Roof Underlayment Calculator
Estimate underlayment rolls, ice and water shield and drip edge for a re-roof, including the double layer required on low slopes.
How it's calculated
The field of the roof gets synthetic or felt underlayment, figured by net coverage per roll after laps. Where ice and water shield goes down at the eaves, it replaces the field layer. Below 4/12 pitch, field coverage is doubled for the two-layer installation the IRC requires under shingles.
Ice and water shield is figured in 3 ft wide courses along the eaves, 3 ft wide down each valley, and about 9 sq ft around each penetration, at about 180 sq ft net per roll.
Worked example
A 1,860 sq ft roof with 104 ft of eaves and one course of ice shield has 1,548 sq ft of field. That is 2 rolls of 10-square synthetic and 2 rolls of ice and water shield, plus 19 pieces of drip edge.
Field notes
- Drip edge goes under the underlayment at the eaves and over it at the rakes.
- Lap underlayment shingle-fashion so water always runs over a seam, never into it.
- Ice barrier rules vary: many cold-climate codes require it to extend 24" inside the warm wall.
- Synthetic underlayment is slippery when wet or dusty. Use toe boards on steep roofs.
Common questions
- How much does a roll of synthetic underlayment cover?
- Large rolls are 10 squares gross, about 900 sq ft after laps. Smaller rolls run 4 squares.
- How much does a roll of #15 felt cover?
- About 4 squares (400 sq ft) after laps. #30 felt covers about 2 squares.
- Where is ice and water shield required?
- In areas with ice dams, at the eaves to 24" inside the exterior wall. It is also good practice in valleys and around chimneys and pipes anywhere.