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Roof Eave Ice Dam Protection Membrane Footprint (and Valley Lining)

Calculate the ice-and-water-shield membrane area required at eaves per IRC cold-climate requirements, or the lining a roof's valleys take.

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A strip along the eaves against ice dams, or a lining down every valley.

Both are the same self-adhering membrane, worked differently: the eave strip is a width up the slope from the drip edge, measured from the wall line; a valley lining is a width laid centred down each valley, along its whole sloping length, under the shingles or the metal.

The combined length of all eaves requiring ice barrier protection.

Add every eave that needs protecting, on every plane, measured along the eave in plan. A simple gable has two; a hip has four; a roof with dormers has an eave under each one. Gable ends are rakes and are not eaves — they shed rather than collect, and including them buys membrane for the two edges least likely to dam. Valleys need their own treatment and are not covered by this.

The horizontal distance the roof overhangs beyond the exterior wall line.

The plan distance from the outside face of the exterior wall to the edge of the roof. This matters here more than it looks: the code requires the barrier to reach a set distance INSIDE the wall line, so the overhang is added to that distance, and every extra 100 mm of overhang is another 100 mm of membrane along the whole eave. Measure to the wall's outer face, not to the fascia.

How far the membrane must extend past the interior face of the exterior wall line, per IRC R905.1.2.

IRC R905.1.2 sets this minimum at 24in for most cold climates; check your local jurisdiction's amendments.

The roof's slope, expressed as rise per 12 units of run — used to convert horizontal coverage distance to along-slope distance.

Pitch is what turns the code's horizontal distance into the material you actually buy: the steeper the roof, the more slope length it takes to cover the same horizontal distance. A 4/12 roof needs about 5% more membrane than the plan distance; a 12/12 roof needs 41% more. On a roof with planes at different pitches, work each plane separately rather than averaging. The field starts at 1 in 12 because a flat roof has no slope to convert. A roof covered in a membrane system rather than shingles or tiles is sealed whole rather than in a strip at the eaves, so this page does not apply to it.

Ice dam membrane area needed

246 sq ft

High confidence
Membrane width (along slope)
3.73 ft
Then change the inputs to see how far the answer moves.

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How this was calculated

Formula source(s)

  • IRC R905.1.2: self-adhering ice barrier membrane must extend from the eave edge to at least 24in inside the building's exterior wall line, measured along the roof slope
  • IRC R905.2.8.2, valleys under asphalt shingles: a closed valley lined with one ply of smooth roll roofing at least 36 in (914 mm) wide, an open valley lined with metal at least 24 in (610 mm) wide; self-adhering polymer-modified bitumen underlayment (ASTM D1970) is permitted in place of the lining

Inputs used

Membrane For
The eaves (an ice barrier strip)
Total Eave Length
66 ft
Roof Overhang
16 in
Required Coverage Inside Wall Line
24 in
Roof Pitch (rise per 12)
6
Total Valley Length (along the slope)
30 ft
Valley Lining Width
36 in

Intermediate steps

Membrane width (along slope)
3.73 ft
Final result245.97 sq ft

What this calculation does not cover

  • The two horizontal distances above are added at "the wall line", and a wall has thickness. R905.1.2 measures its 24in from the INSIDE face of the exterior wall, while an overhang is normally dimensioned from the OUTSIDE face of the framing or sheathing — so the 150 to 300 mm between those two faces belongs in the membrane width and is in neither box. On a thick wall, or one with exterior insulation, the membrane stops short of the point the code actually names.
  • Gives a flat area, and the membrane is bought in rolls of fixed width — commonly 900 mm — that overlap at every side and end lap. The default inputs need a 1.13 m width, which is wider than one roll, so it goes on as two courses with a lap between them, and the sheet area to order runs above this figure by the lap on every seam plus the cut-off at each end of a run.
  • Eaves only. The same membrane is normally run up every valley, behind sidewalls and dormer cheeks, around chimneys and skylights, and across any section shallow enough to need it — none of that is in this number, and on a cut-up roof it is often more area than the eaves themselves.

Estimated cost — your price

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26.6°1266/12
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Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-10-11 · v1.0.1

Regulatory standards & verification citations2
  1. IRC R905.1.2: self-adhering ice barrier membrane must extend from the eave edge to at least 24in inside the building's exterior wall line, measured along the roof slope
  2. IRC R905.2.8.2, valleys under asphalt shingles: a closed valley lined with one ply of smooth roll roofing at least 36 in (914 mm) wide, an open valley lined with metal at least 24 in (610 mm) wide; self-adhering polymer-modified bitumen underlayment (ASTM D1970) is permitted in place of the lining

Which documents these citations point at

  • International Residential Code — R905.1.2, R905.2.8.2 (United States)One- and two-family dwellings and townhouses up to three storeys — structure, fire safety, energy, plumbing, mechanical and electrical in a single document.

Standards referenced: ASTM D1970 (ASTM International, United States).

A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.

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Tools and safety for this job

To lay the roof underlay. Generic types, no brands, no prices.

Protection this work requires

  • Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.
  • Knives cause more site injuries than any power tool: cut away from your body, change blades often, and wear cut-resistant gloves to EN 388 level C for repeated cutting.
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  • Tape measure

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Also needed as materials: replacement blades, staples.

what each roofing tool is for, and the spec that decides which to buy where one does.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Called something else where you work? Ice dam — the term in each market, how close the equivalence really is, and the standard that governs it.

Still deciding? Ice Dam Membrane vs Attic Ventilation — the factors that actually differ, with no invented prices.

Worked example: Shingle roof replacement — step 4 of 11

The data behind it: Production rates by operation

How to calculate roof eave ice dam protection membrane footprint (and valley lining) in 8 steps

  1. Membrane ForA strip along the eaves against ice dams, or a lining down every valley.
  2. Total Eave LengthThe combined length of all eaves requiring ice barrier protection.
  3. Roof OverhangThe horizontal distance the roof overhangs beyond the exterior wall line.
  4. Required Coverage Inside Wall LineHow far the membrane must extend past the interior face of the exterior wall line, per IRC R905.1.2.
  5. Roof Pitch (rise per 12)The roof's slope, expressed as rise per 12 units of run — used to convert horizontal coverage distance to along-slope distance.
  6. Total Valley Length (along the slope)Every valley's length, eave to top, measured along the valley.
  7. Valley Lining WidthThe lining's width across the valley: 36 in (914 mm) under a closed valley, IRC R905.2.8.2.
  8. Ice dam membrane area neededThe tool computes the ice dam membrane area needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Ice dam membrane area needed by total eave length

Page defaults, not your figures above.

Total Eave LengthIce dam membrane area needed (sq ft)
40 ft148
60 ft222
80 ft296
100 ft370
120 ft444

Frequently asked questions

Why does the membrane width need to be measured along the slope, not horizontally?
The overhang and required interior coverage are both horizontal distances, but the membrane itself lies along the sloped roof surface, so the horizontal distance must be scaled up by the slope factor (from the roof pitch) to get the actual membrane width needed.
Where does the 24in requirement come from?
IRC R905.1.2 requires self-adhering ice barrier membrane to extend from the eave edge to at least 24in inside the building's exterior wall line, measured along the roof slope, in areas with a history of ice forming along the eaves.
How much lining does a valley take?
Its length along the valley times the lining's width. A closed valley takes a lining at least 36 in (914 mm) wide under IRC R905.2.8.2, so two valleys of 4.5 m (14.8 ft) each, 9 m (29.5 ft) in all, take 8.23 m² (88.6 sq ft) of lining before laps.
Does this calculator include the overhang itself in the required coverage?
Yes — the membrane width combines the horizontal overhang distance plus the required interior coverage distance, then converts that total horizontal run to an along-slope width using the roof pitch.
Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.