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Roofing Underlayment Calculator - Rolls Needed by Roof Size

Enter the building footprint and roof pitch so the calculator can estimate sloped roof area. This avoids needing to measure or calculate the roof surface area first.

Last updated: August 2026

Unit system Switch between U.S. customary and metric units. Your choice is remembered on other calculators.

Building footprint and roof pitch

Use the building footprint or roof plan dimensions. Pitch is entered as rise per 12 inches of run, such as 6 for a 6/12 roof.

Visual guide

Which measurement goes where?

Use building footprint and roof pitch. The roll count then uses estimated sloped roof area and the roll coverage you enter.

ft
ft
per 12 in run
Enter 6 for a 6/12 roof, 8 for an 8/12 roof, etc. This is treated as a pitch ratio, not a length to convert.
sq ft/roll
Advanced settings
ft each side
Adds this amount to each side of the roof plan footprint. Set to 0 if your dimensions already include overhangs.
sq ft
Add extra surface area for dormers, porch roofs, additions, or sections not included in the main footprint.
%

Buying and sizing notes

How to use this estimate

What this includes

  • Building footprint
  • roof pitch slope factor
  • roll coverage
  • overhang/extra area
  • waste allowance

What it does not include

  • local ice barrier requirements
  • shingle bundles
  • flashing
  • starter
  • ridge cap
  • labor
  • deck repair

Common mistakes

  • using flat footprint without pitch
  • forgetting laps and waste
  • assuming all rolls cover the same area
  • ignoring ice/water barrier rules

Quick questions

How do I estimate roofing underlayment rolls?

Estimate the sloped roof surface from footprint and pitch, then divide by roll coverage and add waste for laps, cuts, valleys, and roof complexity.

Is ice and water shield included?

No. Ice barrier requirements vary by roof design and local rules. This calculator estimates roll coverage only.

Next steps for this project

Use these related paths when the calculator result turns into a real buying decision, budget, or quote comparison.

Project path

Roofing Calculators

Connect underlayment with shingles, pitch, cost, and quote checklist items.

Open → Printable

Roof Replacement Quote Checklist

Check underlayment, ice barrier, flashing, drip edge, ridge cap, and ventilation.

Open →

Related guide

Use the guide with this calculator when you need more context before buying materials or comparing options.

Guide

Roof shingle calculator guide

Understand roof area, pitch, underlayment, accessories, and roofing quote scope.

Read guide →

Related calculators

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Materials & Projects

Shingle Bundles

Estimate roof shingles and bundles from building footprint, roof pitch, bundle coverage, overhang, extra roof areas, and waste allowance.

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Roof Pitch

Calculate roof pitch, slope angle, and slope factor from rise and run.

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