Roof Underlayment Calculator Pro (USA)
Estimate synthetic roofing felt, rolls, and squares based on current US standards.
Visual Representation of Coverage
Expert Guide to Roof Underlayment
Why Accurate Calculation Matters
Estimating underlayment correctly prevents mid-project delays and budget overruns. In the USA, roofing materials are sold in “Squares” (100 square feet), but underlayment rolls vary significantly in coverage. This calculator accounts for waste factor, which is critical for roofs with many valleys, hips, or steep slopes (pitch > 6/12).
Synthetic vs. Felt: What’s Best for You?
Synthetic Underlayment (Polypropylene): Weighs less, resists tearing, and offers superior UV protection. It is the industry standard for premium residential roofing in the US. Typical coverage is ~10-12 squares per roll depending on the weight (#15 vs #30).
Asphalt Felt (Tar Paper): The traditional choice. #15 felt is lighter, #30 is heavier and more durable. Felt is prone to wrinkling if wet and tears easier than synthetics. It is generally more cost-effective for low-slope projects or outbuildings.
Understanding Waste Factor
We recommend a 10% waste factor for simple roofs. However, if your roof has:
- Multiple dormers or skylights
- Complex valleys (dead valleys)
- A steep pitch requiring extra layout time
Increase the waste factor to 15-20% to ensure you have enough material to complete the job without small patches.
