Ice and water shield is a self-adhering rubberised membrane that seals around fasteners. A standard roll is 36 inches wide by 66 feet, covering about 200 square feet — but it is ordered by the linear foot of eave and valley, not by roof area.
Where it is required
In climates with a history of ice damming, code requires it along every eave, running from the roof edge to at least 24 inches inside the exterior wall line measured horizontally.
It also belongs in all valleys, around chimneys and skylights, and along rake edges on low-slope sections. Many roofers run it in valleys regardless of climate.
Deep overhangs need two courses
A 36 inch roll laid along the eave covers 36 inches up the slope. With a 12 inch overhang and a 6/12 pitch, that leaves you short of the 24-inch-past-the-wall requirement.
Measure from where the interior wall face is, work back down the slope to the edge, and see whether one roll width covers it. If not, budget for two courses along the eave.
It only goes down once
The adhesive grabs permanently on contact. There is no repositioning — a course laid crooked comes off in shreds.
Work in cool weather if you can, peel the backing gradually, and roll it as you go. In hot sun the membrane becomes very difficult to handle.
What causes an ice dam in the first place
Ice barrier is a remedy for a symptom. The cause is heat escaping into the attic, melting snow on the upper roof, and that meltwater refreezing when it reaches the cold overhang beyond the heated space.
The ridge of ice that builds up there holds water behind it, and water standing on a roof finds its way under shingles, which are designed to shed water running downhill, not to hold a pond.
Ice barrier stops that water getting into the house. It does not stop the dam. The actual fix is air sealing and insulation in the attic floor, plus enough ventilation to keep the roof deck cold.
Valleys, walls and low slopes
The eave is the well known location, but it is not the only one specified. Valleys concentrate runoff from two roof planes and are commonly required to have ice barrier along their full length.
Where a roof meets a vertical wall, and around chimneys and skylights, the membrane is also standard practice even where it is not strictly required.
Low slope sections — anything under about 4/12 — collect more and drain slower, and some codes require full coverage rather than a strip. Check locally, because these requirements vary more between jurisdictions than almost anything else in roofing.