Eave Ice-Barrier Membrane Courses and Rolls
The self-adhering ice-and-water membrane the eave actually needs.
Example
You enter
- Eave run to protect (ft, all eaves) 40
- Overhang from wall line (in) 12
- Pitch rise (in / 12) 4
- Roll width (in) 36
- Roll length (ft) 66.7
- Course side lap (in) 6
You get
- Slope factor 1.054
- Coverage up slope (in) 37.9
- Courses 2
- Membrane (lf) 80
- Rolls 2
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
ScopeThe up-slope coverage from the overhang and pitch (IRC R905.1.2 runs it 24 in inside the exterior wall line, measured on the slope), the courses when that exceeds one roll width, and the rolls. A deep overhang or a steep pitch quietly pushes the coverage past a single 36 in course, so a one-roll-per-eave guess shorts the order. Required only where the AHJ has adopted it; valley and low-slope coverage is a separate add. A material takeoff, not a flashing plan.
Formula or tableslope_factor = sqrt(pitch_rise^2 + 144) / 12; coverage_in = (overhang_in + 24) x slope_factor; courses = coverage_in <= roll_width_in ? 1 : 1 + ceil((coverage_in - roll_width_in) / (roll_width_in - side_lap_in)); roll_lf = courses x eave_length_ft; rolls = ceil(roll_lf / roll_len_ft).
Source and editionIRC R905.1.2 (the ice-barrier extent: from the eave to 24 in inside the exterior wall line, and at least 36 in along the slope from the eave on a roof of 8:12 or steeper) and ASTM D1970 (self-adhering polymer-modified membrane), by name; the coverage-and-roll relations are first-principles geometry.
Free accessThe ice-barrier extent is stated in the published IRC R905.1.2; the slope-factor and roll relations are public roofing-takeoff arithmetic. The membrane standard is ASTM D1970.
What governsEstimate. AHJ and licensed professional govern.
API field nameseave_length_ft, overhang_in, pitch_rise, roll_width_in, roll_len_ft, side_lap_in, slope_factor, coverage_in, courses, roll_lf, rolls
- Coverage extent coverage = (overhang + 24 in) measured up the slope, and at least 36 in on 8:12 or steeper: the slope factor sqrt(rise^2 + 144)/12 converts the horizontal run to the on-slope distanceIRC R905.1.2
- Courses the first roll covers one roll width; each later course nets (roll width - side lap), so courses = 1 + ceil((coverage - roll width) / (roll width - side lap)) once coverage exceeds one widthfirst principles
- Rolls roll_lf = courses x eave length; rolls = ceil(roll_lf / roll length); the roll dimensions are the product's stated sizefirst principles