HesapHanem

Ice & Water Shield Calculator

Enter your eave length and courses to find the rolls of ice & water shield needed.

How it is calculated

The estimate is one multiplication and a rounded-up division, all in feet. linear feet = eave length × courses and rolls = ceil(linear feet / roll length). A course is one row of membrane running the full length of the eave, so two courses mean the whole eave is covered twice (one row above the other), not that the membrane is twice as thick. Partial rolls are rounded up to a whole roll. The tool uses the roll length you give (66 ft is the preset) and only the length: it does not use the roll width, it does not subtract overlap between courses or side laps, and it adds no waste allowance for valleys, hips, cuts or end laps. All three numbers must be above zero. For a quote, add extra for those items and check the roll size of the product you buy.

Worked example

A house has 85 ft of eaves and the plan calls for two courses of membrane. Linear feet: 85 × 2 = 170.00 ft. A roll of 66 ft covers 66 ft of one course, so 170 / 66 = 2.58. Rounded up, the job needs 3 rolls. The three rolls hold 198 ft, so about 28 ft are left over for overlaps, cuts and valley pieces, which the tool does not otherwise count.

Try this example →

Common mistakes

  • Entering the roof area in square feet instead of the eave length in feet; the first field is a length along the edge.
  • Counting the whole roof perimeter, including rakes (sloped edges), when only the eaves or the areas your code requires get the membrane.
  • Entering the number of rolls per course instead of the number of courses (rows) across the roof, which gives the wrong linear footage.
  • Using a roll length from one product and buying another: rolls of different lengths change the count, so enter the length printed on the roll you will buy.
  • Expecting the count to include overlap and waste; it does not, so a job with several valleys or short eaves often needs a roll more than the tool shows.

About this calculator

Ice and water shield is a self-adhering, rubberized-asphalt roofing underlayment installed along eaves, valleys and other vulnerable areas before shingles go on. In cold climates it’s often required by building code — the IRC generally calls for it to extend from the eave’s edge to at least 24 inches inside the exterior wall line — because it seals around nail penetrations and stops water from ice dams backing up under shingles and leaking into the attic.

This calculator estimates how many rolls you need from your total eave length, the number of courses (rows) the shield must cover and the length of your roll in feet: eave length × courses gives the linear feet needed, divided by the roll length and rounded up. Overlap between courses and waste are not added, so count any extra course or round up if your installation needs them. It’s used by roofing contractors and estimators quoting a job and by homeowners and DIYers planning a roofing project who need to know how much underlayment to buy before ordering materials.

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Comments (1)

  • Editorial team

    Check your local code before deciding how many courses to enter, since the required coverage depends on climate and the roof. One way to use the tool is to run two scenarios and compare the roll counts. With 120 ft of eave and a 66 ft roll, one course needs 2 rolls and two courses need 4. If the plan lands just under a whole roll, one extra is cheap insurance for valleys and cuts. What roll length do you usually find at your supplier?

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