what code actually requires (and where), why a metal roof needs a completely different product, and how to make sure the second roof under your shingles is one you’d have chosen yourself.
What underlayment is actually for
It helps to separate two jobs the roof does, because underlayment exists to cover the gap between them.
The primary water barrier is the shingles (or metal, or tile). It handles essentially all the water, essentially all the time, by shedding it downhill. When the roof is new and undamaged, the underlayment beneath it stays bone dry — it does nothing, day after day, for years.
The secondary water barrier is the underlayment. Its entire purpose is the exception case: the day the primary barrier is compromised. It has four distinct jobs, and only understanding all four explains why the product choice matters.
1. It’s the backup membrane. When water gets past the shingles — a lifted tab, a missing shingle, a nail hole from a satellite dish someone removed — the underlayment catches it and carries it back downhill to the eave, off the roof, before it reaches the wood. A roof with good underlayment survives a blown-off shingle as an inconvenience. A roof with degraded or skipped underlayment survives it as a leak.
2. It protects the deck during construction — the “dried-in” stage. Between tear-off and the last shingle, your roof is open. On a big or complex roof, or when weather rolls in, the deck can sit exposed for a day or more. The moment underlayment is down over the whole deck, the roof is “dried-in” — watertight enough to survive a rainstorm mid-project without soaking the sheathing. This is why a good crew’s first priority after stripping a section is getting felt or synthetic over it, not admiring the view. (More on the choreography in What Actually Happens During a Roof Replacement.)
3. It’s a separator layer. Certain roof coverings react badly with certain decks. The resins in some wood decking can bleed into and stain the back of asphalt shingles; a metal roof expanding and contracting against bare wood telegraphs every movement. Underlayment keeps the covering and the deck from touching directly.
4. It smooths the substrate. A thin layer of underlayment masks minor deck irregularities — a slightly proud plywood seam, a small ridge — so they don’t print through as a visible line in the finished shingle surface.
Notice what’s not on that list: holding the shingles down. That’s the fasteners’ job, and it’s why underlayment is never a substitute for proper nailing. The two systems fail in completely different ways and protect against completely different things.
The three families of underlayment
There are exactly three product families in residential roofing. Every underlayment decision is a choice among them — and increasingly, a combination of them on the same roof.
1. Asphalt-saturated felt — the traditional standard
“Felt paper” is the material most people picture: a roll of black paper, saturated with asphalt, that comes in two weights.
- No. 15 (“15-pound”) felt — the lighter, thinner standard, historically the default under asphalt shingles.
- No. 30 (“30-pound”) felt — heavier, thicker, more tear-resistant, more water-resistant, the traditional upgrade.
The naming is a fossil. The numbers used to mean the weight in pounds of a 100-square-foot “square” of the material, but modern felts are almost always lighter than their label — today’s “15-pound” felt weighs closer to 8 pounds per square. The number is now a product class, not a measurement.
Felt is governed by two old ASTM standards: ASTM D226 (asphalt-saturated organic felt) and ASTM D4869 (the newer standard written specifically for steep-slope roofing underlayment, including a “shower test” for water shedding). Both are composition-based standards — they specify what the material is made of.
Felt’s virtue is cost: a roll of 15-pound runs roughly $0.05–$0.07 per square foot, the cheapest option by a wide margin. Its liabilities are all about how it behaves once it’s on the roof:
- It wrinkles when it gets wet. Organic felt absorbs water, swells, and buckles into ridges. Those wrinkles telegraph straight through the shingles as visible waviness, and a wrinkled sheet sheds water poorly at exactly the moment it’s being asked to.
- It degrades fast in sunlight. Left exposed, felt starts breaking down in 7–14 days. On a job that stalls for weather, an exposed felt underlayment can be cooked before the shingles arrive.
- It tears. It’s brittle, especially in cold, and a crew member’s boot or a gust of wind will rip it.
- It’s heavy and slick. Rolls are short and heavy; the surface is slippery, particularly on steep pitches.
Felt still has a place — it’s inexpensive, code-accepted, and vapor-permeable enough to let a deck breathe — but on new installations it has been losing ground for a decade to the second family.
2. Synthetic underlayment — the modern default
Synthetic underlayment is woven or spun polypropylene or polyethylene — essentially a tough, engineered plastic fabric — usually with a non-slip coating. It rolls out light, wide, and flat, and it has quietly become the standard on quality re-roofs.
For most of its history it had no product standard of its own; manufacturers simply certified that their synthetics met or exceeded the old felt standards, which was awkward because synthetics are made of entirely different stuff. That changed in December 2020, when ASTM published ASTM D8257 — the first U.S. product standard written for synthetic underlayment. It tests things the felt standards never contemplated: fastener pull-through, hydrostatic (water) resistance, accelerated weathering, thermal cycling, and UV resistance — performance-based tests, not composition ones.
What you get for the higher price (roughly $0.08–$0.20 per square foot, still a rounding error on a full roof) is a material that fixes nearly every felt complaint at once:
- It stays flat. Synthetics don’t absorb water, so they don’t wrinkle or buckle. The finished shingle surface stays smooth.
- It survives sun. Depending on the product, synthetics tolerate 60–180 days of UV exposure, so a roof can sit dried-in through a long weather delay without the underlayment failing.
- It’s tough. High tear strength and fastener-pull-through resistance mean it doesn’t rip in the wind or blow off before shingles go down.
- It’s walkable and light. Non-slip surfaces make steep work safer, and one roll covers far more deck than a roll of felt, so it goes down faster with fewer seams.
The one honest caveat: most synthetics are near-vapor-impermeable — perm ratings well below 0.1 — so on a roof assembly that depends on drying upward through the deck, the wrong synthetic can trap moisture. On a properly ventilated roof this is a non-issue, but it’s a real consideration on unvented or complex assemblies, and it’s why “just use synthetic everywhere” isn’t automatically correct on every building.
3. Self-adhered membrane — the waterproof one
The third family isn’t a water-shedding sheet at all — it’s a genuinely waterproof membrane, and it’s the most important underlayment on the roof even though it usually covers the least area.
Self-adhered underlayment (the category most people know by the trade shorthand “ice and water shield”) is a polymer-modified bitumen sheet with an aggressive adhesive backing protected by a release film. You peel the film and press the sheet onto the deck, where it bonds completely and permanently. Its defining property is that it self-seals around fasteners: when a roofing nail is driven through it, the rubberized asphalt closes around the shank so tightly that the penetration doesn’t leak. Felt and synthetic have holes at every nail; a self-adhered membrane effectively doesn’t.
That property is exactly what you want in the places water is most likely to be forced past the shingles rather than merely leaking through:
- The eaves, where ice dams back standing water uphill under the shingles (the mechanism is all physics — see Ice Dams).
- The valleys, where two roof planes dump their combined runoff into one channel at high velocity.
- Around every penetration — chimneys, skylights, vent pipes, walls — where the flashing does the primary sealing but a self-adhered membrane underneath is the backup that catches what the flashing misses.
- Low-slope sections, where water moves slowly enough to find any gap (below a certain pitch, shingles need a fully-adhered underlayment or shouldn’t be used at all — see Low-Slope and Flat Roofs).
It is the most expensive underlayment per square foot, which is why it isn’t used across the whole roof on a standard shingle job — but where it’s needed, nothing else does the job, and skipping it is one of the most consequential corners a contractor can cut.
What code actually requires — and where
Here is where most homeowners are surprised: the building code doesn’t just permit underlayment, it dictates it — and the requirement changes depending on your roof’s slope and your region’s climate. These come from the International Residential Code (IRC), Chapter 9, which the vast majority of U.S. jurisdictions adopt.
Standard underlayment (the whole roof). For asphalt shingles on a normal slope (4/12 and steeper), code requires at least one layer of underlayment across the entire deck. Drop below 4/12 into low-slope territory and the requirement doubles to two layers (or a single fully-adhered membrane), because water moves slower and sits longer on a shallow pitch. This is one more reason roof pitch quietly governs so much of the job.
The ice barrier. This is the requirement people don’t know about until it saves them — or its absence costs them. In any region with “a history of ice forming along the eaves causing a backup of water,” the IRC (Section R905.1.2) requires a self-adhered ice barrier — either two cemented layers of underlayment or a self-adhering polymer-modified sheet — running from the roof’s lowest edge to a point at least 24 inches inside the exterior wall line of the heated space below.
That last detail is the one that trips up bad installs. The membrane doesn’t stop at the edge of the roof — it has to extend far enough up the slope to clear the inside face of the exterior wall, because that’s where the warm wall meets the cold overhang and where an ice dam’s backed-up water pools. On a house with a deep overhang, hitting “24 inches past the wall line” can mean a course and a half of membrane, not just a strip along the very edge. And on steeper roofs (8/12 and up), because the horizontal run covers more slope, that same rule can push the membrane 36 inches or more measured along the roof surface. A crew that runs a single 36-inch-wide roll along the eave and calls it done has, on many houses, not actually met code.
The takeaway for a homeowner in snow country: the ice barrier is not an upgrade you should have to ask for. It’s code. If a quote doesn’t mention it, that’s a question to ask before you sign — not a change order to discover in February.
The special case: metal roofs need a different product entirely
If you’re putting on a metal roof, the underlayment conversation changes completely, and it’s the single most-skipped detail in metal roofing.
Metal gets hot. A dark standing-seam roof in summer sun, with the metal conducting heat straight down onto the underlayment beneath it, routinely drives the sheathing-surface temperature to 240–260°F. Standard self-adhered membranes — the ordinary “ice and water shield” that’s perfect under asphalt shingles — use an asphalt-based adhesive that begins to soften above roughly 250°F. Under a metal roof, that means the adhesive can literally re-liquefy: it loses its grip, oozes, drips out of the eave, and stops sealing around the fasteners it was installed to seal. The failure is invisible for a couple of summers and then shows up as “year-three leaks” that make no sense to anyone who doesn’t know what’s happening underneath.
The fix is a purpose-built high-temperature (HT) underlayment — typically a butyl-based self-adhered membrane rated for continuous service at 240°F and above, and tested (often to ICC-ES AC48 or ASTM D1970) specifically for the metal-roof thermal environment. It costs more than a standard membrane. It is not optional under metal. A contractor who reaches for the same roll they’d use under shingles is setting a clock on the roof, and the homeowner is the one who finds out.
Reading it in your quote — and the corner that gets cut
Underlayment is one of the easiest places for a bid to be quietly cheap, precisely because the homeowner can’t see it once it’s installed and rarely knows to ask. A few things to look for:
The material should be named. “Underlayment” is not a spec. A good quote says what’s going down — a specific synthetic product, felt weight, and the self-adhered membrane at the eaves and valleys — the same way it names the shingle. Vagueness here is where the substitution happens.
The ice barrier should be spelled out, by location. “Ice and water shield at eaves and valleys, extending 24 inches past the interior wall line” is a real spec. “Ice and water as needed” is a blank check, and in a cold climate it’s the wrong one.
The full-adhesion detail matters. A self-adhered membrane only works if it’s actually pressed down onto a clean, dry, primed (where required) deck. Applied over dust, over a damp deck, or in cold weather without the right primer, it won’t bond — and an unbonded “waterproof” membrane is just an expensive sheet lying on the wood. This is craftsmanship you can’t inspect after the shingles are on, which is one more reason to hire on the strength of the crew rather than the bottom-line number (see How to Vet a Roofing Contractor).
Warranties reach down here. Manufacturer “system” warranties — the enhanced coverage that requires several components from the same maker — usually list a qualifying underlayment among the required parts. Mixing a bargain underlayment under a premium shingle can quietly disqualify the very warranty the premium shingle was bought for. If a long system warranty is part of why you’re paying up, the underlayment line is part of earning it.
The economics are lopsided in the homeowner’s favor, which is what makes cutting this corner so indefensible. Upgrading a whole roof from the cheapest felt to a quality synthetic, with a proper self-adhered membrane where code and good practice call for it, adds a small fraction to the total — often a couple hundred dollars on a five-figure job. What it buys is the difference between a roof that shrugs off a blown-off shingle and one where the same shingle becomes a soaked deck, a stained ceiling, and an insurance call. The second roof under your shingles is the cheapest insurance on the whole building. It’s worth knowing it’s there — and worth making sure it’s the one you’d have chosen.
ShowMyRoof lets you see a photorealistic preview of your own home with a new roof before you spend a dollar — and connects you with a vetted local roofer when you’re ready for a real quote. The best time to ask what’s going under your shingles is before the old ones come off.