How many rolls of ice and water shield do I need?

Multiply the eave length by the number of courses, add the valley length, divide by the roll length on the wrapper and round up. In the example above, 270 lf ÷ 66 ft = 4.09, so 5 rolls.

How far up the roof does ice and water shield go?

As far as your local code and the membrane’s instructions require. When that requirement is a level distance inside the exterior wall, the calculator adds the overhang and wall thickness, then converts the total to the length up the slope.

Why does it say 2 courses when the code asks for 24 inches?

The 24 in is counted from inside the wall, not from the eave edge. With an 18 in overhang and a 6 in wall you are already at 48 in level, more than one 36 in roll covers.

Can I use the valley length from a plan drawing?

No. A valley runs down the slope, so its plan length is shorter than the real one. Use the true length, measured on the roof or taken from a report that corrects for pitch.

Does the calculator add waste?

No waste percentage is added. Rolls are rounded up to whole rolls, and you add penetrations, sidewalls and chimneys by hand.

Free calculator

Ice and Water Shield Calculator for eaves and valleys

Enter the pitch, the overhang and your eave and valley lengths. You get the coverage up the slope, the courses at the eave and the rolls to order.

The formula

Every length below is in inches until the lineal feet. The pitch factor is rounded to 3 decimals.

Horizontal coverage: H = overhang + wall thickness + inside distance Measured level, from the eave edge inward.
Distance up the slope: D = H × √(1 + (rise ÷ 12)²) The membrane lies on the roof, so the level distance grows by the pitch factor.
Courses at the eave: 1 if D ≤ roll width, else 1 + ⌈(D − roll width) ÷ (roll width − side lap)⌉ Each extra course adds its width minus the side lap.
Lineal feet: eaves × courses + valleys Valleys get one course centred on the valley line.
Rolls: ⌈total lineal feet ÷ roll length⌉ Rounded up to whole rolls.
Square feet: total lineal feet × roll width ÷ 12 Roll width in inches, so ÷ 12 turns it into feet.

Worked example

A 6/12 roof with 120 lf of eave, 30 lf of valley and a 24 in overhang. Defaults for the rest: 6 in wall, 24 in inside the wall, 36 in × 66 ft rolls, 3 in side lap.

Horizontal coverage: 24 in overhang + 6 in wall + 24 in inside = 54 in
Pitch factor at 6/12: √(1 + (6 ÷ 12)²) = √1.25 = 1.118
Distance up the slope: 54 × 1.118 = 60.4 in
Courses: 60.4 in is more than one 36 in roll, so 1 + ⌈(60.4 − 36) ÷ (36 − 3)⌉ = 1 + ⌈0.74⌉ = 2 courses
Eave lineal feet: 120 × 2 = 240 lf
Valley lineal feet: 30 lf (one course)
Total lineal feet: 240 + 30 = 270 lf
Rolls: 270 ÷ 66 = 4.09, rounded up to 5 rolls
Square feet covered: 270 × 36 ÷ 12 = 810 sq ft

Order 5 rolls: two courses along 120 lf of eave plus one down 30 lf of valley, 270 lf and 810 sq ft in all.

Courses needed by pitch

18 in overhang + 6 in wall + 24 in inside the wall = 48 in level. With 36 in rolls and a 3 in side lap, one course covers 36 in up the slope and two cover 69 in.
Pitch	Pitch factor	Up the slope	Courses
3/12	1.031	49.5 in	2
4/12	1.054	50.6 in	2
5/12	1.083	52.0 in	2
6/12	1.118	53.7 in	2
7/12	1.158	55.6 in	2
8/12	1.202	57.7 in	2
9/12	1.250	60.0 in	2
10/12	1.302	62.5 in	2
11/12	1.357	65.1 in	2
12/12	1.414	67.9 in	2

What ice and water shield is for

Ice and water shield is a self-adhering membrane that goes straight on the deck, before the underlayment and shingles. When snow melts on a warm roof and refreezes at the cold eave, it builds an ice dam, and the water behind it backs up under the shingles. The membrane seals around the nails and keeps that water off the deck.

Many building codes, such as the International Residential Code, call for ice barrier from the eave edge to a point at least 24 inches inside the exterior wall line where ice dams are a risk; your local code and the manufacturer decide. Past the eave, it also goes where water gathers: valleys, and around penetrations.

Why the slope distance is longer than the level one

The coverage line is set level, from the eave edge to a point inside the wall. The membrane lies on the roof, so it has to cover the sloped length above that level distance. Multiply the horizontal coverage by the pitch factor to get it.

At 6/12, 54 in level becomes 60.4 in on the deck. At 12/12, the same 54 in becomes 76.4 in (54 × 1.414). Two 36 in courses with a 3 in lap only reach 69 in, so that roof needs a third course.

When a second course comes in

One 36 in course runs out fast. Add the overhang and the wall to the inside distance and you are near 36 in before the pitch counts. On a 4/12 roof with a 6 in wall and 24 in inside, a 4 in overhang gives 34 in level and 35.8 in up the slope: one course. Make it a 6 in overhang and you get 36 in level and 37.9 in up the slope: two courses.

A low pitch doesn’t save much. At 3/12 the pitch factor is only 1.031, so the level distance does nearly all the work. Deep overhangs, thick walls and steep pitches are what push you into a second or third course. Measure the overhang level, from the wall face to the eave edge, and use the side lap printed on the membrane’s instructions: a bigger lap means each extra course covers less.

Valleys, penetrations and sidewalls

The calculator gives each valley one course centred on the valley line. Enter the valley’s true length along the slope, not its plan length from a drawing; the plan length comes up short on any pitched roof.

Penetrations (vents, stacks, skylights), sidewalls and chimneys are sized by hand. Add a few lineal feet for each to the total, or plan to use the leftover from the last roll, and round up again if it pushes you past a full roll.

If you don’t have eave and valley lengths yet, trace the roof: RoofStruct gives you free, unlimited self-serve measuring and PDF reports, no card, with every eave and valley totalled.

Ice and water shield questions

How many rolls of ice and water shield do I need?: Multiply the eave length by the number of courses, add the valley length, divide by the roll length on the wrapper and round up. In the example above, 270 lf ÷ 66 ft = 4.09, so 5 rolls.
How far up the roof does ice and water shield go?: As far as your local code and the membrane’s instructions require. When that requirement is a level distance inside the exterior wall, the calculator adds the overhang and wall thickness, then converts the total to the length up the slope.
Why does it say 2 courses when the code asks for 24 inches?: The 24 in is counted from inside the wall, not from the eave edge. With an 18 in overhang and a 6 in wall you are already at 48 in level, more than one 36 in roll covers.
Can I use the valley length from a plan drawing?: No. A valley runs down the slope, so its plan length is shorter than the real one. Use the true length, measured on the roof or taken from a report that corrects for pitch.
Does the calculator add waste?: No waste percentage is added. Rolls are rounded up to whole rolls, and you add penetrations, sidewalls and chimneys by hand.