Roof underlayment is the water-resistant layer between the roof deck and the roof covering. Under 2021 IRC Section R905.1.1 it must meet the ASTM standards listed for the covering, carry a label showing the standard, and be applied and attached per Tables R905.1.1(1) to (3).
A roof underlayment is a sheet material, rolled out over the sheathing, that sheds water that gets past the shingles, tiles or panels above it. The 2021 International Residential Code (IRC) sets its types in Table R905.1.1(1), its layers and laps in Table R905.1.1(2), and its fastening in Table R905.1.1(3). The code edition your jurisdiction has adopted, plus any local amendments, governs, and the roof covering and underlayment manufacturers' instructions apply on top of it.
Key facts
- 2021 IRC Section R905.1.1 requires underlayment for asphalt shingles, tile, metal roof shingles, roll roofing, slate, wood shingles, wood shakes, metal roof panels and photovoltaic shingles to conform to the standards listed in Chapter 9.
- 2021 IRC Section R905.1.1 requires underlayment made to ASTM D226, D1970, D4869 and D6757 to bear a label stating the standard and, where it applies, the type classification.
- 2021 IRC Section R905.2.2 allows asphalt shingles only on slopes of 2:12 or greater and requires double underlayment from 2:12 up to 4:12.
- 2021 IRC Table R905.1.1(2) calls for a 19-inch strip at the eave followed by 36-inch sheets overlapping 19 inches on asphalt shingle roofs from 2:12 up to 4:12.
- 2021 IRC Table R905.1.1(2) sets end laps at 4 inches, offset by 6 feet, for asphalt shingle underlayment.
- 2021 IRC Section R905.1.1 Exception 1 permits self-adhering polymer-modified bitumen underlayment labeled to ASTM D1970 as an alternative.
What roof underlayment does the IRC require?
The IRC requires an underlayment under every roof covering it names, of a type listed for that covering, labeled to its ASTM standard.
Section R905.1.1 covers asphalt shingles, clay and concrete tile, metal roof shingles, mineral-surfaced roll roofing, slate and slate-type shingles, wood shingles, wood shakes, metal roof panels and photovoltaic shingles. Each of those has its own subsection (R905.2 through R905.16 as applicable), and each underlayment subsection (for example R905.2.3 and R905.3.3) simply sends you back to Section R905.1.1.
Two things follow. First, the code names materials by standard and type, so the label on the roll is what an inspector reads. Second, the type depends on whether the site falls in an area where wind design is required under Figure R301.2.1.1, and the wind-design column is stricter. Your jurisdiction's wind map determines which column you read.
What are the underlayment types for each roof covering?
Table R905.1.1(1) assigns allowed ASTM standards and types to each covering, in two columns: areas where wind design is not required, and areas where it is required.
2021 IRC Table R905.1.1(1), underlayment types, selected rows. The printed table lists further standards for some coverings (for example tile and photovoltaic shingles); read the full table for your covering.
| Roof covering | Section | Wind design not required | Wind design required |
|---|---|---|---|
| Asphalt shingles | R905.2 | ASTM D226 Type I or II; ASTM D6757 | ASTM D226 Type II; ASTM D4869 Type III or Type IV |
| Metal roof shingles | R905.4 | ASTM D226 Type I or II; ASTM D4869 Type I, II, III or IV | ASTM D226 Type II; ASTM D4869 Type III or Type IV |
| Mineral-surfaced roll roofing | R905.5 | ASTM D226 Type I or II; ASTM D4869 Type I, II, III or IV | ASTM D226 Type II; ASTM D4869 Type III or Type IV |
| Slate and slate-type shingles | R905.6 | ASTM D226 Type I; ASTM D4869 Type I, II, III or IV | ASTM D226 Type II; ASTM D4869 Type III or Type IV |
| Wood shingles | R905.7 | ASTM D226 Type I or II; ASTM D4869 Type I, II, III or IV | ASTM D226 Type II; ASTM D4869 Type III or Type IV |
| Wood shakes | R905.8 | ASTM D226 Type I or II; ASTM D4869 Type I, II, III or IV | ASTM D226 Type II; ASTM D4869 Type III or Type IV |
| Metal panels | R905.10 | Manufacturer's instructions | ASTM D226 Type II; ASTM D4869 Type III or Type IV |
The asphalt shingle row, and the clay and concrete tile and photovoltaic rows, list further standards that are not reproduced here. Check the printed Table R905.1.1(1) for the full list for those coverings.
Is synthetic underlayment allowed, and how does it compare with felt?
The code does not use the words synthetic or felt as categories. It lists ASTM standards and types, so any product whose label shows a listed standard and type for your covering, installed to its manufacturer's instructions, qualifies.
The application text in Table R905.1.1(2) does say "underlayment felt" when describing the 19-inch starter strip and the sheet layout, but the types in Table R905.1.1(1) are what govern what you may buy. Whether a particular roll complies is a label question, not a marketing one.
Self-adhering membranes are handled separately. Section R905.1.1 Exception 1 permits a self-adhering polymer-modified bitumen underlayment labeled to ASTM D1970 if it is installed to both the underlayment manufacturer's and the roof covering manufacturer's instructions for the deck, ventilation configuration and climate exposure. Roof ventilation is covered in attic ventilation requirements. Eave ice barriers are a different requirement under Section R905.1.2, covered in the ice and water shield article.
How many layers of underlayment does a shingle roof need?
Asphalt shingles need one layer at 4:12 or steeper and two layers from 2:12 up to 4:12; they are not permitted below 2:12 (2021 IRC Section R905.2.2).
2021 IRC Table R905.1.1(2), asphalt shingles, areas where wind design is not required under Figure R301.2.1.1.
| Roof slope | Application |
|---|---|
| 2:12 up to 4:12 | Two layers. A 19-inch strip of underlayment felt parallel to and starting at the eaves, then 36-inch-wide sheets starting at the eave, overlapping successive sheets 19 inches. End laps 4 inches, offset by 6 feet |
| 4:12 or greater | One layer applied shingle fashion, parallel to and starting from the eave, lapped 2 inches. End laps 4 inches, offset by 6 feet |
The table adds that distortions in the underlayment shall not interfere with the ability of the shingles to seal. The ice and water shield article explains how this double layer differs from the eave ice barrier. For how slope is expressed and measured, see roof pitch and slope.
How is underlayment applied under tile, metal, slate and wood coverings?
Clay and concrete tile follow a layered pattern like shingles, while metal roof shingles, roll roofing, slate, wood shingles, wood shakes and metal panels defer to the manufacturer where wind design is not required.
In Table R905.1.1(2), clay and concrete tile on slopes from 2-1/2:12 up to 4:12 need not fewer than two layers: a 19-inch strip at the eave, then 36-inch strips overlapping 19 inches, with 4-inch end laps offset by 6 feet. At 4:12 or greater, not fewer than one layer applied shingle fashion and lapped 2 inches.
For metal roof shingles, mineral-surfaced roll roofing, slate and slate-type shingles, wood shingles, wood shakes and metal panels, the table says to apply in accordance with the manufacturer's installation instructions where wind design is not required. Where wind design is required, it calls for two layers: a 19-inch strip of underlayment felt parallel to and starting at the eaves, then 36-inch-wide sheets overlapping successive sheets 19 inches, with end laps of 4 inches offset by 6 feet. The cedar shake roof installation article covers the shake and shingle sections in more detail.
How is roof underlayment fastened?
Table R905.1.1(3) sets fastening by whether wind design is required: enough fasteners to hold it in place where it is not, and a specified cap-nail grid where it is.
2021 IRC Table R905.1.1(3), underlayment attachment, areas where wind design is required under Figure R301.2.1.1.
| Item | Requirement |
|---|---|
| Pattern | Corrosion-resistant fasteners in a grid pattern of 12 inches between side laps, with 6-inch spacing at side and end laps |
| Fastener type | Annular ring or deformed shank nails with 1-inch-diameter metal or plastic caps |
| Metal caps | Not less than 32-gage sheet metal |
| Power-driven metal caps | Minimum thickness 0.010 inch |
| Plastic caps | Minimum thickness of the outside edge 0.035 inch |
| Cap nail shank | Not less than 0.083 inch; length sufficient to penetrate through the roof sheathing or not less than 3/4 inch into it |
Where wind design is not required, the table reads "Fastened sufficiently to hold in place" for asphalt shingles, clay and concrete tile and photovoltaic shingles, and "Manufacturer's installation instructions" for the other coverings. Underlayment goes on the deck, so the deck itself has to be adequate first; panel thickness and span rating are in roof sheathing thickness.
What is the alternative with a strip over the deck joints?
Exception 2 to Section R905.1.1 lets you cover every joint in the roof decking with a self-adhering strip first, then lay a normal underlayment over the whole roof.
The strip must be at least 4 inches wide, a self-adhering polymer-modified bitumen membrane labeled to ASTM D1970, installed per the manufacturer's instructions for the deck material. An approved underlayment complying with Table R905.1.1(1) for the covering, using the "wind design not required" column, then goes over the entire roof, applied per Table R905.1.1(2) in that column and attached per Table R905.1.1(3).
Summary
Section R905.1.1 of the 2021 IRC requires labeled underlayment of a type listed in Table R905.1.1(1) for the roof covering, applied per Table R905.1.1(2) and attached per Table R905.1.1(3). Asphalt shingles need two layers from 2:12 up to 4:12 and one layer at 4:12 or steeper, with 4-inch end laps offset by 6 feet. Wind-design areas have stricter types and a cap-nail grid. The adopted code and the manufacturers' instructions govern.
Framing Plan draws joist layouts, beams and openings and exports PNG, PDF and DXF. It does not check roof coverings or any other code requirement: the editor works in millimeters and its sample profile uses demo span figures and metric member sizes, so treat its output as a preliminary drawing aid. To lay out the roof framing under your underlayment, open the framing studio. The full code text is at codes.iccsafe.org.