Roof Underlayment Calculator

Rolls of synthetic or felt for the deck, plus ice and water barrier for eaves and valleys.

Underlayment rolls

Coverage per roll

Underlayment roll coverage (check the product label)
Product	Gross per roll	Net after laps (typical)	Notes
Synthetic (10-square roll)	1,000 sq ft	≈900–950 sq ft	Light, tear-resistant; roll sizes vary by brand
#15 felt	432 sq ft	≈400 sq ft (4 squares)	Traditional; can wrinkle when wet
#30 felt	216 sq ft	≈200 sq ft (2 squares)	Heavier felt, common under tile
Ice & water (self-adhered)	200 sq ft (3 × 66.7 ft)	≈180–190 sq ft	Eaves, valleys, low slopes, penetrations

Underlayment rolls = ⌈ roof area ÷ net coverage per roll ⌉ Ice barrier area = eave length × band width + valley length × 3 ft Ice barrier rolls = ⌈ ice barrier area ÷ net coverage per roll ⌉

The calculator covers the whole deck with underlayment and adds the ice barrier on top of that. Many crews skip underlayment where ice and water is applied, which saves a little; the conservative count avoids running short.

Code basics (IRC)

Ice barrier: where there is a history of ice forming at the eaves, the IRC requires an ice barrier from the eave edge to at least 24 inches inside the exterior wall line. With a 1 to 2 ft overhang that usually takes two 3 ft courses, hence the 6 ft default.
Low slope: for shingles on roofs from 2/12 up to 4/12, the IRC calls for two layers of underlayment (or an approved alternative). For that, double the roll count or choose "ice & water over the whole roof".
Valleys: a 36-inch-wide strip centered in the valley is common practice and required by many manufacturers.

Local amendments vary. Your building department and the shingle manufacturer's instructions control.

Worked example

2,000 sq ft roof, 120 ft of eaves, 30 ft of valleys. Synthetic: 2,000 ÷ 950 = 2.1 → 3 rolls. Ice barrier: 120 × 6 + 30 × 3 = 810 sq ft ÷ 185 = 4.4 → 5 rolls. With #15 felt instead, 2,000 ÷ 400 = 5 rolls.

Frequently asked questions

How many squares does a roll of synthetic underlayment cover?

Common rolls are 10 squares (1,000 sq ft gross), with roughly 9 to 9.5 squares of net coverage after laps. Smaller 4- and 5-square rolls also exist.

How much does a roll of ice and water shield cover?

A typical roll is 3 ft × 66.7 ft, or 200 sq ft gross, and covers about 180 to 190 sq ft after laps.

Do I need ice and water shield?

In areas where ice dams form, the IRC requires an ice barrier at the eaves extending at least 24 inches inside the exterior wall. It is also good practice in valleys and around chimneys and skylights everywhere.

Last reviewed 2026-10-10. Results are estimates for planning; confirm quantities with your supplier and requirements with your local building code and the manufacturer's instructions.