How do you calculate how far up the eave ice and water shield must go?
Short answer. Where an eave ice barrier is required, measure horizontally from the roof edge to 24 inches inside the exterior wall, then multiply by your roof's pitch factor to get the distance up the slope. With a typical overhang, that distance usually exceeds 36 inches, so one row of membrane falls short and two rows are needed.
The formula
Distance up the slope = (overhang + wall thickness + 24 inches) x pitch factor.
- Overhang: the horizontal distance from the fascia to the outside of the wall.
- Wall thickness: the line falls inside the exterior wall, so include the wall, roughly 4 to 6 inches on frame houses.
- Pitch factor: about 1.054 for 4/12, 1.118 for 6/12, 1.202 for 8/12, 1.302 for 10/12 and 1.414 for 12/12.
Worked example
- 12 inch overhang + 6 inch wall + 24 inches = 42 inches horizontal.
- On a 6/12 roof: 42 x 1.118 = about 47 inches up the slope.
- One 36 inch row covers 36 inches; two rows with a 3 to 3.5 inch side lap cover about 68 inches, so two rows it is.
- An 18 inch overhang on an 8/12 roof: (18 + 6 + 24) x 1.202 = about 58 inches, still within two rows.
Rolls for the eaves
Multiply the total eave length by the number of rows, add about 10 percent for end laps and waste, and divide by the roll length. For 120 feet of eave at two rows: 240 feet plus 10 percent is about 264 feet, or four rolls of about 66 feet.
Is it required where you live?
The eave ice barrier applies only where the local building department designates a history of ice damming, which is uncommon in most of Georgia, so ask your jurisdiction, especially in the mountains. Many roofers still run membrane along eaves under problem areas. See ice and water shield in Georgia and how roofers measure a roof.