Ice and Water Shield Calculator

Eave coverage by pitch for the 24 in inside-the-wall rule

Horizontal run is overhang plus a 6 in wall plus the 24 in the IRC asks for, so the four columns are a flush eave, 12 in, 24 in and 36 in of overhang. Cells give the slope distance and the courses a 36 in roll with a 3 in side lap needs, where course one covers 36 in and every course after it adds 33. Watch the 8/12 row on a flush eave, which tips from one course to two at 36.1 in. Under the 2024 IRC an 8/12 or steeper roof also has a straight 36 in along-the-slope minimum, so take the larger figure. Each course lays 3 ft of width, so membrane per 100 ft of eave is the course count times 300 sq ft.

Ice and water shield rolls: size, laps and net coverage

Gross roll area is the number printed on the label. GAF adds the footnote excludes laps to both its products. Net after side lap is roll length times the exposed width once the side lap comes off, which is the same arithmetic Owens Corning used to publish 183.4 sq ft for WeatherLock Mat (66.7 ft x 33 in). End laps take roughly another half foot of length off every roll that butts the one before it. Laps and roll sizes come from each manufacturer sheet separately and are never averaged across brands.

The 24 inches the code asks for doesn’t climb with the roof

IRC R905.1.2 is short. Where Table R301.2 flags a history of ice forming along the eaves, an ice barrier goes in under asphalt shingles, metal roof shingles, mineral-surfaced roll roofing, slate and slate-type shingles, wood shingles and wood shakes. It has to be two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet used in place of normal underlayment. Either one runs from the lowest edges of all roof surfaces to a point not less than 24 inches inside the exterior wall line of the building.

The wall line is a plan-view line. So the 24 inches is measured flat, the way you would scale it off a floor plan. The roof above it is sloped, so the membrane has to travel further than 24 inches to reach that point on the plan.

Two more things stretch it. The eave overhang sits outside the wall entirely and the barrier starts at the drip edge, so the overhang is added before you even reach the wall. And the wall has thickness, roughly 6 inches for a 2x6 stud with half-inch sheathing. GAF writes the target as 24 in past the interior warm inside wall. CertainTeed writes it as no less than 24 inches beyond the interior wall line of the exterior walls. Both put the finish line at the inside face, and that reading is the conservative one, so it is what this calculator uses by default.

Add it up and the horizontal run is overhang plus wall thickness plus 24 inches. Multiply by the pitch multiplier for the slope distance. On a 6/12 roof with a 16 in overhang and a 6 in wall the run is 46 in and the membrane has to reach about 51.4 in up the slope. A single 36 in course falls roughly 15 inches short of that, which is the mistake this page exists to stop.

Ask your inspector which face they measure from before you order. Some read the exterior wall line as the outside face, which trims 6 inches off the run. Set the wall thickness field to zero and the calculator follows that reading instead.

What changed between the 2021 and 2024 IRC

Less than the trade press reported. Both editions carry the same opening sentence, the same list of roof coverings and the same 24 in requirement. Both also carry a second requirement for steep roofs, where a slope of 8 units vertical in 12 units horizontal or greater gets the barrier applied not less than 36 inches measured along the roof slope from the eave edge.

The 2021 wording says the barrier shall also be applied. The 2024 wording drops the word also and adds a paragraph break. Several write-ups treated the 8:12 rule as deleted for 2024. The published 2024 section still prints it, so plan on both numbers and take whichever is larger.

Earlier editions kept the rule somewhere else. Up to the 2012 IRC it sat at R905.2.7.1, inside the asphalt shingle section, with no steep-slope sentence at all. The 2015 IRC promoted it to R905.1.2 so it covered every roof covering in the list, and the 8:12 sentence first shows up there. If you are working to an older adopted edition, check which section number your jurisdiction cites.

The exception hasn’t moved in any of them. Detached accessory structures that contain no conditioned floor area are out, which usually covers a plain garden shed and usually doesn’t cover a heated detached garage.

None of this fires unless your jurisdiction says so. Table R301.2 carries an ice barrier underlayment required column, and states fill it in county by county. Indiana, for instance, publishes a table where the answer is yes across the north and no further south. Where the table says no, the manufacturer instructions can still require the membrane, because R905.1 makes those instructions binding.

Courses, side laps and why one row rarely does it

Rolls are 36 inches wide, near enough universally. The first course covers a full 36 inches up the slope. Every course after that only adds 36 inches minus the side lap, because it has to overlap the one below to shed water.

Side laps aren’t the same across brands, and this is where estimates drift. Grace Ice and Water Shield asks for 3.5 in. GAF asks for 3 in on both StormGuard and WeatherWatch. Owens Corning laps the second WeatherLock course 3 in onto the first. CertainTeed publishes 4 in for WinterGuard Granular and 6 in for WinterGuard Sand, which is double the GAF figure on the same roof.

Run the arithmetic and the difference bites. Two courses of a 3 in lap product cover 69 in. Two courses of WinterGuard Sand cover 66. On a 9/12 roof with a 24 in overhang the required reach is about 67.5 in, so the GAF sheet finishes in two courses and the Sand needs three. A third course on a 100 ft eave is another 300 square feet of membrane and one more roll, five instead of four.

End laps are steadier. Every sheet compared here wants 6 in where one roll ends and the next begins. CertainTeed adds a detail worth copying, which is to offset those end laps at least 2 ft from course to course so the joints don’t stack.

Courses round up, always. Half a course of coverage protects nothing.

Gross roll area, net coverage, and the gap between them

Every roll on the shelf is sold by its gross area. GAF prints 200 square feet on the WeatherWatch and StormGuard rolls, then adds a two-word footnote underneath, excludes laps. Grace lists 225, 200 and 108 square foot rolls, all 36 in wide. CertainTeed lists 195 square feet for WinterGuard Sand and Granular, and 200 for WinterGuard PRO.

Owens Corning is the one brand that publishes both figures. The WeatherLock Mat data sheet gives gross coverage of 200 square feet and net coverage of 183.4. Take the 66.7 ft roll length, take 33 in of exposed width once the 3 in side lap is deducted, and you get 183.4 square feet. It lands on the published number to the tenth, which is a useful check that the arithmetic is the same one the manufacturer used.

So a published net figure typically accounts for the side lap and nothing else. End laps come out on top. A 6 in end lap costs half a foot of length every time one roll meets the next, so a 66.7 ft roll advances roughly 66.2 ft along the run. Call it another 1 percent gone.

Three 200 square foot rolls won’t cover a 200 foot eave in a single course. They give 200.1 ft gross, two end laps take a foot back, and you finish a foot short with a truck already gone. Buy the fourth.

This calculator counts rolls on linear feet of 36 in course rather than on area, because area hides lap losses and linear feet don’t. The side lap is already inside the course count, the end lap is inside the usable roll length, and the waste slider is left to do only what waste sliders should do.

Valleys, rakes and hips

Valleys get the full length, every time, from the low point up and past the peak. Width is settled practice rather than one code line, though three sources land in the same place. CertainTeed requires a minimum 36 in in the valley. Owens Corning says keep the roll width intact and run 18 in either side of the center. IRC R905.2.8.2 lets a closed valley be lined with one ply of smooth roll roofing not less than 36 in wide, and permits an ASTM D1970 self-adhering underlayment in its place.

One roll width, centered, is the answer from all three directions. Sequence matters as much as width. Apply the valleys before the eaves, starting at the low point and working up, so the eave course laps over the valley sheet rather than under it. Grace and Owens Corning both put that order in writing.

Rakes aren’t in R905.1.2 at all. The rule names the lowest edges of roof surfaces, and a rake is a sloping edge rather than a low one. Manufacturers still show rake application, and on an exposed gable in wind-driven rain country it is cheap insurance. Treat it as one 36 in course and price it separately, which is what the rake field does here.

Hips and ridges take membrane on some jobs too. CertainTeed calls for 12 in minimum on ridges. Different width, running a different way, so keep it out of the eave arithmetic and buy the narrow 6, 9, 12 and 18 in strip rolls Grace sells for detail work.

Skylights, chimneys, dormers and sidewalls are usually the leakiest part of a roof and none of them are in this calculator. Add them to the waste allowance or measure them as extra rake feet.

Low slope, and when the membrane covers the whole deck

Asphalt shingles stop at 2:12 under IRC R905.2.2. From 2:12 up to below 4:12 the underlayment doubles. Table R905.1.1(2) sets out the method, which is a 19 in strip parallel to the eaves first, then full 36 in sheets overlapping successive sheets 19 in, with 4 in end laps offset 6 ft.

The IRC low-slope rule asks for two layers of ordinary underlayment. It doesn’t ask for membrane over the whole roof. Shingle manufacturers often do. CertainTeed requires WinterGuard or its equivalent over the entire deck surface for shingles on a low slope deck of 2 to below 4 inches per foot, with a short list of acceptable equivalents. Owens Corning calls full deck coverage a one of two accepted low-slope deck preparations in its shingle instructions, the other being double-lapped synthetic underlayment.

The two rules stack rather than cancel. On a 3:12 roof in an ice barrier county you owe the eave barrier under R905.1.2 and doubled underlayment across the rest, and where the shingle warranty asks for membrane everywhere then membrane everywhere is what you buy. To size that, set the eave and valley fields to zero and put your roof area divided by three into the rake field, which lays exactly one 36 in course. To get that area, take your plan footprint, multiply it by the rafter multiplier from the roof pitch page, and divide by 3 ft per course.

Full deck coverage brings its own condition. These sheets are vapor barriers, and every manufacturer says so. Cover a whole deck and the ventilation has to be right, or you have traded an ice dam for condensation on the underside of the sheathing.

One footnote for Canada. Under NBC 9.26.5.1 eave protection isn’t required on asphalt shingle roofs laid to Subsection 9.26.8 (which covers asphalt shingles generally, so confirm the reading with your authority having jurisdiction), which is the cemented low-slope method for slopes under 1 in 3.

Canada: eave protection under the NBC

Canada writes the same idea with different numbers. NBC 9.26.5.1 requires eave protection on shingle, shake or tile roofs, extending from the edge of the roof a minimum of 900 mm up the roof slope to a line not less than 300 mm inside the inner face of the exterior wall.

Two differences matter for ordering. The horizontal target is 300 mm, roughly 11.8 in, against 24 in in the IRC. And Canada sets a floor measured up the slope, 900 mm or about 35.4 in, which the IRC has no equivalent of at ordinary pitches. On a shallow roof with a modest overhang the 900 mm floor is usually the binding number rather than the 300 mm line, and the calculator applies whichever is larger.

The exemptions run broader too. Eave protection isn’t required over unheated garages, carports and porches, where the roof overhang exceeds 900 mm measured along the roof slope from the edge of the roof to the inner face of the exterior wall, on asphalt shingle roofs laid to Subsection 9.26.8 (which covers asphalt shingles generally, so confirm the reading with your authority having jurisdiction), on slopes of 1 in 1.5 or greater, or in regions with 3 500 or fewer degree-days. The steep-roof exemption is the reverse of the IRC, which asks for more membrane at 8:12 and steeper rather than none.

Materials come from 9.26.5.2, which accepts two plies of No. 15 asphalt-saturated felt lapped 480 mm and cemented, Type M or S roll roofing with 100 mm laps, coated base sheets, or self-sealing composite membranes of modified bituminous coated material. The self-adhered sheets sold as ice and water shield sit in that last group, and most of them also carry CSA A123.22.

Provincial codes adopt the NBC with amendments, so the wording above is the model rather than the last word. Check yours.

Temperature, cold decks and how long it can sit exposed

Cold is the usual reason a membrane fails to stick. Every sheet here is an adhesive and adhesives stiffen.

Grace asks for 40 degrees F or higher for the air, the roof deck and the membrane, and for the roof covering to go on at 40 degrees F or higher too. Owens Corning wants over 40 and under 100, and warns that below 40 the product won’t fully adhere until things warm up. GAF prints 45 to 120 degrees F on the StormGuard and WeatherWatch instructions. CertainTeed puts it bluntly, saying WinterGuard won’t adhere to wet or cold surfaces, and that on WinterGuard PRO it allows only roofing nails for cold-weather tack-down and bans staples outright. The Sand and Granular sheet still permits either.

A cold deck can still be roofed. GAF publishes a cool-weather bulletin for this case. Store the rolls overnight at 55 degrees F minimum, bring them out one at a time, get the membrane to at least 45 degrees F at the moment it goes down, pre-cut into 10 to 20 ft lengths, back nail every 18 in in the selvage edge and hand roll every lap. Stop work if the laps aren’t adhering, because a lap that admits dirt may never seal.

Dry beats warm. The deck has to be clean and dry with no ice, frost, dew, moisture, dust or dirt. Frost at eight in the morning on a bright 45 degree day is still frost, and the sheet will lift off it in a month.

Exposure limits differ enough to matter on a slow job. Grace says cover within 90 days. GAF rates StormGuard at up to 90 days and WeatherWatch at up to 60. CertainTeed allows six months on WinterGuard Sand and Granular and four on WinterGuard PRO. Owens Corning is the tightest of the group, at 30 days. If the shingles are weeks away, that spread is worth checking before you pick a brand.

The fields, and why these ones

Five numbers set the answer, and the one most calculators drop is the wall.

Eave length is total, not per side. Add up every eave the barrier runs along, so a simple gable is two runs and an L-shaped house is usually four. Measure along the wall rather than along the fascia if the two differ.

Pitch drives the multiplier and nothing else here. Enter it as x/12, the same way the roof pitch calculator on this site reports it.

Overhang is horizontal, from the outside face of the wall to the roof edge. It is not the length of the rafter tail, which is longer on any sloped roof. Measuring the tail and entering that is a quiet way to over-order by a course.

Wall thickness is the field to argue about. Roughly 6 in covers a 2x6 stud wall with half-inch sheathing, and about 4.5 in covers a 2x4. Set it to zero if your inspector reads the exterior wall line as the outside face. Six inches of horizontal run is about 7 in up the slope on a 6/12, and on a deep overhang it can push you into another course.

Valley and rake are lengths rather than checkboxes. A toggle would need a length anyway, and roofs rarely have one tidy valley. Enter zero for either and its row drops out of the results.

Waste sits at 10 percent by default. It covers offcuts at the ends of runs, the odd ruined sheet and detail work around a chimney or skylight. It isn’t covering the laps, because the side lap is already inside the course count and the end lap is already inside the usable roll length.

Common questions

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Same job, next question: work out pitch, slope and rafter length, lay out a stair stringer, board feet in a lumber order or work out common rafter cuts.