How do you calculate how much ice and water shield you need?
Short answer. To estimate rolls, add up the square feet you need to cover (the length of each eave, valley and wall area times its covered width), add about 10 to 15 percent for laps and waste, and divide by the square footage on the roll label, commonly around 195 to 225 square feet. Round up to whole rolls.
The formula
Rolls needed = (total covered square feet x 1.10 to 1.15) / square feet per roll, rounded up.
Total covered square feet is the sum of:
- Eaves: eave length x rows x 3 feet (only if you are covering eaves).
- Valleys: valley length x 3 feet.
- Walls: wall joint length x strip width, often about 1.5 feet.
- Penetrations: an allowance per pipe, more for chimneys and skylights.
Worked example
A two-story Georgia home with two valleys, one dormer and four pipes, with no eave ice barrier required:
- Valleys: 2 x 25 feet x 3 feet = 150 square feet
- Walls: 30 feet x 1.5 feet = 45 square feet
- Pipes: 4 x 9 square feet = 36 square feet
- Total: 231 square feet, plus 15 percent = about 266 square feet
- At 200 square feet per roll: 1.3, so buy 2 rolls
Add two rows along 120 feet of eaves (120 x 2 x 3 = 720 square feet) and the total rises to about 1,094 square feet with waste, or 6 rolls.
Adjustments
- Measure valleys and slopes along the roof, not from the floor plan.
- Use the higher waste factor on cut-up roofs.
- Low-slope sections may need full coverage; add their full area.
- Check the roll label, since coverage differs by product.
For measuring the roof itself, see how roofers measure a roof, and for where membrane belongs, ice and water shield in Georgia.