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IRC R905.1.2 eave band — 24 in inside the wall line, converted along the slope, then rolls from published 200 sq ft coverage
This is an ice-barrier coverage band, not a shingle takeoff and not attic airflow. Eave length vs squares: Roofing Square & Shingle Bundle Calculator. Heat that melts the snow pack: attic ventilation calculator. Why dams form and how to measure: guide. Stains vs ventilation: troubleshooting. Peel-and-stick vs felt: comparison. Roofing index: roofing calculators hub. Field underlayment upslope: underlayment calculator. Lock pitch first: roof pitch / slope calculator.
Calculator Inputs
Eave length, X/12 pitch, overhang, and wall thickness. IRC R905.1.2 wants the membrane from the lowest roof edge to a point 24 in inside the exterior wall line — measured horizontally, then converted along the slope. Not a whole-roof square count.
The drip-edge run you are protecting — one eave, or the sum of eaves getting ice barrier. Related roof measurement, not sloped squares: Roofing Square & Shingle Bundle Calculator uses plan length × width for shingles. Bring the eave (the horizontal edge) here.
Same X/12 as the shingle and rafter calculators. Here it converts the horizontal 24-in-inside-the-wall reach into distance along the deck: multiplier = √(1+(X/12)²). At 8/12 and steeper, R905.1.2 also wants at least 36 in measured along the slope from the eave.
Overhang is fascia to the exterior wall face. Wall thickness is the exterior wall (a 2×6 plus sheathing is often about 6 in — Fine Homebuilding’s worked 2×6 examples). Horizontal reach = overhang + wall + 24 in.
2021 and 2024 IRC R905.1.2 require the barrier only “in areas where there has been a history of ice forming along the eaves … as designated in Table R301.2.” That is a local climatic checkbox, not a nationwide climate-zone map. Older editions used a January 25°F trigger. Detached unconditioned accessory structures are excepted.
Default 200 sq ft = GAF WeatherWatch 2.0-square roll (36 in × 66.7 ft) and Grace Ice & Water Shield 36 × 66.6 ft. 150 = WeatherWatch 1.5-square. 108 = Grace 36 × 36 ft. 225 = Grace 36 × 75 ft. All from manufacturer datasheets; coverage excludes laps. Side lap in the course math is Grace’s 3.5 in minimum.
Results
Required width along slope
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Enter eave length, pitch, overhang, and wall thickness
Width along slope—
Horizontal from eave—
Code sectionIRC R905.1.2
Pitch slope factor—
Net code area—
36" courses—
Installed area—
Rolls to order—
Roll coverage used—
IRC 2021/2024 §R905.1.2: ice barrier from the lowest edges of all roof surfaces to a point not less than 24 inches (610 mm) inside the exterior wall line, required where Table R301.2 designates a history of ice forming along the eaves. Exception: detached accessory structures not containing conditioned floor area. On slopes ≥ 8/12, also at least 36 inches measured along the roof slope from the eave. Horizontal 24 in is taken from the interior face of the exterior wall (NRCA / Fine Homebuilding). Roll coverage: GAF WeatherWatch sell sheet RESUL103 — 2.0 square roll 200 ft² excluding laps; Grace Ice & Water Shield datasheet — 200 / 225 / 108 ft² by length. Side lap 3.5 in (Grace). Valleys and penetrations are extra. Why ice dams form: guide. Shield gap vs attic heat: troubleshooting. Self-adhering vs felt: comparison. Attic NFA: attic ventilation calculator.
How to Use This Ice & Water Shield Coverage Calculator
Enter the eave length you are protecting, pitch as X/12, the overhang past the wall, and the exterior wall thickness. Mark whether your jurisdiction’s Table R301.2 actually requires an ice barrier. The tool builds the horizontal reach (overhang + wall + 24 in), converts it along the slope, floors steep roofs at 36 in along the eave, then converts width × eave into square feet and 36 in courses / rolls.
H (in) = overhang + wall thickness + 24
width along slope = H × √(1+(X/12)²)
if pitch ≥ 8/12: width = max(width, 36 in along slope)
net sq ft = eave (ft) × width (ft)
courses of 36 in membrane with 3.5 in side lap, then rolls = ceil(installed sq ft ÷ roll coverage)
width along slope = H × √(1+(X/12)²)
if pitch ≥ 8/12: width = max(width, 36 in along slope)
net sq ft = eave (ft) × width (ft)
courses of 36 in membrane with 3.5 in side lap, then rolls = ceil(installed sq ft ÷ roll coverage)
This is an eave band, not a shingle takeoff. Squares live in the Roofing Square & Shingle Bundle Calculator. Ice dams still start with attic heat: attic ventilation calculator. Why the wall-line measurement exists: guide. Stains vs ventilation: troubleshooting. Peel-and-stick vs felt: comparison.
What This Number Is (and Is Not)
The headline is how far up the deck the ice barrier must reach, in inches along the slope, plus how many published rolls that eave consumes. It is not 24 inches up from the gutter, and it is not the whole roof in squares.
Assumptions baked into every result
One pitch, one overhang, one wall thickness, one eave length (or the sum you typed). The 24 in is applied horizontally from the interior face of the exterior wall, then multiplied by the geometric slope factor. Steep roofs (≥8/12) also meet a 36 in along-slope floor. Courses assume 36 in roll width and a 3.5 in side lap. Roll count uses installed course area, not the thinner net code band, so you are not short a course. Valleys, skylights, and sidewalls are extra. Local Table R301.2 decides whether the code requires the barrier at all.
One 36 in strip is often short.Fine Homebuilding’s 6/12, 12 in overhang, 2×6 wall example is about 47 in along the slope — two courses. The driveway view of a single yellow roll at the fascia is not the code measurement.
When this tool is the wrong tool
Shingle squares belong in the roofing square calculator. Attic net free area belongs in the attic ventilation calculator — membrane does not replace soffit intake. Gutter flow belongs in the gutter sizing calculator.
Attic still cooking the snow pack?Size IRC R806.2 intake and exhaust. Ice barrier without ventilation is a bandage on a warm roof.
Open Attic Ventilation Calculator →Any trade
Checkout opens here — you stay on this page.
Ice Barrier Gear
The calculator returns a slope width and a roll count. A self-adhering membrane and a hook knife are how that band gets on the deck. CSV rows 400–410 were nailers, vents, and gutter gear — not ice barrier — so these two SKUs are new.
Grace Ice & Water Shield 36 in x 36 ft Roll
GCP 36 in x 36 ft roll = 108 sq ft (datasheet). The calculator defaults to a 2.0-square / 200 sq ft roll (GAF WeatherWatch and Grace 36x66.6 ft). Switch roll size to match the carton in your hand.
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Stanley 10-510 FatMax Hook Knife
Hook blade for cutting membrane and shingles without gouging the deck. Size the ice-barrier band first; this is how you trim the course.
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Run the Eave-Band Math
Overhang + wall + 24 in, converted along the slope. Net area, 36 in courses, rolls from published coverage.
Open Ice Barrier Calculator →
Attic Heat Feeds Ice Dams
Membrane at the eave does not cool the attic. Size soffit intake and ridge exhaust with IRC R806.2 NFA.
Open Attic Ventilation Calculator →
Need Eave Length?
The square calculator uses plan length × width. Bring the eave (horizontal edge) here for the ice-barrier band — not the sloped squares.
Open Roofing Square Calculator →As an Amazon Associate, TestTalkHQ earns from qualifying purchases.