Calculator
Ice & Water Shield Calculator
In climates where ice dams form at the eave, residential building codes commonly require a self-adhered ice barrier membrane to extend from the roof's edge to a point measured horizontally a set distance inside the exterior wall line — a flat, horizontal dimension that has nothing to do with the roof's slope. But the membrane itself is installed on the slope, climbing up the actual pitched surface, not across a horizontal line, and that mismatch is why the coverage this calculator produces is almost always more than a single standard-width course.
This calculator takes the horizontal code dimension — overhang, wall thickness and the required distance inside the wall, added together — and projects it up the roof's actual slope using the pitch multiplier, the same conversion used elsewhere on this site to turn a flat measurement into a sloped one. What comes out is how many courses of membrane the eave actually needs and how much material that represents, rather than assuming one course at the eave satisfies a horizontal code requirement it was never measured against.
Calculate your quantity
24 in is the common code minimum. Check your local amendment.
Ice barrier rolls
3 rolls
- Required horizontal width
- 48 in
- Pitch multiplier
- 1.118 x
- Width measured up the slope
- 4.47 ft
- Courses of membrane
- 2 courses
- Eave coverage
- 440 sq ft
- Valley coverage
- 0 sq ft
- Total membrane
- 440 sq ft
- A single 36 in course only reaches 2.75 ft up the slope, so this eave needs 2 courses to satisfy the 24 in inside-the-wall requirement.
Shopping summary
- • 3 rolls of ice and water shield (200 sq ft each)
This is an estimate — confirm structural work with a professional.
How this calculation works
The code dimension is built from three horizontal distances added together: how far the roof overhangs past the wall, the wall's own thickness, and the required distance the membrane has to reach past the interior face of that wall. All three are flat, horizontal measurements, and their sum is the total horizontal distance the membrane has to cover, measured from the eave edge inward.
That horizontal total then gets multiplied by the pitch multiplier to find how far up the actual roof slope the membrane has to run to satisfy it — because the membrane doesn't travel horizontally, it travels along the pitched surface, and a steeper roof needs more sloped membrane to cover the same horizontal distance than a shallow one does. Divide that required slope distance by what one course of membrane actually exposes, after its own head lap is subtracted, and the result is how many courses the eave needs — commonly more than one, since a single standard-width course frequently doesn't reach far enough up a typical pitch to satisfy a typical overhang and wall thickness combined.
- Overhang, wall thickness and the required interior distance are all horizontal measurements added together before any pitch correction is applied — none of the three is measured on the slope.
- A wider roll width reaches farther up the slope per course, which can reduce the number of courses needed even on a demanding pitch and overhang combination.
- Valleys need their own membrane coverage, calculated separately from the eave courses since a valley runs up the slope in a different direction and to a different extent than the eave-parallel courses do.
- The horizontal distance requirement, and whether it applies to this roof at all, is set by local code and climate — this page calculates coverage against whatever distance you enter, but does not determine that distance for you.
The formula
requiredHorizIn = overhang + wallThickness + insideWallLine; requiredSlopeIn = requiredHorizIn × pitchMultiplier; courses = ceil(requiredSlopeIn ÷ (rollWidth − headLap))
- overhang + wallThickness + insideWallLine
- The total horizontal distance the membrane must cover, from the eave edge to the required point inside the exterior wall — three flat measurements added together, none of them yet corrected for slope.
- pitchMultiplier
- √(1 + (rise/12)²) — the ratio that converts the horizontal code distance into the actual length the membrane travels up the pitched roof surface to cover that same horizontal span.
- rollWidth − headLap
- How far up the slope one course of membrane actually reaches once the portion overlapped by the next course above it is subtracted — the same exposure logic used for rolled underlayment.
- insideWallLine
- The required horizontal distance inside the wall's interior face the membrane must reach — a code minimum where required, though local amendments can set a different figure, so verify the requirement for your specific jurisdiction rather than assuming the field's default applies everywhere.
Where these numbers come from
- 24 in as a commonly cited minimum distance inside the exterior wall line
- Widely referenced as a common requirement in residential building codes for regions with ice-dam risk, but local amendments vary and some jurisdictions set a different distance or apply the requirement differently — confirm the exact figure and its applicability with your local building department rather than treating this as universal.
- 36 and 48 in roll widths for self-adhered ice barrier membrane
- Manufacturer-published standard roll widths for ice and water shield products, used here as the base width from which head lap is subtracted to find effective exposure per course.
- Pitch multiplier as √(1 + (rise/12)²)
- A trigonometric ratio rather than a looked-up figure — the same slope-to-run relationship used elsewhere on this site, applied here to convert the horizontal code distance into how far up the actual pitch the membrane has to run.
- 3 in default head lap between courses
- Trade convention for self-adhered ice barrier membrane, typically narrower than the head lap used on rolled felt underlayment because the membrane bonds directly rather than relying purely on mechanical overlap.
Worked examples
A house with a modest 16 in overhang and an 8 in wall, 6-in-12 pitch
| Eave length | 80 ft |
|---|---|
| Eave overhang | 16 in |
| Exterior wall thickness | 8 in |
| Required distance inside the wall | 24 in |
| Roof pitch (rise per 12) | 6 in |
| Roll width | 36 in |
| Roll coverage | 200 sq ft |
| Head lap | 3 in |
| Valley length | 0 ft |
| Ice barrier rolls | 3 rolls |
|---|---|
| Required horizontal width | 48 in |
| Pitch multiplier | 1.118 x |
| Width measured up the slope | 4.47 ft |
| Courses of membrane | 2 courses |
| Eave coverage | 440 sq ft |
| Valley coverage | 0 sq ft |
| Total membrane | 440 sq ft |
16 in of overhang, 8 in of wall and a 24 in required distance inside it add to 48 in of required horizontal coverage. At a 6-in-12 pitch, that becomes about 53.7 in measured up the actual slope — already more than a single 36 in roll's width, before any lap is even subtracted. Once the 3 in head lap is applied, one course only reaches 33 in, so this eave needs two courses to satisfy the requirement, which comes to three rolls across 80 linear feet of eave.
That two-course result is the thing worth flagging before ordering by feel: a fairly ordinary overhang, wall thickness and pitch combination, nothing unusual about any one of the three numbers, still adds up to more coverage than a single standard-width course provides. Assuming one course at the eave satisfies a horizontal code requirement it was never actually measured against is the most common way this material comes up short.
A house with a deep 24 in overhang, a thick 12 in wall, a steeper 10-in-12 pitch, and a valley
| Eave length | 60 ft |
|---|---|
| Eave overhang | 24 in |
| Exterior wall thickness | 12 in |
| Required distance inside the wall | 24 in |
| Roof pitch (rise per 12) | 10 in |
| Roll width | 48 in |
| Roll coverage | 300 sq ft |
| Head lap | 3 in |
| Valley length | 25 ft |
| Ice barrier rolls | 2 rolls |
|---|---|
| Required horizontal width | 60 in |
| Pitch multiplier | 1.3017 x |
| Width measured up the slope | 6.51 ft |
| Courses of membrane | 2 courses |
| Eave coverage | 450 sq ft |
| Valley coverage | 100 sq ft |
| Total membrane | 550 sq ft |
This roof asks for more horizontal coverage than the first example — 60 in against 48 — and climbs a steeper pitch, yet needs fewer rolls, because it pairs that demand with a wider 48 in roll rather than the standard 36 in one. A single course here reaches 45 in up the slope after the head lap, which still isn't quite enough to cover the full 78.1 in the pitch-corrected distance requires, so it still needs two courses — but the wider roll's higher coverage per roll and the shorter 60 ft eave together bring the roll count down to two despite the more demanding geometry.
The 25 ft valley in this example is a genuinely separate quantity from the eave courses, adding 100 sq ft of membrane on top of the eave's 450 — valleys run up the slope in their own direction and need their own coverage regardless of how the eave courses work out, which is why the two figures are tracked and reported separately rather than folded into a single total.
Common mistakes
- Assuming one standard-width course at the eave satisfies the code requirement without checking the actual horizontal distance against the roof's pitch. As both worked examples show, an ordinary overhang and wall thickness combination frequently needs two courses, not one.
- Applying the required distance as if it were measured up the slope rather than horizontally inside the wall line. The code dimension is horizontal; the membrane travels on the pitch, and skipping the pitch correction understates how far up the slope the membrane actually needs to run.
- Forgetting to add overhang and wall thickness to the required interior distance, and using the interior distance alone as if it were the full horizontal requirement. The membrane has to reach past the wall's interior face, which means covering the overhang and the wall's own thickness first before the interior distance even starts.
- Treating valley coverage as included in the eave-course calculation. A valley runs up the slope in its own direction, independent of the eave's coverage requirement, and needs its own membrane counted separately.
- Assuming the 24 inch figure applies uniformly everywhere. Local code amendments vary, and some jurisdictions set a different required distance or apply the requirement to a different set of roof types — confirm the actual local requirement rather than treating any single number as universal.
Shopping summary
Order rolls at the total this page reports, and confirm the required horizontal distance for your specific jurisdiction with your local building department before finalizing the insideWallLine figure this calculation runs on — local amendments genuinely change this number in some areas.
Ice barrier membrane typically installs before the drip edge at the eave and is overlapped by standard underlayment further up the roof — check your membrane manufacturer's instructions for the exact sequence, since it varies by product and by whether drip edge is specified above or below the membrane at the eave specifically.
Where the roof also needs step flashing along a sidewall in the same ice-dam-prone area, the roof flashing calculator covers that separately, since step flashing is counted by shingle course rather than by the horizontal code dimension this page works from.
FAQ
Why does my eave need more than one course of ice barrier when the roll is 36 inches wide?
Because the required coverage is a horizontal distance — overhang, wall thickness, and the code-required distance inside the wall, all added together — and that horizontal distance gets longer once it's projected up the actual roof slope. A single 36 inch course, especially after its head lap is subtracted, often doesn't reach far enough up the slope to cover a horizontal requirement that already added up to 40 or 50 inches before the pitch correction was even applied.
Does a steeper roof need more or less ice barrier for the same overhang and wall?
More, generally — a steeper pitch means the same horizontal distance translates into a longer run up the actual slope, which either needs more courses or a wider roll to satisfy in the same number of courses. The two worked examples above compare a moderate and a steep pitch directly for exactly this reason.
Is the 24 inch inside-the-wall figure the same everywhere?
Not always, and not only by code — a shingle manufacturer's own installation instructions can require ice barrier membrane, or specify a longer horizontal distance than your local code does, as a condition of the shingle product's warranty staying valid, entirely separate from what your building department requires. Satisfying the code minimum doesn't automatically satisfy the warranty terms, so check the shingle's own installation literature alongside the local code amendment rather than assuming one covers the other.
Do valleys need ice barrier membrane even if my code minimum is satisfied at the eave?
Typically yes where ice-dam risk exists — valleys concentrate runoff and are a separate ice-damming risk from the eave itself, which is why this page calculates valley coverage as its own quantity rather than assuming eave coverage extends automatically into the valleys.
Does every eave on the house need ice barrier membrane, or just the ones over living space?
Code and manufacturer guidance are generally aimed at eaves over conditioned, occupied space, where an ice dam backing water up under shingles causes interior damage. An eave over a detached garage or an unheated shed roof often falls outside the practical concern even where a literal reading of a local requirement might technically reach it — check how your specific jurisdiction scopes the requirement rather than assuming it applies uniformly to every eave on the property.
Does this calculator tell me whether my roof actually needs ice barrier membrane at all?
No — whether the requirement applies is a climate and code question specific to your location and sometimes your roof's design, which is outside what this page calculates. It assumes the requirement applies and works out how much membrane satisfies it; confirm applicability with local code before ordering.
Where to go next
The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.