Do I need underlayment under shingles?

Yes. Underlayment is required by virtually all building codes and is mandatory for shingle manufacturer warranties. It acts as your secondary moisture barrier — if shingles are damaged by wind or impact, the underlayment prevents water from reaching the sheathing and causing rot or interior damage.

Felt vs. synthetic underlayment — which should I use?

Synthetic underlayment is now the preferred choice for most professional roofers. It is 5 to 10 times stronger than felt, does not absorb water or wrinkle, provides better traction for walking, and can be left exposed for up to 6 months. Felt is less expensive but tears easily, absorbs moisture, and wrinkles in hot weather.

How many rolls of underlayment do I need?

Divide your roof area by the roll coverage (400 sq ft for 15-pound felt, 200 sq ft for 30-pound felt, or 1,000 sq ft for synthetic) and add 15% for overlaps and waste. A 1,500 sq ft roof needs about 4-5 rolls of 15-pound felt, 8-9 rolls of 30-pound felt, or 2 rolls of synthetic underlayment.

What is ice and water shield and do I need it?

Ice and water shield is a self-adhered rubberized asphalt membrane that provides a waterproof barrier in vulnerable roof areas. It is required by code in cold climates along eaves (to prevent ice dam damage), in valleys, and around penetrations. Even in moderate climates, it is recommended in valleys and around skylights and chimneys.

Can I use underlayment as a temporary roof?

Felt underlayment should not be left exposed for more than 30 days, as UV exposure degrades it rapidly. Synthetic underlayment can withstand 4 to 6 months of UV exposure (check the manufacturer's rating). Neither is designed as a permanent roof covering and will eventually fail if shingles are not installed.

How much overlap is needed between underlayment courses?

Standard practice is 2 to 4 inches of horizontal overlap (upper course lapping over lower course) and 6 inches of vertical overlap at end-of-roll joints. In high-wind areas or on low-slope roofs, increase the horizontal overlap to 6 inches. Ice and water shield requires a minimum 6-inch overlap on all seams.

Roofing Underlayment Calculator

How many rolls of roofing underlayment? Free calculator for breathable membrane & felt — accounts for overlap and waste.

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Enter Your Measurements

How to Use This Calculator

1Enter total roof area.
2Select the underlayment type you are using.
3Click Calculate.

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About This Material

Roofing underlayment is the secondary moisture barrier installed over the roof sheathing before shingles or other roofing materials are applied. There are three main types: 15-pound asphalt felt (the traditional choice, sold in rolls covering 400 sq ft, costs $15-25 per roll), 30-pound felt (heavier and more durable, 200 sq ft per roll, $20-35 per roll), and synthetic underlayment (modern non-woven polypropylene, 1,000 sq ft per roll, $50-100 per roll). Synthetic underlayment has largely replaced felt in professional roofing because it is lighter, stronger, does not absorb moisture, and can be left exposed longer during construction. In cold climates (zones 5-8), ice and water shield — a self-adhered rubberized asphalt membrane — is required along the eaves for at least the first 24 inches past the interior wall line. Most shingle manufacturers require underlayment for their warranty to be valid, and building codes universally mandate it as part of the roof assembly.

Next Steps:

Shingle Calculator →Drip Edge Calculator →Roof Sheathing Calculator →

Installation Tips

•Install horizontally, starting at the eave.
•Overlap each course 2–4 inches over the one below.
•Use ice and water shield in the first 2 courses in cold climates.
•Fasten with cap nails or staples per manufacturer instructions.

Common Mistakes to Avoid

Installing in the wrong direction (should go horizontal).
Not using ice and water shield near eaves — required in climate zones with freezing.
Using staples in high-wind areas instead of cap nails.

Roofing Underlayment Calculator

How to Use This Calculator

About This Material

Installation Tips

Common Mistakes to Avoid

Frequently Asked Questions

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