Quick answer
Roof underlayment is a sheet laid over the whole roof deck before the shingles or other roof covering go on. It keeps the wood deck dry if water gets past the covering, and it protects the deck while the roof is being built.
The three main types are asphalt-soaked felt, synthetic underlayment made of plastic sheets, and self-adhered membrane that sticks to the deck. The building code requires underlayment under shingles, tile, slate, wood and metal roofs 1.
Quick facts
Key takeaways
- Underlayment is the sheet between the wood roof deck and the shingles. It is a backup layer for water that gets past the roofing.
- There are three main types: felt (No. 15 or No. 30), synthetic, and self-adhered membrane.
- The code requires underlayment under shingles, tile, slate, wood and metal roofs, and sets how it is laid and fastened.
- On asphalt shingle roofs from 2/12 up to 4/12, the code calls for two layers instead of one.
- On a quote, look for the product name, the type, how many layers, and where self-adhered membrane goes.
What underlayment is
A sloped roof is built in layers. At the bottom is the roof deck, the plywood or board sheathing nailed to the rafters. On top is the roof covering, such as shingles, tile or metal.
Underlayment is the thin layer in between. It is rolled out across the deck in rows, from the bottom edge of the roof up to the ridge.
The shingle makers' trade association describes underlayment as a barrier that keeps the shingles from touching the wood deck and shields the deck from moisture 2. That is the short answer to what it does.
It is not the main waterproof layer. The shingles do most of the work. Underlayment is the second line of defense for the water that the shingles miss.
People use many names for it. Older roofers say tar paper or felt paper. Supply houses list it as roofing felt, synthetic underlayment or peel-and-stick. The building code just calls it underlayment and lists the standards each kind must meet 1.
How it works
Water runs downhill. Underlayment is laid like shingles for that reason. The first row goes along the eave, the bottom edge of the roof.
Each row above laps over the one below it, so water that lands on any row runs onto the next one down, not under it. The code calls this applying it "shingle fashion" 1.
Rain can get past shingles in a few ways. Wind can lift a tab and drive water under it. A shingle can crack, blow off or wear thin.
Water can back up behind ice at the eave. When that happens, the underlayment catches the water and sends it down to the gutter, so it does not soak into the wood.
Underlayment also matters before the shingles go on. A torn-off roof is open to the weather.
ARMA notes that underlayment protects the deck from rain if the shingles cannot be put on right away, though it still advises putting the shingles on as soon as possible after the underlayment 2.
It also notes that underlayment helps stop resin in the wood from reaching the shingles, and reduces a defect called picture framing, where the edges of the deck panels show through the shingles 2.
- Drip edge at the eavesGoes on first, on the bare deck 1
- Ice barrier where requiredAt the eaves in ice-dam areas 1
- Underlayment from the bottom upEach row laps the one below, so water flows over the laps 1
- Drip edge at the rakesGoes over the underlayment on the sloped edges 1
- Roof coveringShingles or other covering, ideally the same day 2
The sequence shows the one rule behind every lap: the upper layer always sits on top of the lower one.
Types of underlayment
ARMA lists four kinds used under asphalt shingles 2. The first three are the ones you will see on most quotes.
| Type | What it is | Standard |
|---|---|---|
| Felt (No. 15 or No. 30) | Paper-like felt soaked in asphalt. Type I is No. 15, the lighter weight. Type II is No. 30, the heavier weight. | ASTM D226 2; the code also lists ASTM D4869 1 |
| Synthetic | A woven or spun plastic sheet, often polypropylene, with a coating. Lighter, so a roll covers more roof. | ASTM D8257 2, or an ICC-ES report as an alternative to D226 4 |
| Self-adhered membrane | A sticky, rubbery asphalt sheet that bonds to the deck and seals around nails. See ice and water shield. | ASTM D1970 2 |
| Inorganic fiber felt | Like regular felt, but made with inorganic fibers so it holds its shape better. | ASTM D6757 2 |
"No. 15" and "No. 30" are old weight grades. They tell you which felt you are getting, but the label to look for is the ASTM standard and type, since the code requires underlayment to be labeled that way 1.
Felt: the traditional choice
Felt is the oldest kind still in wide use. It is a felt sheet soaked in asphalt, which ARMA says makes it resist water, though some vapor can still pass through it 2. That small amount of breathing lets a damp deck dry out a little.
No. 30 felt is the heavier grade. It is harder to tear underfoot and stands up a bit better if it is left uncovered for a short while.
No. 15 is lighter and easier to handle. In areas where the code requires wind design, the code calls for the heavier Type II grade under asphalt shingles 1.
Felt has weak points. It can wrinkle when it gets wet and then dries, and the code says any wrinkles must not stop the shingles from sealing 1. It also tears more easily than synthetic sheets, which matters when a crew walks on it.
Synthetic: lighter and tougher
Synthetic underlayment is a plastic sheet instead of felt. One maker's product, as described in its ICC-ES evaluation report, has a woven polypropylene base, a layer of nonwoven polyolefin sheet and a polymer coating on the back 4.
ICC-ES is the code council's evaluation service. Its reports show building departments that a product not named in the code can be used in place of one that is.
That report treats the product as an alternative to ASTM D226 Type I and Type II felt, limited to use under asphalt shingles 4.
It comes in rolls 48 inches wide and 250 feet long 4. ARMA's main point about synthetics is the same: they are lighter, so each roll covers more roof 2.
Synthetic sheets come with their own rules. The report above requires cap nails, which are roofing nails with 1-inch plastic washers, spaced 8 inches apart at the laps and 24 inches apart in the field 4.
It limits the product to roofs with ventilated attics, solid decks and slopes of at least 2/12 4. Every synthetic brand has its own report or instructions, and those govern how it goes on.
Self-adhered: for the weak spots
Self-adhered membrane is a peel-and-stick sheet made to ASTM D1970. ARMA says these membranes seal around nail shanks and protect against ice dams 2. Because they stick to the deck over their whole surface, water cannot run underneath them.
Roofers mostly use it at the eaves, in valleys and around chimneys and skylights. Where the code requires an ice barrier, a self-adhered sheet is one of the two ways to meet it. Our page on ice barrier membrane covers that rule in detail.
The code also lets self-adhered membrane replace the regular underlayment, if it is put on to the maker's instructions for the deck, ventilation and climate 1. The catch is moisture. ARMA notes that these membranes act as vapor retarders, which means they block water vapor, so the attic needs good ventilation 2.
Worked example: two layers on a 3/12 roof
The code sets out how two layers go on a low-slope asphalt shingle roof 1. Here is how that adds up on one rafter run.
| Step | What goes down | Result |
|---|---|---|
| 1 | A 19-inch strip along the eave | Starts the double layer at the edge |
| 2 | A 36-inch sheet over it, from the eave | Two layers at the eave |
| 3 | Each next 36-inch sheet laps the one below by 19 inches | Each sheet adds 17 inches of new roof |
| 4 | End laps of 4 inches, offset by 6 feet | Joints do not line up row to row |
Because each 36-inch sheet only gains 17 inches up the roof, a low-slope roof uses a little more than twice the underlayment of a steep one.
A steep roof at 4/12 or more gets one layer with 2-inch laps, so each 36-inch sheet gains 34 inches. Use our pitch calculator to see which side of 4/12 your roof falls on.
Where it goes on the roof
Underlayment covers the whole deck, not just part of it. The code requires it under asphalt shingles, clay and concrete tile, metal shingles, roll roofing, slate, wood shingles and shakes, metal panels and solar shingles 1.
Some spots get extra. The ICC-ES report above, for example, calls for a single 24-inch-wide strip down the center of every valley before the field underlayment, and over hips and ridges after it 4. Many roofers use self-adhered membrane in valleys and around chimneys, skylights and pipes, where water collects.
At the edges, the order of the layers matters. At the eaves, the underlayment goes over the drip edge. At the rakes, the sloped side edges, it goes under the drip edge 1. That way, water always flows from one layer onto the one below it.
Benefits
- Catches water that wind, ice or a damaged shingle lets past the roof covering
- Keeps the deck dry during the install, between tear-off and shingles
- Separates the shingles from the wood, so resin does not reach them 2
- Helps the finished roof look flat by reducing picture framing 2
- Is required by code, so the roof can pass inspection 1
- Can be part of a fire-rated roof assembly when the listed product is used 4
Limitations
- Felt and synthetic sheets do not seal around nails, so they cannot stop standing water on their own.
- It is not a roof. Most products must be covered within a time limit set by the maker.
- Felt can wrinkle after it gets wet, and wrinkles can stop shingles from sealing 1.
- Self-adhered sheets block vapor, so they depend on good attic ventilation 2.
- A synthetic product's approval may limit it to certain roof coverings and slopes 4.
What drives the cost
Underlayment is usually a line on the quote, priced by the square (100 square feet of roof). We do not list prices. These are the things that move the number. For a full estimate, try our roof cost calculator.
- Type. Felt, synthetic and self-adhered sheets are priced differently, and self-adhered is usually used only in some areas.
- Roof area. Measured in roofing squares, plus waste at hips and valleys.
- Slope. A roof under 4/12 needs two layers, so it uses more material and labor 1.
- Wind zone. Where wind design is required, the code calls for heavier grades and a tighter nailing pattern 1.
- Extra membrane. How far self-adhered sheet runs at the eaves, valleys and around chimneys.
- Fasteners. Cap nails at set spacing take more time than staples or plain nails.
How it is installed
A licensed roofer does this work. These are the steps you can expect and check, based on the code and one product's ICC-ES report 1 4.
- The old roofing and old underlayment come off, down to the bare deck.
- The deck is checked. It must be dry and free of frost, dirt and loose nails, and damaged sheathing is replaced 4.
- Drip edge goes on at the eaves. Ice barrier goes on where the local code requires one.
- Underlayment is rolled out parallel to the eave, starting at the bottom, each row lapping the one below 1.
- It is fastened to the code table and the maker's instructions; under asphalt shingles outside wind-design areas the code says "fastened sufficiently to hold in place" 1.
- Drip edge goes on over the underlayment at the rakes, then the shingles go on, ideally the same day 2.
Keeping it working
You cannot see underlayment once the roof is on, and it is not something you service. What keeps it working is a sound roof covering on top and a dry, vented attic below. Underlayment holds up best when it only sees an occasional drip, not a steady leak.
Attic airflow matters most. The code requires at least a 1-inch air space between the insulation and the roof sheathing at the vents, and says insulation must not block the free flow of air 5.
When the deck stays dry, the underlayment and the wood both last longer. Our ventilation calculator checks the vent area for your attic.
Fix missing or damaged shingles quickly. Underlayment exposed to the sun and rain for months will break down, and once it fails, water reaches the deck.
Signs the underlayment may have failed
- Water stains on the ceiling or on the attic side of the roof deck
- Daylight or dark, wet spots on the deck seen from the attic
- Soft or spongy spots when the roof is walked by a roofer
- Shingles with wavy lines or ridges that follow the rows of underlayment
- Leaks that start after a shingle blew off, before it was replaced
- If Stain on a ceiling
- Then Possible roof leak, but plumbing and condensation look the same. Ask a roofer to trace it.
- If Wet or dark spots on the deck from the attic
- Then Note where they are and photograph them for the roofer.
- If Wavy lines in the shingles that follow the rows
- Then Possible wrinkled underlayment. Ask about it before the warranty period ends.
- If A leak after a shingle blew off
- Then Get the shingle replaced fast; underlayment is not made to be the only layer for long 2.
These are possible causes, not findings. Only an inspection confirms them. Source: [2]
These signs point to water getting through. A roof inspection can find out where. Do not walk on the roof yourself to look.
What the code requires
These are the rules in the 2021 International Residential Code, read as adopted in Maryland. Your state or city may use a different edition or change the rules, so check with your building department.
| Rule | What the code says |
|---|---|
| Where it is required | Under asphalt shingles, clay and concrete tile, metal shingles, mineral-surfaced roll roofing, slate, wood shingles and shakes, metal panels and photovoltaic shingles |
| Labeling | The underlayment must be labeled with the standard it meets and its type |
| Wind-design areas | Type II grades: ASTM D226 Type II, or ASTM D4869 Type III or IV |
| Asphalt shingles, 4/12 and up | One layer, shingle fashion from the eave, lapped 2 inches; end laps 4 inches, offset 6 feet |
| Asphalt shingles, 2/12 to 4/12 | Two layers: a 19-inch starter strip, then 36-inch sheets lapped 19 inches |
| Attachment, no wind design | Fastened sufficiently to hold in place |
| Attachment, wind design | Corrosion-resistant cap fasteners on a 12-inch grid, 6 inches apart at laps |
| Alternatives | A self-adhered ASTM D1970 sheet, or 4-inch self-adhered strips over all deck joints plus approved underlayment |
| At the edges | Over the drip edge at the eaves, under the drip edge at the rakes |
Sources: [1]
Felt vs synthetic vs self-adhered
| Felt | Synthetic | Self-adhered | |
|---|---|---|---|
| Made of | Asphalt-soaked felt 2 | Woven or spun plastic with a coating 4 | Rubberized asphalt sheet 2 |
| Standard | ASTM D226 or D4869 | ASTM D8257 or an ICC-ES report | ASTM D1970 |
| How it is attached | Nails or staples | Cap nails at set spacing 4 | Sticks to the deck |
| Seals around nails | No | No | Yes 2 |
| Lets vapor through | Some 2 | Depends on the product | No, it is a vapor retarder 2 |
| Typical use | Whole deck | Whole deck | Eaves, valleys, chimneys; sometimes the whole deck |
| Counts as an ice barrier | Only two layers cemented together | Only two layers cemented together 4 | Yes, in one layer |
Common misconceptions
- Myth Underlayment is the waterproof layer.
- Reality The roof covering does most of the work. Underlayment is the backup for water that gets past it.
- Myth Synthetic is always better than felt.
- Reality Synthetic is lighter and tears less, but each product has its own limits on slope, covering and venting. The right choice depends on the roof.
- Myth One layer is fine on any shingle roof.
- Reality From 2/12 up to 4/12 the code calls for two layers under asphalt shingles.
- Myth Peel-and-stick over the whole roof is the best upgrade.
- Reality It blocks vapor, so it needs good ventilation and must follow the maker's rules for your deck and climate.
Local rules and climate
The rules above come from the model code. Each state or city adopts a version of it, sometimes years old, and may change parts of it. Your local building department decides which edition applies to your address.
Two local settings change what goes under your shingles. First, whether your area requires wind design, which brings in the heavier grades and tighter nailing in the code tables 1.
Second, whether your area has a history of ice dams, which brings in the ice barrier at the eaves. Our ice barrier study looks at how that second rule plays out.
We do not list local rules town by town; ask your building department or your roofer which apply.
What to ask your roofer
- Which underlayment product will you use, and what ASTM standard or ICC-ES report does it meet?
- Is our roof under 4/12? If so, will you put down two layers?
- Does our area require wind design, and does the nailing pattern meet it?
- Where will self-adhered membrane go: eaves, valleys, chimney, skylights?
- Will you remove the old underlayment down to the deck and replace bad sheathing?
- Does the shingle maker's warranty require a certain underlayment?
Keep going
- Roof replacementUnderlayment is installed at the deck stage
- Roofing materialsHow coverings compare on top of the underlayment
- Roofing cost guideWhat moves the price of a new roof
- Choosing a rooferHow to compare quotes line by line
Questions about roof underlayment
What is the purpose of roof underlayment?
Underlayment is a backup layer that keeps the roof deck dry when water gets past the shingles. It also protects the deck during the install, keeps wood resin away from the shingles, and helps the roof look flat. The building code requires it under shingles, tile, slate, wood and metal roofs.
Is synthetic underlayment better than felt?
It depends on the roof. Synthetic sheets are lighter and tear less underfoot, and one roll covers more roof. Felt lets a little vapor through and is the traditional choice. Each synthetic product has its own approval limits, so the right pick follows the roof covering, slope and the shingle maker's instructions.
What is the difference between No. 15 and No. 30 felt?
No. 30 is the heavier grade. Under ASTM D226, No. 15 felt is Type I and No. 30 felt is Type II. The heavier felt resists tearing better. In areas where the code requires wind design, the code calls for Type II grades under asphalt shingles.
Do you need two layers of underlayment?
On asphalt shingle roofs from 2/12 up to 4/12, yes. The 2021 IRC calls for a 19-inch starter strip at the eave, then 36-inch sheets lapped 19 inches, which makes two layers everywhere. At 4/12 and steeper, one layer lapped 2 inches is the code minimum outside wind-design areas.
Can underlayment be left exposed?
Only for a short time. Underlayment is not a finished roof, and each product sets how soon it must be covered. ARMA advises putting shingles on as soon as possible after the underlayment. Check the maker's instructions for the product on your roof.
Does underlayment go over or under the drip edge?
Both, depending on the edge. The code says underlayment goes over the drip edge along the eaves and under the drip edge along the rakes. That order makes water flow from one layer down onto the next instead of behind the metal.
Is ice and water shield the same as underlayment?
It is one type of underlayment. Ice and water shield is a self-adhered sheet made to ASTM D1970 that sticks to the deck and seals around nails. Felt and synthetic underlayment are fastened down and do not seal around nails. Most roofs use self-adhered sheet only at the eaves and other weak spots.
Should old underlayment be removed when reroofing?
On a full replacement it usually is. One synthetic product's ICC-ES report says that on a reroof, the old roof covering and old felts come off to expose the deck before the new underlayment goes on. That also lets the roofer find and replace damaged sheathing.
Can you put peel-and-stick underlayment on the whole roof?
The code allows it if the sheet meets ASTM D1970 and goes on to the maker's instructions for the deck, ventilation and climate. These sheets block vapor, so the attic must be well vented for the deck to stay dry. Ask whether the shingle maker's warranty allows it.
How is synthetic underlayment fastened?
Usually with cap nails, which are roofing nails with plastic washers. One product's ICC-ES report calls for 1-inch caps spaced 8 inches apart at the laps and 24 inches apart in the field. Wind-design areas follow the tighter pattern in the code table instead.
Sources
- International Code Council, 2021 International Residential Code §R905.1.1 Underlayment and Tables R905.1.1(1)–(3), §R905.2.8.5 (Maryland adoption, read at UpCodes), retrieved .
- Asphalt Roofing Manufacturers Association, "Steep Slope Roofing Underlayment" (May 6, 2025), retrieved .
- Asphalt Roofing Manufacturers Association, "Installation of Asphalt Shingles on Lower Sloped Roofs" (July 14, 2025), retrieved .
- ICC Evaluation Service, ESR-4103, RoofRunner and RoofRunner Air Synthetic Roof Underlayments (CertainTeed LLC, reissued August 2026), retrieved .
- International Code Council, 2021 International Residential Code §R806.3 Vent and insulation clearance (read at UpCodes), retrieved .
Expert review
Written by the HyreRoof editorial team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Code text read in the 2021 IRC as adopted in Maryland; product details from one ICC-ES evaluation report, labelled as that product's; lap arithmetic calculated from the code table.
Suggest a correction. We fix errors and say what changed.