Roofing
Roofing underlayment types: felt, synthetic, and self-adhered field guide
The layer between the deck and the covering, type by type: asphalt felt, synthetic, and self-adhered ice and water shield, with the slope rule, fastening, exposure, and where each one belongs.
Direct answer
Roofing underlayment is the layer between the roof deck and the covering, a secondary water barrier that protects the deck if water gets past the shingles and a temporary dry-in before the covering goes on. The three types are asphalt felt, synthetic, and self-adhered ice and water shield. The manufacturer instructions and adopted code control.
Key takeaways
- Roofing underlayment is the layer between deck and covering, serving as the secondary water barrier and the temporary dry-in; the three types are asphalt felt, synthetic, and self-adhered membrane.
- Self-adhered membrane is waterproof and seals around fasteners; felt and synthetic only shed water and leak at fastener holes when water stands.
- Ice and water shield goes at eaves, valleys, penetrations, and low-slope sections, commonly reaching at least 24 inches inside the exterior wall line per code.
- Fasten underlayment with cap nails or cap staples, not bare staples, often about 6 inches at laps and 12 inches in the field with a roughly 1 inch cap.
- Slope drives the underlayment: single layer at roughly 4:12 and up, double layer at 2:12 to 4:12 for shingles, low-slope membrane below about 2:12.
What roofing underlayment is and the job it does
Roofing underlayment is the layer that goes between the roof deck and the roof covering. It is the sheet you roll out across the clean deck before the shingles, the metal, or the tile go on. Two jobs ride on it. First, it is the secondary water barrier, the backup that protects the deck if water ever gets past the covering. Second, it is the dry-in, the temporary skin that keeps the building dry while the covering is still being installed.
The covering is the primary barrier. The shingles, the panels, the tiles do the daily work of shedding rain. Underlayment is what catches the water the covering misses, and water does get past a covering. Wind drives rain sideways under a shingle butt. A nail backs out. Ice dams up at the eave and pushes meltwater uphill under the courses. When that happens, the underlayment is the layer between the wet and the wood.
This guide is about the underlayment itself, type by type. The shingle system that sits on top of it is covered in the steep-slope asphalt shingle guide, and the deck it rolls out over is covered in the roof deck substrate guide. Read those alongside this one. The covering and the deck both drive which underlayment you run, and none of the three layers makes sense without the other two.
Why does a roof need underlayment?
A roof needs underlayment because the covering is a shedding layer, not a sealed one, and the deck behind it is wood or board that rots the moment it stays wet. Underlayment is the secondary barrier that buys you the margin between a covering that lets a little water through and a deck that fails.
Run through what the covering cannot do on its own. Asphalt shingles lap each other and shed water down the slope, but they are full of nail holes and butt joints, and wind-driven rain works backward under the laps. A metal panel sheds beautifully until a fastener gasket ages or condensation forms on the underside. Tile is a rain screen that expects water to get behind it. In every one of those systems, the design assumes some water reaches the layer below, and the layer below is the underlayment.
The dry-in job matters just as much on the schedule. You strip a roof, you sheath it, and you are not getting the covering on before the next weather. The underlayment is what keeps that open deck dry overnight and through a shower. A felt or synthetic sheet is a temporary roof. A self-adhered membrane is closer to a permanent one. Either way, the deck does not sit bare, because a deck that gets soaked before the covering goes on is a deck that telegraphs, swells, and grows mold under a brand-new roof.
Asphalt-saturated felt: #15 and #30
Asphalt-saturated felt is the traditional underlayment, the black paper roll the trade ran for a century before synthetic showed up. It is an organic or fiberglass mat saturated with asphalt so it sheds water, and it comes in two common weights. No. 15 is the lighter, thinner roll. No. 30 is heavier, thicker, and tougher, and it is what most pros reach for when they run felt at all. The numbers came from an old weight-per-square convention, not a strict gauge, so what is in the wrapper varies by maker.
Felt is cheap and proven. An inspector has seen it on ten thousand roofs and knows how it behaves. That is its case, and it is a real one. The downsides are why the trade moved off it. It tears, especially at the fasteners and in wind before the covering is on. It wrinkles and buckles when it gets wet, and those wrinkles do not lie back down. It is heavy and the rolls are short, so you are hauling more rolls up the ladder and seaming more often. And it has a short tolerance for sun, drying out and cracking if it sits exposed.
The wrinkle problem is the one that bites after the roof looks done. Felt absorbs water, swells, and ridges up, and if it dried wrinkled under the shingles, those ridges telegraph through the covering as visible lines across the field. You see it most on a low-angle morning when the light rakes across the roof. The shingles are fine. The felt under them moved. That is a callback you cannot fix without pulling the covering, and it is the single best argument for the next material.
Synthetic underlayment, and why it took over
Synthetic underlayment is a woven or spun polypropylene or polyethylene sheet, and it is the modern default on most shingle and metal work. It does the same secondary-barrier and dry-in job as felt, but it is built like a tarp instead of like paper, and that changes how it behaves on the roof.
The reasons it took over are practical, not marketing. It is far lighter, on the order of 2 to 4 pounds per square against roughly 20 to 30 for No. 30 felt, so a crew that needed 14 rolls of felt to cover a house covers it with about 3 rolls of synthetic. It does not tear the way felt does, so it holds the fasteners and survives a wind that would shred felt before the shingles are on. The rolls are wider and cover more per roll, so there is less seaming and faster coverage. Most products have a printed, slip-resistant walking surface, which matters when a wet felt roof is an ice rink and a wet synthetic roof has traction.
Two more reasons close the case. Synthetic does not absorb water, so it does not wrinkle and buckle, which kills the telegraphing problem felt creates. And it tolerates more sun, holding up for a stated number of months of exposure where felt cracks in weeks. It costs more per roll than felt. It is usually cheaper installed once you count the labor, the rolls, and the callbacks you did not get, which is why most production crews stopped buying felt for the field.
What is the difference between felt and synthetic underlayment?
The difference between felt and synthetic underlayment is the material and the way it behaves under stress. Felt is asphalt-saturated paper, cheap and familiar but heavy, tear-prone, and quick to wrinkle when wet. Synthetic is woven plastic, lighter, far stronger against tearing, walkable, and stable when it gets wet or sits in the sun. Both are shedding layers, not sealed ones.
Cost is where felt still wins on the sticker. A roll of felt is cheaper than a roll of synthetic, full stop. The math flips when you count coverage and labor. Synthetic covers far more per roll, goes up faster, and weighs a quarter of what felt weighs, so the installed cost is often a wash or in synthetic's favor once the crew time is in the number.
The head-to-head below is the version a foreman carries. The water story is the part people oversell. Synthetic repels water better and survives exposure longer, but neither felt nor synthetic is waterproof and neither seals around a fastener. For that you need the self-adhered membrane covered further down. If you remember one thing, it is that synthetic beats felt on weight, tear, traction, and exposure, and felt only beats synthetic on the price of the roll itself.
| Property | Asphalt felt (#15 / #30) | Synthetic |
|---|---|---|
| Material | Asphalt-saturated paper or mat | Woven polypropylene or polyethylene |
| Weight per square | Roughly 20 to 30 lb for #30 | Roughly 2 to 4 lb |
| Tear resistance | Low, tears at fasteners and in wind | High, holds fasteners and foot traffic |
| Behavior when wet | Absorbs, wrinkles, buckles | Sheds, stays flat |
| Sun exposure tolerance | Short, cracks in weeks | Longer, months per the maker |
| Traction underfoot | Slick, worse when wet | Printed slip-resistant surface |
| Coverage per roll | Less, more seams | More, wider rolls |
| Roll cost | Lower | Higher |
| Waterproof | No, shedding layer | No, shedding layer |
Self-adhered membrane: ice and water shield
Self-adhered membrane, the product most people call ice and water shield, is a different animal from felt and synthetic. It is a rubberized asphalt or butyl sheet with a peel-off release film on the back, and you stick it straight to the deck. The thing that sets it apart is that it is waterproof and it self-seals around fasteners. Drive a nail through it and the compound closes around the shank instead of leaving an open hole.
That seal is the whole point. A felt or synthetic underlayment sheds water but leaks at every fastener the moment water stands on it. A self-adhered membrane holds water like a bathtub and grips the nails that go through it, so it protects the deck in the exact places where water backs up and sits instead of running off. It is the ice-dam barrier in cold climates, the layer that stops meltwater from getting under the shingles at the eave when ice piles up and dams the gutter.
It does not go everywhere on a normal roof, because it is the most expensive of the three and the field of a steep roof does not need a sealed layer. It goes in the high-risk areas, and on low-slope or high-wind roofs it can go over the whole deck. Where it goes is covered in the next section, and the ice-dam detailing at the eave is also covered in the steep-slope shingle guide. Match the two so the eave is built the same way in both.
Where does ice and water shield go?
Ice and water shield goes in the spots where water concentrates, backs up, or gets driven in, not across the whole field of a normal steep roof. The high-risk areas are the eaves, the valleys, around every penetration, the low-slope sections, and the rakes and edges on high-wind or coastal work. Those are the places a shedding underlayment is not enough.
At the eaves it is the ice-dam barrier. In a cold climate the code-referenced detail runs the membrane from the lowest edge of the roof to a point a set distance inside the warm-wall line of the building, commonly stated as at least 24 inches past the interior face of the exterior wall. The reason is physics. Ice dams form where the warm roof meets the cold overhang, so the membrane has to reach back past the warm wall to cover where the meltwater actually backs up. Confirm the distance and the trigger against the adopted code, because the climate threshold and the dimension are set there.
In the valleys it lines the channel before the metal or the woven shingles, because a valley carries the runoff from two planes and is where leaks start. Around penetrations it wraps pipes, the chimney curb, and skylights, sealing the cuts where the field is interrupted. On low-slope sections and on high-wind and coastal roofs, the smart move is often to run it over the entire deck, because the whole roof is now a high-risk area and a sealed layer is cheap insurance against a covering that will see wind-driven rain its whole life.
High-temperature self-adhered under metal and tile
Under metal and under tile, the heat on the underlayment is a problem the standard self-adhered membrane is not built for. A dark metal roof in the sun runs far hotter on its underside than a shingle roof, and the asphalt compound in an ordinary ice and water shield can soften, flow, and lose its grip at those temperatures. The fix is a high-temperature self-adhered membrane made for it.
High-temp membrane uses a compound and a facer rated for the higher service temperature you see under metal panels and clay or concrete tile. Many products carry a stated temperature range, and the high-temp versions sit well above what a shingle-rated membrane tolerates. If you put a standard membrane under a hot metal roof, you can find it slid, bled, or stuck to the panel in a way that telegraphs and fails, and that is a tear-off to fix.
The rule is to match the membrane to the heat of the covering, not to grab whatever roll is on the truck. Metal and tile both call for a high-temp self-adhered where a self-adhered layer is used, and they often call for synthetic rather than felt in the field for the same heat reason. The covering manufacturer's instructions state the temperature rating the underlayment has to meet, and on a warranted roof that instruction is the one that controls.
Waterproof vs water-resistant: what is the difference?
The difference is whether the layer seals or only sheds. Self-adhered membrane is waterproof and seals around the fasteners that go through it. Felt and synthetic are water-resistant, meaning they shed water that runs across them but do not seal at the fasteners and are not built to hold standing water. That single distinction decides where each one belongs.
Think about a nail through each material. Through synthetic or felt, the nail leaves a hole, and as long as water sheds past it nothing happens, but let water pond or back up over that hole and it leaks. Through self-adhered membrane, the compound flows around the shank and closes the hole, so even standing water stays out. That is why the membrane is the only one of the three you trust where water sits: the eave behind an ice dam, a valley, a low-slope plane.
Do not let a product's water-repellent marketing blur this line. A synthetic that repels water well is still a shedding layer with open fastener holes. It is the right material for the field of a steep roof, where water runs off fast and never stands. It is the wrong material for a spot where water backs up. Use the shedding layer where the slope does the work, and the sealed membrane where the slope cannot.
Underlayment by roof covering
The covering and the slope drive the underlayment, not crew habit. Asphalt shingles run synthetic or felt in the field with self-adhered membrane at the eaves, valleys, and penetrations. Metal and tile run hotter and demand more of the layer below, so they lean on synthetic in the field and high-temperature self-adhered where a sealed layer is used. Match the underlayment to what the covering needs and what the covering manufacturer requires for the warranty.
Metal is the case people get wrong. A standing-seam or exposed-fastener metal roof transmits heat to the underlayment and lives a long time, so the underlayment under it has to outlast a shingle-grade product and tolerate the heat. That means a high-temp synthetic or a high-temp self-adhered, not a cheap felt that will be cooked and cracked while the metal is still good. Tile is the same story with added weight and a covering that openly expects water behind it, so the underlayment is doing more of the real water work than it does under shingles.
The table below is a starting point, not the spec. The covering manufacturer's published instructions and the adopted code set the actual requirement, and on a warranted system the manufacturer's underlayment call is what keeps the warranty intact. Read it before you order, because the wrong underlayment under the right covering voids the coverage and shows up years later as a deck failure nobody can warranty.
| Covering | Field underlayment | High-risk areas | Heat note |
|---|---|---|---|
| Asphalt shingles | Synthetic or felt | Self-adhered at eaves, valleys, penetrations | Standard temperature |
| Metal panel | Synthetic, often high-temp | High-temp self-adhered | Runs hot, needs heat rating |
| Clay or concrete tile | Synthetic, often high-temp | High-temp self-adhered | Hot and heavy, water behind tile |
| Low-slope section | Self-adhered or doubled | Self-adhered over whole plane | Water stands, seal it |
Does the roof slope decide the underlayment?
Yes. The slope decides whether one layer of underlayment is enough, whether you double it, or whether you go to a sealed membrane, because slope controls how fast water leaves the roof. The steeper it runs, the faster water sheds and the less the underlayment has to do. The flatter it gets, the longer water sits, and a single shedding layer stops being enough.
On a standard steep slope, the common asphalt-shingle floor of roughly 4:12 and up, a single layer of underlayment in the field is the norm, with self-adhered membrane in the high-risk areas. Drop into the reduced-slope range, commonly stated as 2:12 up to 4:12 for shingles, and the code-referenced detail calls for a double layer of underlayment, lapped so it behaves like a doubled, tighter skin under a covering that now sheds more slowly. Below about 2:12 you are off shingles entirely and onto a low-slope membrane system, where the underlayment question becomes a low-slope membrane question.
This tracks the slope rule in the steep-slope shingle guide, where the slope sets whether shingles even apply. The same boundary that says use shingles or switch to a membrane also says use one layer, two layers, or a sealed membrane underneath. Confirm the exact slope thresholds and the doubling requirement against the adopted code and the shingle manufacturer's instructions, because the numbers live there and they move between code cycles.
Fastening: cap nails, not bare staples
Fasten underlayment with cap nails or plastic-cap staples, not bare staples. A bare staple or a smooth roofing nail pulls a tiny head through the sheet under wind load and foot traffic, and the underlayment tears free and blows off before the covering is on. The cap is the whole reason the fastener holds. It spreads the load over a wide plastic or metal disc so the sheet cannot tear past it.
For synthetic in particular, plastic cap nails are the standard, and the manufacturers print the pattern right on the product or in the instructions. A common pattern runs the caps closer together along the laps and edges and wider in the field, often stated as around 6 inches at the laps and 12 inches in the body, with a minimum cap diameter the maker specifies, commonly about 1 inch. Confirm the spacing and the cap size against the product instructions, because the wind rating and the warranty depend on the exact pattern.
The blunt version: bare staples through synthetic are a callback waiting on a windy night. The sheet looks fastened, the crew moves on, and the first real gust unzips a section because every staple sliced through. If the covering is not going on the same day, every fastener gets a cap and the pattern gets tightened, because that exposed sheet is your only roof until the shingles arrive.
How long can underlayment stay exposed?
Underlayment can stay exposed for the window the manufacturer states, and that window is short for felt and longer for synthetic, but the rule on every roof is the same: cover it as soon as you can. Underlayment is a dry-in, not a finished roof, and the sun is what kills it while it waits.
Felt has the shortest patience. Left in the sun it dries out, loses its oils, curls, and cracks in a matter of weeks, and once it has cracked it is no longer a barrier. Synthetic holds up far longer, with products stating exposure windows measured in months, because the plastic resists the sun in a way asphalt paper does not. Self-adhered membrane has its own exposure limit on the facer, and the high-temp products tolerate more. None of these numbers is a license to leave the deck dried-in all season.
Treat the stated exposure time as a maximum under ideal conditions, not a plan. Wind, foot traffic, and ponding all shorten it, and an underlayment that sat out near its limit has spent margin you wanted for the life of the roof. The reason to cover it fast goes past UV. An exposed dry-in is one wind event away from peeling, and a peeled dry-in over an open deck is a wet deck. Confirm the exposure rating against the product, and schedule the covering so you never test it.
Laps: shingle the underlayment so it sheds
Underlayment goes on shingle-style, lapped so that every upper sheet overlaps the one below it and water runs down the laps instead of into them. Run the first course along the eave, then each course above laps over the top of the course below. Get the lap direction backward and you have built a funnel that channels water under the sheet at every seam.
The two laps to hold are the head lap and the side lap. The head lap is the horizontal overlap where the next course up covers the top of the course below, commonly a few inches and printed as a line on the sheet. The side lap, or end lap, is the vertical overlap where one roll meets the next along the run, also a few inches and also printed. Most underlayment carries those overlap lines right on the surface so the crew lands the next course on the line instead of guessing. On reduced slope the laps widen, because water moves slower and the seams have to be tighter.
The printed lines are there because the lap is the part that fails when a crew is moving fast. A short lap, or a course landed below the line, leaves a seam that sheds backward under wind-driven rain. Follow the lines, keep the laps shingled down-slope, and the sheet behaves like the shedding layer it is supposed to be. Confirm the lap dimensions against the product and the slope, because the reduced-slope laps are larger and the manufacturer states them.
The deck under the underlayment
Underlayment goes over a clean, dry, sound deck, and it does not fix a bad one. It is a thin sheet. It follows whatever is under it, so a deck with high fasteners, swollen edges, gaps between panels, or rot will telegraph through the underlayment and the covering both. The deck has to be right first.
Clean means swept free of the old roof's debris, the torn felt, the nails, and the grit, because anything left under a self-adhered membrane stops it from bonding and anything sharp under any underlayment is a puncture waiting to happen. Dry matters most for the self-adhered membrane, which will not stick to a damp or dusty deck and will let go later if you force it. Sound means the panels are fastened, the edges are supported, and there is no soft, rotted, or delaminated sheathing left to roof over.
What the deck is made of also drives the underlayment and the fastener, and that is its own subject. The roof deck substrate guide covers the deck types, the nailable versus non-nailable split, and the moisture problem with new concrete. Read it before you spec the underlayment, because a cap nail that grabs plywood does nothing on a concrete or steel deck, and the attachment method changes with the deck under your feet.
High-wind attachment and code
In high-wind and coastal zones the underlayment attachment is not a default pattern, it is a rated detail, and the code and the manufacturer both tighten it. The underlayment is part of the wind-uplift path until the covering is on, and in a hurricane zone it is also a backup if the covering itself comes off, so how it is fastened and what it is made of both get stricter.
Two things change in high wind. The fastener pattern tightens, with caps spaced closer at the laps and in the field than the standard pattern, and the cap size and type get specified rather than assumed. And the material leans toward synthetic or self-adhered, because felt does not survive the wind exposure during the dry-in and a sealed membrane gives a real secondary barrier if the covering is lost. Many high-wind specs run self-adhered over the entire deck for exactly that reason.
The numbers here belong to the adopted code and the manufacturer, not to a rule of thumb. Wind-borne-debris regions, the design wind speed, and the specific assembly all set the requirement, and a roof in a high-velocity hurricane zone is governed by detailed product approvals. Confirm the attachment, the material, and any whole-deck membrane requirement against the adopted code, the local amendments, and the product listing before you bid it, because guessing the pattern in a wind zone is how a roof fails its inspection or its first storm.
Underlayment is not a vapor retarder
Underlayment and a vapor retarder are not the same layer and do not do the same job. Underlayment is a water barrier on the outside of the deck that stops liquid water from the weather. A vapor retarder is a layer that slows water vapor moving through the assembly from the inside, and in a roof system it usually lives below the insulation, on the warm side, not on top of the deck under the covering. Mixing them up causes real moisture problems.
The distinction matters because a self-adhered membrane is also a vapor barrier whether you wanted one there or not. Run a fully self-adhered, impermeable membrane over a whole deck and you have put a vapor-tight layer on the cold side of the assembly, and in the wrong climate or the wrong assembly that traps moisture trying to dry outward and rots the deck from below. That is a different failure from a leak and it fools people, because the roof never leaked from the sky.
Roof moisture and vapor control is its own subject, and the right answer depends on the climate zone, the insulation, and where the dew point lands in the assembly. Treat underlayment selection and vapor control as two separate decisions, and on a fully-adhered or low-slope assembly bring in the vapor question deliberately rather than discovering it after the deck is wet. Confirm the assembly against the building science for the climate zone and the adopted code.
Commercial, low-slope, and data-center roofs
On commercial and low-slope roofs the underlayment question changes shape, because the covering is usually a membrane system rather than shingles and water is expected to sit, not shed. The steep-slope idea of a shedding underlayment under a shedding covering does not carry over cleanly. What rides under a single-ply or built-up roof is more often a self-adhered base sheet, a slip sheet, a cover board, or an air and vapor barrier, each chosen for the specific assembly.
Where steep-slope underlayment thinking does apply on commercial work is the transition and the low-slope plane on an otherwise steep roof. A low-slope porch, a connector, or a section under 2:12 on a building that is shingles elsewhere needs the sealed treatment, commonly a self-adhered membrane over the whole plane, because the slope no longer sheds. That is the same slope rule from earlier, applied to the one plane that broke the threshold.
Data-center and mission-critical roofs raise the stakes on the secondary barrier, not the basic physics. A leak over a white-space or a power room is a far more expensive event than a leak over a warehouse, so the design tends to over-build the redundancy, the sealed layers, and the detailing at penetrations, and the manufacturer and the specifier drive that. The roof deck substrate guide covers the commercial deck types those assemblies attach to. Treat the commercial underlayment package as part of the membrane assembly it belongs to, specified with it, not bolted on after.
What to document
A roof's underlayment disappears the moment the covering goes on, so the record is the only proof of what is under there. When a leak shows up in year three, the question is what underlayment was run, where the self-adhered membrane went, and whether it matched the covering manufacturer's requirement. If nobody wrote it down, nobody can answer it, and the warranty conversation goes nowhere.
Capture the field underlayment type and product, the self-adhered membrane product and where it went, the fastener type and pattern, the slope of each plane and whether it triggered a double layer, the exposure time before the covering went on, and the covering manufacturer's required underlayment for the warranty. Photograph the eaves, valleys, and penetrations before the covering covers them, because those photos are the evidence the high-risk areas were sealed the way the spec called for.
| Field to record | Why it matters |
|---|---|
| Field underlayment type and product | Sets the covering warranty and the exposure limit |
| Self-adhered membrane and locations | Proves the high-risk areas were sealed |
| Fastener type and pattern | Ties to the wind rating and the manufacturer pattern |
| Slope per plane, single or double | Shows the slope rule was applied |
| Exposure time before covering | Flags underlayment that sat near its UV limit |
| Covering manufacturer requirement | The instruction that controls the warranty |
Common mistakes
- Fastening synthetic with bare staples instead of cap nails, so the sheet tears off in the first wind.
- Leaving felt exposed for weeks until it dries, curls, and wrinkles, then roofing over the wrinkles so they telegraph.
- Skipping ice and water shield at the eaves in a cold climate, so an ice dam pushes meltwater under the shingles.
- Running a standard self-adhered membrane under metal or tile, where the heat softens the compound and it flows.
- Putting a single layer of underlayment on a reduced-slope plane that the code says needs a double layer.
- Lapping the underlayment the wrong way, so the seams channel water under the sheet instead of shedding it.
- Running a fully impermeable self-adhered membrane over a whole deck without checking the vapor and dew-point question.
- Spec-ing the underlayment by crew habit instead of the covering manufacturer's required product for the warranty.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Standards and references
The material specifications come from ASTM. Asphalt-saturated organic felt is covered by ASTM D226 and, for steep-slope underlayment specifically, ASTM D4869, which sorts felt into types by weight and includes a water-shedding test. Self-adhered ice and water membrane is covered by ASTM D1970, written for self-adhering polymer-modified bituminous sheet used for ice-dam protection. Synthetic underlayment got its own product standard, ASTM D8257, for mechanically attached polymeric underlayment, after years of being qualified against the felt standards. A glass-fiber underlayment standard, ASTM D6757, also appears in the code tables. Confirm the current designation and revision, because these standards are revised on their own cycles.
The installation requirements come from the adopted building code and the covering manufacturer. The residential code chapter on roof assemblies sets the underlayment requirement, the double-layer rule for reduced slope, and the ice-barrier detail at the eaves, and it references the ASTM material standards in its tables. The exact section numbers and the ice-barrier trigger and dimension live in that chapter and shift between code editions, so confirm them against the edition the jurisdiction has actually adopted and any local amendments before you cite them on a submittal.
On a warranted roof, the covering manufacturer's published instructions are the document that controls. They state the required underlayment, the temperature rating for metal and tile, the fastener type and pattern, and the exposure limit, and installing anything other than what they require can void the warranty. Where the code and the manufacturer both speak, follow the stricter of the two, and let the project specification override a rule of thumb whenever it is tighter.
Units, terms, and conversions
Roofing underlayment carries its own vocabulary, and the same idea reads differently across a spec, a product sheet, and a code table, so the terms below are the ones that cause confusion on the job.
Coverage is measured in squares, where one square is 100 square feet of roof area, and felt weight is described per square, which is where No. 15 and No. 30 come from. Slope is written as rise over run in inches per foot, such as 4:12, meaning 4 inches of rise for every 12 inches of run. Self-adhered membrane is the same product the trade calls ice and water shield, peel-and-stick, or self-adhering membrane. Dry-in is the act of getting the deck weather-tight with underlayment before the covering goes on.
- Underlayment
- The layer between the deck and the covering, the secondary water barrier and the temporary dry-in
- Square
- 100 square feet of roof area, the unit roofing coverage and felt weight are measured in
- #15 / #30 felt
- Asphalt-saturated felt by an old weight-per-square convention, #30 being the heavier, tougher roll
- Self-adhered membrane
- Peel-and-stick rubberized asphalt or butyl sheet that is waterproof and seals around fasteners, the ice and water shield
- Dry-in
- Getting the deck weather-tight with underlayment before the roof covering is installed
- Slope (pitch)
- Rise over run in inches per foot, such as 4:12, which drives single, double, or membrane underlayment
- Head lap / side lap
- The horizontal and vertical overlaps between underlayment courses, printed as lines on most sheets
FAQ
What is roofing underlayment?
Roofing underlayment is the layer between the roof deck and the covering. It is a secondary water barrier that protects the deck if water gets past the shingles, and a temporary dry-in that keeps the deck weather-tight before the covering goes on. The three types are asphalt felt, synthetic, and self-adhered membrane.
What is the difference between felt and synthetic underlayment?
Felt is asphalt-saturated paper: cheap, heavy, tear-prone, and quick to wrinkle when wet. Synthetic is woven plastic: lighter, far stronger, walkable, and stable when wet or sun-exposed. Both are shedding layers, not sealed ones. Synthetic costs more per roll but often less installed, which is why it took over the field.
What is ice and water shield?
Ice and water shield is a self-adhered, peel-and-stick membrane of rubberized asphalt or butyl that sticks to the deck. Unlike felt or synthetic, it is waterproof and seals around the fasteners driven through it. It goes at eaves, valleys, and penetrations, and over whole low-slope or high-wind decks, where water backs up and sits.
Do you need underlayment under a roof?
Yes. The covering sheds water but is not sealed, and the deck behind it rots when it stays wet. Underlayment is the secondary barrier that catches water the covering misses and the dry-in that protects the open deck before the covering goes on. Confirm the required type against the adopted code and the covering manufacturer.
What underlayment goes under a metal roof?
Metal runs hot on its underside, so it needs an underlayment rated for the heat, commonly a high-temperature synthetic or a high-temperature self-adhered membrane. A standard ice and water shield can soften and flow under metal. The metal manufacturer's instructions state the required temperature rating, and on a warranted roof that requirement controls.
Can you use staples for synthetic underlayment?
No. Bare staples pull through synthetic under wind and foot traffic and the sheet tears off before the covering is on. Use plastic cap nails or cap staples on the manufacturer's pattern, commonly tighter at the laps and edges than in the field, with a minimum cap diameter the maker specifies, often about 1 inch.
How long can roofing underlayment stay exposed?
It depends on the product. Felt cracks and curls in the sun within weeks, while synthetic states exposure windows measured in months and self-adhered membrane has its own limit. Treat the stated time as a maximum under ideal conditions, not a plan, and cover it sooner in wind. Confirm the rating against the manufacturer.
Does roof slope change the underlayment?
Yes. Slope controls how fast water sheds. Standard steep slope, roughly 4:12 and up, takes a single field layer with membrane at the high-risk areas. Reduced slope, commonly 2:12 to 4:12 for shingles, takes a double layer. Below about 2:12 you move to a low-slope membrane system. Confirm the thresholds against the adopted code.
Where does ice and water shield need to go?
At the eaves past the warm-wall line in cold climates, in the valleys, around every penetration, and on low-slope sections. On high-wind and coastal roofs it often runs over the entire deck. The eave dimension and the climate trigger come from the adopted code, commonly stated as reaching at least 24 inches inside the exterior wall line.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.