How it works
The calculator finds how far the membrane has to run up the slope, counts the courses needed with the side laps and then turns the total length into rolls.
Distance up the slope = (Overhang + Distance inside the wall) × √(1 + (Rise ÷ Run)²)
Courses = 1 if the distance ≤ roll width, else ⌈ (Distance − Side lap) ÷ (Roll width − Side lap) ⌉
Length = (Courses × Eave length + Valleys + Other pieces) × (1 + Extra ÷ 100)
Rolls = ⌈ Length ÷ (Roll length − End lap) ⌉
⌈ ⌉ means rounding up to the next whole course or roll. The square-root term is the slope factor, about 1.118 for a 6:12 roof.
How far the barrier has to reach
In areas with a history of ice forming along the eaves, IRC R905.1.2 requires an ice barrier – two layers of underlayment cemented together or a self-adhering polymer-modified bitumen sheet – from the lowest edges of all roof surfaces to a point not less than 24 in (610 mm) inside the exterior wall line. Owens Corning’s instructions add that the distance is measured horizontally. Local codes change the distance and where it applies, so the calculator takes the distance as an input; the filled-in figure is only an example.
From level distance to slope distance
The membrane is laid on the roof, so it has to cover the overhang and the distance inside the wall along the slope. On a 6:12 roof with a 1 ft overhang and 24 in inside the wall, that is 3 ft × 1.118 = 3.35 ft, or about 40 in – more than one 36 in roll.
Courses and laps
The first course runs along the eave edge. Each course above overlaps the one below by the side lap – at least 3.5 in for Grace Ice & Water Shield and 3 in for WeatherLock G – so a 36 in roll adds 32.5 in or 33 in. Where two pieces meet in a course they overlap by the end lap, 6 in for both products, so each roll covers its length less 6 in.
Valleys and other areas
Owens Corning and GCP both put the membrane in valleys before the eaves. Owens Corning keeps the roll width intact, 18 in on either side of the valley, so the calculator adds one roll width for every foot of valley. Pieces around chimneys, skylights and walls depend on your roof – enter them as a length if you want them in the count.
Worked example in words
On 80 ft of eaves with a 1 ft overhang, 24 in inside the wall and a 6:12 pitch, the membrane has to run 40.2 in up the slope. One 36 in course is not enough, so two courses are needed, covering 36 + 32.5 = 68.5 in. Two courses of 80 ft are 160 ft. A 75 ft Grace roll covers 74.5 ft after the end lap, so 160 ÷ 74.5 = 2.15 – you need 3 rolls.
Worked example
Inputs
- Total eave length
- 80 ft
- Eave overhang (horizontal)
- 1 ft
- Distance inside the wall line
- 24 in
- Roof pitch
- 6:12
- Total valley length
- 0 ft
- Other pieces (length)
- 0 ft
- Ice and water shield
- GCP Grace Ice & Water Shield – 36 in × 75 ft, 3.5 in / 6 in laps
- Extra allowance
- 0 %
Result
- Distance up the slope
- 40.25 in (3.35 ft)
- Courses along the eaves
- 2
- Width covered by the courses
- 68.5 in (5.71 ft)
- Length of membrane incl. extra
- 160 ft
- Rolls
- 3
Common mistakes
- Measuring the distance along the slope instead of level. The code distance is measured inside the exterior wall line; on a pitched roof the membrane has to run further up the slope to get there.
- Forgetting the overhang. The barrier starts at the eave edge, so the overhang comes before the distance inside the wall.
- Dividing the eave length by the roll length without the end laps. Each join in a course overlaps 6 in with the products used here.
- Leaving out the valleys. Owens Corning applies WeatherLock in valleys first, with the roll width intact – 18 in on either side.
- Assuming one course is enough. A 36 in roll covers only 36 in up the slope; with a deep overhang or a steep roof you need a second course.
Assumptions and limits
- Distance up the slope = (overhang + distance inside the wall line) × √(1 + pitch²). The overhang and the distance are measured level.
- The first course covers a full roll width; each further course covers the roll width less the side lap. Courses are rounded up to whole courses.
- Each valley takes one roll width along its length; other pieces are added as length. Rolls = total length × (1 + extra) ÷ (roll length − end lap), rounded up.
- All eaves have the same overhang and pitch. Individual pieces and offcuts are not planned.
Frequently asked questions
How far up the roof does ice and water shield go?
IRC R905.1.2 says the ice barrier extends from the lowest edges of all roof surfaces to a point not less than 24 inches (610 mm) inside the exterior wall line, in areas with a history of ice forming along the eaves. Owens Corning’s WeatherLock G instructions say the same, measured horizontally. Local codes can require more – check with your building department.
How many rolls of ice and water shield do I need?
Multiply the eave length by the number of courses, add the valleys, and divide by the roll length less the end lap. On 80 ft of eaves with a 1 ft overhang at 6:12, two courses are needed, so 160 ft of membrane – three 75 ft Grace rolls.
How much does a roll of Grace Ice & Water Shield cover?
GCP lists 36 in wide rolls of 75 ft (225 sq ft), 66.6 ft (200 sq ft) and 36 ft (108 sq ft). Side laps take at least 3.5 in and end laps at least 6 in, so each roll covers a little less on the roof.
What size is a roll of Owens Corning WeatherLock G?
Owens Corning lists a 2-square roll, 66.7 ft long and 3 ft wide, covering 200 sq ft. Its instructions overlap courses 3 in and end laps 6 in.
Do valleys need ice and water shield?
Owens Corning applies WeatherLock in valleys before the eaves, with the roll width intact so it extends 18 in on either side of the valley, and GCP also applies its membrane in valleys before the eaves. Whether valleys need it on your roof is set by local code and your roofing manufacturer.
Sources
- Seattle Residential Code 2015, Chapter 9 – Roof Assemblies (2015 IRC with amendments) — City of Seattle, Department of Construction and Inspections. R905.1.2: where there is a history of ice forming along the eaves (Table R301.2(1)), an ice barrier of two cemented layers or a self-adhering polymer-modified bitumen sheet shall "extend from the lowest edges of all roof surfaces to a point not less than 24 inches (610 mm) inside the exterior wall line of the building".
- GRACE ICE & WATER SHIELD Data Sheet (last updated 2026-03-20) — GCP Applied Technologies (copy hosted by BuildSite). Product data: rolls 36 in wide × 75 ft (225 ft²), 66.6 ft (200 ft²) or 36 ft (108 ft²); "Side laps must be a minimum of 3.5 in. (90 mm) and end laps a minimum of 6 in. (150 mm)"; apply the membrane in valleys before the eaves.
- WeatherLock G Granulated Self-Sealing Ice & Water Barrier – Data Sheet — Owens Corning. Typical values, 2 sq. roll: length 66.7 ft (21.3 m), width 3 ft (0.9 m), area 200 sq ft.
- WeatherLock G – Installation Instructions — Owens Corning. "overlapping additional courses 3 inches on the selvedge edge"; "Overlap the end lap where the 2 rolls meet by 6 inches"; "should extend from the eaves to a point 24 inches inside the exterior wall, measured horizontally. Consult local building code"; valleys: roll width intact, 18 inches on either side, applied before the eaves.
- Estimating How Much Roofing Is Required — IKO. Actual area = horizontal area × area/rake factor; 6/12 → 1.118. The factor equals √(1 + (rise/run)²) (Pythagoras).
Method reviewed November 6, 2026