Free estimate — verify against local code before building
Ice & Water Shield Calculator
Calculate membrane area and whole rolls for entered eave, valley, and penetration zones with visible pitch and lap assumptions.
What this calculator includes
Estimate self-adhering roof membrane as specific eave, valley, and penetration zones instead of applying a blanket percentage to the entire roof. Horizontal eave-zone width is converted to sloped width with the entered pitch, valley coverage is measured as its own strip, and roll coverage stays editable because required laps reduce usable area. The calculator does not determine where membrane is required or whether a selected product is compatible with the roof assembly.
Next step in your project
Continue from roof edge to runoff capacity
Use contributing roof area and user-entered rainfall intensity to evaluate planning flow and gutter/downspout capacity.
Open Gutter Size Calculator →How to use this ice & water shield calculator
- 01
Measure each protected zone
Add the eave lengths receiving membrane, the total valley length, and the number of penetrations that need a measured allowance.
- 02
Convert the eave zone to slope
Enter horizontal eave-zone width and roof pitch. The calculator converts that width to its approximate distance along the roof surface.
- 03
Use effective roll coverage
Enter usable installed square footage after the selected product's side, end, and transition laps rather than nominal roll area.
- 04
Confirm the final layout
Apply an editable cut allowance, round to whole rolls, and verify required locations and detailing against approved documents and current product instructions.
Worked example
Example: 80 ft of eaves and one valley
At 6-in-12 pitch, a 3 ft horizontal eave zone becomes about 3.35 ft along the slope, covering about 268 square feet over 80 ft of eaves. Adding a 20 by 3 ft valley and 10% waste produces about 361 square feet, or two rolls at 195 square feet of effective coverage each.
Practical buying and overage guidance
Use one compatible membrane system and verify substrate preparation, primer, temperature range, adhesion, lap dimensions, exposure limits, sequencing, flashing integration, and roll storage. Order by effective coverage and keep enough material for transitions and repairs without assuming every offcut is reusable.
Continue the project
Upgrade Roof Drainage
Plan roof area, gutter runs, downspouts, fittings, extensions, drainage stone, grading, excavation, supplies, and contractor pricing.
Open the project workflow →Calculation sources and review
Primary references and formula assumptions are linked so you can verify them against the selected product, supplier, and adopted local requirements.
Internal formula review completed July 13, 2026. What this review covers
- Asphalt Roofing Manufacturers Association - technical bulletins (opens in a new tab)
Industry information about asphalt roofing assemblies and underlayment considerations.
- GAF - roofing document library (opens in a new tab)
Manufacturer instructions illustrate product-specific coverage, substrates, laps, and roof-edge details.
- OSHA - residential fall protection (opens in a new tab)
Roof work requires appropriate fall-hazard planning and protection.
Frequently asked questions
How wide should ice and water shield be at an eave?
That requirement varies by adopted rules, climate, roof geometry, wall line, product, and assembly. Enter the width from approved details or qualified local guidance; this calculator does not prescribe one.
Why is horizontal eave width different from membrane width on the roof?
The roof surface is longer than its horizontal projection when pitched. Multiplying horizontal width by the pitch factor estimates the sloped membrane distance.
Should I cover the entire roof deck?
Some assemblies use full-deck self-adhering membrane and others use targeted zones or a different underlayment. Compatibility, drying, ventilation, climate, substrate, and manufacturer requirements must be reviewed for the actual assembly.
Does roll coverage include overlaps?
Use effective installed coverage after required laps. Nominal roll width times length can overstate coverage when side laps, end laps, and transition detailing consume material.
Are valleys and penetrations included automatically?
Only when you enter their combined length, width, count, and allowance. Complex intersections should be measured and detailed individually.