Where is ice and water shield required on a roof?
Where ice damming is a concern — the IRC triggers it where the average January temperature is 25°F or colder — an ice barrier must extend from the eave's edge to a point at least 24 inches inside the exterior wall line (2018 IRC §R905.1.2). It's a self-adhering membrane, or two cemented layers of underlayment, replacing normal underlayment in that zone.
Source: 2018 IRC §R905.1.2 · Verify against your AHJ and the adopted code edition.
An ice barrier (often called ice-and-water shield) stops meltwater that backs up behind an ice dam from leaking through the deck at the eaves.
Under IRC §R905.1.2, in areas where there’s been a history of ice damming — the code’s trigger is a locality where the average January temperature is 25°F (-4°C) or lower — the barrier must:
- Run from the lowest edge of the roof (the eave) up the slope to a point at least 24 inches inside the exterior wall line of the heated space.
- Be either a self-adhering polymer-modified bitumen membrane, or two layers of underlayment cemented together, in place of ordinary underlayment in that band.
On low-slope or deep-overhang roofs the 24-inch-inside-the-wall measurement can push the membrane well up the roof, so figure it along the slope, not horizontally. Whether your area requires it is a local call — verify with your AHJ.
Work it out: the roofing calculator gives the roof area and squares; the eave band of ice barrier is a strip along each lower edge, so price it by the linear foot of eave.
Sources
- ICC — International Codes (IRC) — 2018 IRC §R905.1.2