Underlayment ships by the roll, and each roll covers a set number of square feet. Convert your roof squares to square feet, divide by the roll coverage, and round up.
A 22-square roof is 2,200 sq ft; at 1,000 sq ft per roll that is 2.2, which rounds up to 3 rolls.
The example
Three roofs measured in squares, with each roll's coverage in square feet.
| A | B | C | D | |
|---|---|---|---|---|
| 1 | Roof | Squares | Roll (sq ft) | Rolls |
| 2 | House A | 22 | 1,000 | 3 |
| 3 | House B | 30 | 1,000 | 3 |
| 4 | House C | 45 | 800 | 6 |
The formula
The same formula sizes every order:
How it works
From squares to a roll count:
B2*100converts roofing squares to square feet (one square is 100 sq ft).- Dividing by
C2, the square feet a roll covers, gives the raw number of rolls. ROUNDUP(…,0)rounds to whole rolls, because you cannot buy part of one.
Synthetic underlayment commonly covers about 1,000 sq ft per roll; felt covers less. Confirm the figure on the label.
Try it: interactive demo
Enter the roof size in squares and the coverage per roll.
Variations
Add a waste factor
Account for overlaps and cuts by padding the square footage first.
Pitfalls & errors
Skip the *100 and you will divide squares by square feet — a nonsense number. Convert to square feet first.
Roll coverage already assumes the standard overlap; if you double-lap valleys, add a small waste factor.
Practice workbook
Frequently asked questions
How much does a roll cover?
Do I need a waste factor?
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