Ice and water shield, called an ice barrier in the 2021 International Residential Code (IRC), is required by Section R905.1.2 where local ice damming is recorded in Table R301.2. It runs from the eave to at least 24 inches inside the exterior wall line, and to at least 36 inches along the slope on roofs of 8:12 or steeper.
An ice barrier is a waterproof layer under the roof covering at the eaves. It stops water that backs up behind an ice dam from reaching the deck and the wall below. In the code it is two cemented layers of underlayment or a single self-adhering polymer-modified bitumen sheet. This article covers where the 2021 IRC requires it, how far it extends, what qualifies, and how the measurements connect to the exterior wall line, overhang, and slope on a roof framing plan. The adopted code edition and local amendments govern.
Key facts
- An ice barrier is required only where the jurisdiction has designated a history of ice forming along the eaves and causing water backup in Table R301.2 (2021 IRC Section R905.1.2).
- The barrier extends from the lowest edge of every roof surface to a point not less than 24 inches inside the exterior wall line (2021 IRC Section R905.1.2).
- On roofs with a slope of 8 units vertical in 12 units horizontal or greater, the barrier must also cover at least 36 inches measured along the roof slope from the eave edge (2021 IRC Section R905.1.2).
- The barrier is either not fewer than two layers of underlayment cemented together or a self-adhering polymer-modified bitumen sheet used in place of normal underlayment (2021 IRC Section R905.1.2).
- Detached accessory structures that contain no conditioned floor area are exempt (2021 IRC Section R905.1.2).
- Self-adhering underlayment bearing an ASTM D1970 label is a permitted alternative underlayment (2021 IRC Section R905.1.1, Exception 1).
When does the code require an ice and water shield?
The code requires an ice barrier only where the jurisdiction says so. Section R905.1.2 applies "in areas where there has been a history of ice forming along the eaves causing a backup of water as designated in Table R301.2."
Table R301.2 is the climatic and geographic design criteria table that each jurisdiction fills in. Footnote h to that table tells the jurisdiction to enter "YES" where there has been a history of local damage from the effects of ice damming, and "NO" otherwise. So the first step on any project is to read your jurisdiction's Table R301.2 entry, not to assume from the climate.
Which roof coverings need an ice barrier?
Section R905.1.2 names six coverings, and each has its own cross-reference that points back to the same rule. Photovoltaic shingles carry the same cross-reference in R905.16.3.1.
2021 IRC Chapter 9, ice barrier cross-references and minimum roof slope by roof covering.
| Roof covering | Ice barrier section | Minimum slope section | Minimum slope |
|---|---|---|---|
| Asphalt shingles | R905.2.7 | R905.2.2 | 2:12 |
| Metal roof shingles | R905.4.3.1 | R905.4.2 | 3:12 |
| Mineral-surfaced roll roofing | R905.5.3.1 | R905.5.2 | 1:12 |
| Slate and slate-type shingles | R905.6.3.1 | R905.6.2 | 4:12 |
| Wood shingles | R905.7.3.1 | R905.7.2 | 3:12 |
| Wood shakes | R905.8.3.1 | R905.8.2 | 3:12 |
| Photovoltaic shingles | R905.16.3.1 | R905.16.2 | 2:12 |
Two points on this table. Clay and concrete tile (R905.3) has no ice barrier cross-reference in Chapter 9, and neither does the R905.1.2 list name it. Metal roof panels (R905.10) are not named in R905.1.2 either, so follow the panel manufacturer's instructions and your local rules there. Building-integrated photovoltaic (BIPV) roof panels have their own Section R905.17.4, which repeats the two-layer or self-adhering rule and the 24 inch extent. It does not repeat the 36 inch rule for steep roofs, and it carries the same detached-accessory exception.
How far does an ice barrier extend up the roof?
The ice barrier must reach at least 24 inches inside the exterior wall line, and at least 36 inches along the slope on roofs at 8:12 or steeper. Section R905.1.2 gives two tests, and on steep roofs both apply.
2021 IRC Section R905.1.2, ice barrier extent.
| Condition | Required extent | How it is measured |
|---|---|---|
| Any roof requiring an ice barrier | Not less than 24 inches | From the lowest edges of all roof surfaces to a point inside the exterior wall line of the building |
| Slope of 8:12 (67-percent slope) or greater | Also not less than 36 inches | Measured along the roof slope from the eave edge of the building |
The 24 inch figure is a point inside the exterior wall line, fixed by the wall, while the 36 inch figure is a length along the slope. Read both against the drawing rather than converting one to the other.
Where the ice barrier replaces normal underlayment at the eave, the rest of the roof still gets the underlayment that Table R905.1.1(1) and Table R905.1.1(2) require for the covering.
How does overhang depth affect the ice barrier?
The overhang does not change the 24 inch rule, but it changes how long the barrier run is. The barrier starts at the lowest roof edge and ends at a point tied to the exterior wall line, so a deeper overhang means a longer run before the barrier reaches that point. Mark both the wall line and the slope on the plan.
What counts as ice and water shield under the code?
The code accepts two constructions in R905.1.2: not fewer than two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet used in place of normal underlayment. Section R905.1.2 does not name an ASTM standard for either one.
The standard appears in Section R905.1.1. It lists ASTM D1970 among the underlayment standards that must bear a label on the material. Exception 1 permits self-adhering polymer-modified bitumen underlayment with a D1970 label as an alternative to the Table R905.1.1(1) underlayments, installed in accordance with both the underlayment manufacturer's and the roof covering manufacturer's instructions for the deck material, roof ventilation configuration, and climate exposure. Exception 2 uses a D1970 strip of at least 4 inches wide over all joints in the roof decking, with an approved underlayment over the whole roof, as another route for areas where wind design is not required under Figure R301.2.1.1.
Tables R905.1.1(1) through (3) set the other underlayment types and fastening by roof covering and wind region; check the printed tables for your covering.
Does ice and water shield replace double underlayment on low-slope roofs?
No. Double underlayment and the ice barrier are separate requirements that can apply to the same roof. Section R905.2.2 says asphalt shingles are used only on slopes of 2:12 or greater, and that from 2:12 up to 4:12 double underlayment is required under Section R905.1.1.
2021 IRC Table R905.1.1(2), asphalt shingles, areas where wind design is not required under Figure R301.2.1.1.
| Roof slope | Underlayment 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 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 |
A low-slope asphalt shingle roof in an ice-damming area therefore carries both the eave ice barrier and the double underlayment rule. For the slope ranges by roof covering, see roof pitch and slope on a framing plan and the shed roof framing guide, which lists minimum slopes.
Where does the ice barrier sit relative to the drip edge and valleys?
Under Section R905.2.8.5, for shingle roofs, the underlayment is installed over the drip edge along eaves and under the drip edge along rake edges. Where a self-adhering sheet is used in place of normal underlayment at the eave, apply that order and the manufacturer's instructions. The same section requires drip edge segments to overlap not less than 2 inches.
In closed valleys, Section R905.2.8.2 item 3 allows self-adhering polymer-modified bitumen underlayment complying with ASTM D1970 in lieu of the valley lining material. Valley geometry and lining options are in the roof valley guide.
Does an ice barrier need solid sheathing?
For wood shingles and shakes in cold climates, yes. Sections R905.7.1.1 and R905.8.1.1 require solid sheathing on the portion of the roof requiring an ice barrier where the average daily temperature in January is 25°F (-4°C) or less. Asphalt shingles are already fastened to solidly sheathed decks under Section R905.2.1. Sheathing panel thickness is covered in roof sheathing thickness.
What happens at a reroof?
Section R908.3 requires roof replacement to remove existing layers of roof covering down to the roof deck. Its exception allows an existing ice barrier membrane that is adhered to the deck to stay in place and be covered with an additional layer of ice barrier membrane in accordance with Section R905.
How do you show an ice barrier on a roof framing plan?
Show the ice barrier as a note and a boundary tied to the framing, not as a product callout. A reviewer needs to see three things on the plan or the roof section.
- The exterior wall line. The 24 inch test is measured to a point inside it, so dimension or label the wall line the barrier refers to.
- The eave edge and overhang. Dimension the rafter tail or truss overhang so the barrier's starting edge is clear.
- The slope. Give each roof plane's slope, because the 8:12 threshold adds the 36 inch along-slope extent. The roof framing plan guide covers slope arrows and notes, and the rafter span chart covers sizing the rafters themselves.
Add a general note such as "Ice barrier per 2021 IRC R905.1.2 where required by Table R301.2, extent per code" and cite your local amendment if one exists. Hips, gables, and dormers each create eaves of their own, so the barrier applies to the lowest edges of all roof surfaces, as covered in gable and hip roof framing.
Summary
Section R905.1.2 of the 2021 IRC requires an ice barrier where the jurisdiction designates ice damming in Table R301.2, for asphalt shingles, metal roof shingles, mineral-surfaced roll roofing, slate, wood shingles, and wood shakes. It extends from the lowest roof edges to at least 24 inches inside the exterior wall line, plus at least 36 inches along the slope at 8:12 or steeper. It is two cemented layers or a self-adhering polymer-modified bitumen sheet, and detached unconditioned accessory structures are exempt. Local amendments and manufacturer instructions govern the details.
Framing Plan is a preliminary drawing aid. It draws joist layouts, beams, and openings and exports PNG, PDF, and DXF, but it does not check code or size members, and its editor works in millimeters. Open the framing studio to draw the roof layout, then add the ice barrier notes by hand for your reviewer.