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Our area. New roof about 5 years ago and as I recall we had some ice barriers installed in some spots 36" or more due shady roof valleys etc. Can't for certain say- I know it was discussed.
R905.2.7.1 Ice barrier.
See the attached Montana Department of Labor and Industry Technical Advisory T1-16 In areas where there has been a history of ice forming along the eaves causing a backup of water as designated in Table R301.2(1), an ice barrier shall be installed for asphalt shingles, metal roof shingles, mineral-surfaced roll roofing, slate and slate-type shingles, wood shingles and wood shakes. The ice barrier shall consist of not fewer than two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet shall be used in place of normal
underlayment and 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. On roofs with slope equal to or greater than eight units vertical in 12 units horizontal (67-percent slope), the ice barrier shall also be applied not less than 36 inches (914 mm) measured along the roof slope from the eave edge of the building.
Exception: Detached accessory structures not containing conditioned floor area.
Here in MA, 36" is code. Past the first joint in the plywood (about 48") is good practice.
I had my rear roof on dormer of my Cape style house fully covered with ice & water shield. The additional cost was minimal in the grand scheme of things.
How ever wide the roll is and a bit wider than the overhang of the roof. It's for ice dams, so I don't know what purpose it would serve for covering an entire roof that had been installed correctly, except for upping the cost tremendously because ice shield is expensive.
If you put the heat strips along the overhang while the roof is being installed then you don't have to worry about ice dams, for as long as the heat strips are still working. I'd still do the ice shield even with heat strips.
If you have to use heating strips, you have a bigger issue. If the attic is warm and melts the snow, it runs over the cold overhang and freezes. If you can prevent that, you don't need heat strips.
When we were building up north where snowfall is measured in feet, not inches... we were instructed 36" would meet code. And after a decade the roof we put up is as pretty as the day we laid it.
The key is up there, the snow loads are high and length of the Winter season is long.
Though the insurance company told somebody who was building in the same area that they wouldn't insure their build if they didn't use ice shield on the whole roof.
When we were building up north where snowfall is measured in feet, not inches... we were instructed 36" would meet code. And after a decade the roof we put up is as pretty as the day we laid it.
The key is up there, the snow loads are high and length of the Winter season is long.
Though the insurance company told somebody who was building in the same area that they wouldn't insure their build if they didn't use ice shield on the whole roof.
I think that's a bit overkill.
You mention snow load. The primary purpose of ice and water shield is to prevent water intrusion due to ice dams. Minimum code is 36" but for a little more money, one can go beyond minimum and ensure it never leaks.
Location: East of Seattle since 1992, 615' Elevation, Zone 8b - originally from SF Bay Area
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We are in the Seattle area where 10-20F is common in winter, and we get snow 3-4 times a year. Our code calls for a minimum of 24", and 36" if there is less slope to the roof. Our problem that is not helped by this is the metal chimney on the family room gas fireplace. It's triple-wall but apparently still gets warm enough to melt the snow, which then runs over the frozen rain gutter, down the downspout and covers it with over an inch of ice all the way to the ground.
Here in Anchorage, AK, minimum is 3' past where the wall meets the roof. Meaning, if your roof has a 2' overhang, you need at least 5' of ice shield. If it were me, I'd have the whole roof done.
For what it's worth, we have gotten over 10 feet of snow this season.
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