Roof Underlayment Calculator

About the Roof Underlayment Calculator

Roof underlayment is a critical secondary weather barrier installed directly on the roof deck before shingles, tiles, or metal roofing. Whether you use traditional #15 or #30 felt, or modern synthetic underlayment, you need to account for overlap loss, which reduces the effective coverage per roll.

This calculator takes your total roof area and the underlayment product's specifications (roll coverage and overlap) to determine how many rolls to order. Synthetic underlayment typically covers 1,000 sq ft per roll with 3–4 inches of overlap, while #15 felt covers 400 sq ft with 2–4 inches of horizontal overlap.

Ordering the right number of rolls prevents mid-project trips to the supply house and ensures continuous coverage without gaps that could allow water intrusion during construction.

When This Page Helps

Roll coverage on the label assumes zero overlap. In reality, you lose 10–20% of coverage to horizontal and vertical overlaps. This calculator adjusts for real-world overlap loss to give an accurate roll count.

How to Use the Inputs

1. Enter the total roof area (after slope factor adjustment).
2. Select the underlayment type (#15 felt, #30 felt, or synthetic).
3. Review the effective coverage per roll after overlap deduction.
4. Note the roll count — round up and add 1 spare.
5. Order ice and water shield separately for eaves and valleys (see related calculator).

Example Calculation

Tips & Best Practices

• Synthetic underlayment is lighter, easier to walk on, and resists tearing better than felt.
• #30 felt is required by many codes for slopes less than 4:12 and under tile/slate.
• Install underlayment from eave to ridge, overlapping each course above the one below.
• Horizontal overlaps should be at least 4 inches; vertical (end) overlaps at least 6 inches.
• Cap nails or staples are required — smooth nails pull out easily in wind.
• Do not leave underlayment exposed for more than 30–90 days (check manufacturer specs).

Felt vs. Synthetic Underlayment

Traditional asphalt felt paper has been used for over a century but is being rapidly replaced by synthetic underlayment. Synthetic products made from polypropylene or polyethylene are stronger, lighter, and provide better traction for workers on steep roofs. Price per square is often comparable, making synthetic the better choice for most installations.

Overlap Requirements

Horizontal (side) overlaps should be at least 4 inches. Vertical (end) overlaps should be 6 inches minimum. At hips and valleys, underlayment layers should overlap at least 6 inches past the hip or valley line. These overlaps are where the 10–20% coverage loss comes from.

Ice and Water Shield

In cold climates, the first 3 feet from the eave should be covered with self-adhering ice and water shield membrane, not standard underlayment. This prevents ice dam damage. See the separate ice and water shield calculator for that specific material.

Frequently Asked Questions

• What is the difference between #15 and #30 felt?

  #15 felt weighs about 15 lbs per 100 sq ft and is used under shingles on steep roofs. #30 felt weighs about 30 lbs per 100 sq ft, is thicker and more durable, and is required under tile, slate, and on lower-slope roofs. Many codes now require #30 or synthetic as minimum.

• How much does a roll of underlayment cover?

  #15 felt: approximately 400 sq ft per roll (4 squares). #30 felt: approximately 200 sq ft per roll (2 squares). Synthetic: 500–1,000 sq ft per roll depending on the product and width.

• Is synthetic underlayment worth the extra cost?

  Yes for most applications. Synthetic is lighter (easier to carry up a ladder), more durable, resists tearing and wrinkling, provides better traction on wet surfaces, and has a longer UV exposure tolerance. The per-square cost difference is minor relative to the total roof cost.

• Do I need underlayment under metal roofing?

  Yes. Underlayment under metal roofing provides a secondary moisture barrier and reduces noise. Use synthetic underlayment rated for use under metal (high-temp rated if the metal gets hot).

• How long can underlayment be left exposed?

  #15 and #30 felt should not be left exposed for more than 30 days. Synthetic underlayment can typically be exposed for 60–180 days depending on the product (check the spec sheet). Extended UV exposure degrades all underlayment types.

• Can I install a second layer of underlayment?

  Yes, code often requires double-layer #15 felt on slopes below 4:12 and in ice dam regions. Two layers of #15 felt or one layer of #30 felt provide equivalent protection in most applications.