TradesHow this works
Two dry-in takeoffs come off the roof edges. Along the eaves, ice-and-water membrane has to reach 24 inches past the interior wall line, so the coverage up the slope is the overhang plus two feet, and at least one roll width. Past a 36 inch roll it takes a second course, which doubles the rolls. Each roll is about 200 square feet.
Along the peak, ridge vent runs the length of the ridge and provides a net free area per foot for the exhaust side of the attic. The tool gives the membrane rolls and the ridge-vent length and NFA, and it flags when a deep overhang pushes you into a second course. Balance the ridge exhaust with intake at the eaves using the attic-ventilation engine.
Common questions
How much ice-and-water shield do I need?
It depends on the eave length and the overhang. Code requires the membrane to run from the eave to at least 24 inches inside the exterior wall line, so the coverage up the slope is the overhang plus 24 inches, and at least one 36 inch roll width. A shallow overhang fits one course, a deep one needs two. Rolls are about 200 square feet, or roughly 66 feet at 36 inches wide.
How far up the roof does ice-and-water go?
To 24 inches past the interior wall line, measured up the slope. On a house with a 12 inch overhang that is 36 inches, which one roll width covers. On a deeper 24 inch overhang it becomes 48 inches, which needs a second course lapped over the first, so the eave rolls double.
How long a ridge vent do I need?
Run ridge vent the full length of the ridge, less a bit for the end caps, plus two end plugs. Each foot of vent provides a net free area, typically around 18 square inches, which is the exhaust side of attic ventilation. It only works paired with enough intake down at the eaves, so balance the two with the attic-ventilation engine.
Sources: Ice-and-water roll: 36 in x ~66.7 ft = ~200 sq ft; IRC R905.1.2 eave (24 in past wall) · Ridge-vent net free area ~18 sq in per foot (typical, manufacturer-dependent). Engine version 1.0.0. Risk tier 1.