Roof Underlayment Calculator

Calculate how many rolls of underlayment you need for your roofing project with overlap factor applied.

Part of the Roofing & Structure calculators — see all tools in this category.

Roof Area
sq ft

Use our Roof Area Calculator if you need to calculate this first

Underlayment Type
in

Standard 4" overlap on 36" roll

Need to calculate your roof area first? Use our Roof Area Calculator.

For eaves and valleys, consider ice & water shield membrane in addition to standard underlayment. Ice & water shield is not calculated here — consult local building codes for requirements.

Results

Rolls Needed
3

Synthetic Underlayment

Effective Coverage
916.7 sq ft

Per roll after overlap

Total Coverage
2,750 sq ft

Slight overage from rounding up

Frequently Asked Questions

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How It Works

  1. 1. Enter Roof Area

    Enter the total surface area of your roof in square feet. If you don't know this, use our Roof Area Calculator to compute it from dimensions and pitch.

  2. 2. Select Underlayment Type

    Synthetic underlayment is the most popular and covers 10 squares (1,000 sq ft) per roll. #15 felt covers 4 squares (400 sq ft) per roll. #30 felt covers 2 squares (200 sq ft) per roll.

  3. 3. Set Overlap

    Standard 4" overlap on a 36" roll reduces effective coverage by approximately 11%. The calculator accounts for this reduction automatically. Increase overlap for steeper pitches or where local codes require it.

  4. 4. Get Roll Count

    The calculator divides your roof area by the effective coverage per roll (after overlap reduction) and rounds up to whole rolls. Formula: Effective Coverage = Nominal Coverage × (1 − Overlap ÷ Roll Width).

Understanding Overlap Reduction

For a standard #15 felt roll with 4" overlap on a 36" wide roll: 400 × (1 − 4/36) = 355.6 sq ft effective coverage per roll. The overlap ensures waterproofing continuity but reduces the usable coverage of each roll.

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Shingles Calculator

Convert roof area into shingle bundles, squares, and ridge cap quantities for accurate roofing material orders.

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Calculate gutter sections, downspouts, elbows, and end caps from roof perimeter, corners, and building height.

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Calculate ridge cap and hip cap shingle pieces and bundles from ridge and hip lengths.

Related Guides

Roofing & Exterior Guide

Shingles, gutters, siding, and exterior envelope material planning.

Complete Estimation Guide

Site-wide guide to measurement workflows and estimation best practices.

Roof Underlayment Calculator | How Many Rolls of Felt or Synthetic | ConstructCalculators