Residential Building Inspector B1Roof-Ceiling ConstructionHard
A builder is constructing a roof in a region where the average January temperature is 25°F (-4°C). The roof has an eave overhang of 18 inches. According to the IRC, what is the minimum required dimension for the ice barrier membrane measured from the lowest edges of the roof surface?
- A24 inches (610 mm)
- B36 inches (914 mm)
- C72 inches (1829 mm)
- D48 inches (1219 mm)
Show answer & explanationAnswer & explanation
Correct answer: D. 48 inches (1219 mm)
The IRC requires ice barrier membranes to extend from the lowest edges of the roof to a point at least 24 inches (610 mm) inside the exterior wall line of the building. With an 18-inch overhang, the membrane must cover the 18 inches of overhang plus 24 inches inward from the wall, totaling 18 + 24 = 42 inches. Since the options are in standard roll widths, the next standard width is 48 inches.
Why the other options are wrong
- A. 24 inches is the required distance inside the wall line, not the total length from the eave.
- B. 36 inches is a common membrane width, but not sufficient to meet the 24-inch inside the wall requirement with an 18-inch overhang.
- C. 72 inches would provide more than adequate coverage but is not the minimum required dimension.
Ice Barrier Installation
An ice barrier membrane is required in certain climate zones to prevent water penetration from ice damming, extending a specified distance from the eave.
- Required in locations where average January temperature is 25°F or less.
- Must extend from the lowest roof edge.
- Must reach a point at least 24 inches inside the exterior wall line.
Memory trick: Ice barrier: Overhang PLUS 24 inches inside the wall, then round up for the roll.