The number of roofing felt rolls needed depends on the roof’s actual surface area, the coverage printed on each roll, and how much material is lost to overlaps, cuts, and roof features. A quick estimate can be useful, but checking the product label and measuring each roof plane will produce a more dependable order. The method below works for traditional asphalt-saturated felt and many synthetic underlayments sold in the United States.
Quick Estimate: Rolls of Roofing Felt
Use this formula to estimate the quantity:
Rolls needed = roof area ÷ usable coverage per roll, then round up.
To allow for cuts and waste, increase the roof area before dividing. For example, a straightforward roof might need an additional 10 percent; a roof with several hips, valleys, dormers, or other details may need more. Use the roll’s stated coverage, not just its dimensions, because overlapping courses reduce the area a roll can cover.
As a rough illustration, a 2,000-square-foot roof using felt with about 400 square feet of usable coverage per roll needs five rolls before waste. Adding 10 percent makes the estimate 2,200 ÷ 400 = 5.5, so the order rounds up to six rolls. Actual coverage varies by product.
Step One: Measure the Roof Area
Roof area is not always the same as the home’s footprint. A pitched roof covers more surface than the horizontal outline of the building, and garages, porches, and additions may add separate roof planes. The most reliable approach is to measure the length and slope length of each plane, then add the plane areas together.
For a rectangular plane, multiply its length by its slope length. For example, a plane that measures 30 feet along the eave and 20 feet from eave to ridge has an area of 600 square feet. Measure each plane separately and total them. Irregular sections can be divided into rectangles and triangles, whose areas are then added.
Estimating From the Building Footprint
When only the home’s footprint is available, estimate the sloped roof area with a slope factor:
Roof area ≈ horizontal footprint × slope factor.
The slope factor is calculated as √[1 + (rise ÷ 12)²], where “rise” is the number of inches the roof rises for every 12 inches of horizontal run. This estimate assumes a relatively simple roof. It may not account for overhangs, complex shapes, or differences between roof sections.
| Roof pitch | Approximate slope factor |
|---|---|
| 2/12 | 1.014 |
| 4/12 | 1.054 |
| 6/12 | 1.118 |
| 8/12 | 1.202 |
| 10/12 | 1.302 |
| 12/12 | 1.414 |
For instance, a 1,500-square-foot footprint with a 6/12 pitch gives an estimated roof area of 1,500 × 1.118, or about 1,677 square feet, before accounting for overhangs and waste. Measure the actual roof when possible; a footprint-based estimate is a starting point, not a substitute for a roof plan.
Step Two: Check the Roll’s Usable Coverage
Roofing felt is commonly sold in rolls, but the coverage is not universal. Traditional asphalt-saturated felt is often available in 15-pound and 30-pound product categories, while synthetic underlayments come in a range of roll sizes and coverage claims. The number in a product name does not tell a buyer exactly how many square feet the roll will cover.
A roll’s dimensions give its gross area. For example, a 3-foot-wide roll that is 144 feet long has 432 square feet of gross material. The installed coverage will be lower because adjacent courses and roll ends overlap. A product label or installation guide may state a net coverage figure or provide instructions for calculating coverage. Use that product-specific information whenever it is available.
Some common planning estimates are about 400 square feet of usable coverage for certain 15-pound felt rolls, and about 200 square feet for certain 30-pound rolls. These are only examples, not guaranteed specifications. Synthetic products can cover substantially different areas from one another. Confirm the roll dimensions, overlap instructions, and coverage on the exact product being purchased.
Roofing suppliers may describe roof area in “squares.” One roofing square equals 100 square feet. A 2,000-square-foot roof is therefore 20 squares. If a roll provides 400 square feet of usable coverage, it covers about four squares—but the package’s stated coverage should take precedence over a general rule of thumb.
Step Three: Account for Overlap and Waste
Underlayment is installed in courses, with edges and ends overlapped according to the product instructions and applicable code. Those laps are necessary for the roofing assembly, but they mean the roll’s full physical area cannot be counted as exposed coverage. Slopes, valleys, ridges, eaves, penetrations, and irregular roof edges also affect how much material is needed.
For a simple gable roof with few obstacles, a 10 percent allowance is a useful preliminary estimate. A more complex roof may require 15 percent or more, depending on its layout and the installer’s cutting plan. There is no single waste percentage that fits every roof. For a measured roof with a detailed material layout, the contractor can estimate waste more precisely.
Overlap distances should not be guessed from a general online estimate. They can vary with the underlayment product, roof pitch, exposure conditions, and local requirements. Follow the manufacturer’s installation instructions and confirm local building-code requirements. If the instructions specify a particular overlap, use it when estimating coverage.
Worked Examples
Example: A Simple Roof With 15-Pound Felt
Suppose the measured roof area is 2,000 square feet, the selected roll provides 400 square feet of usable coverage, and the roof is simple enough for a 10 percent allowance.
- Increase the measured area by 10 percent: 2,000 × 1.10 = 2,200 square feet.
- Divide by usable coverage per roll: 2,200 ÷ 400 = 5.5 rolls.
- Round up to a whole roll: six rolls.
This result assumes the product really provides 400 square feet of installed coverage. If its label gives a different coverage figure, recalculate using that number.
Example: A Steeper Roof With Synthetic Underlayment
Assume a 1,500-square-foot building footprint, a uniform 8/12 pitch, and a synthetic roll rated for 1,000 square feet of usable coverage. The slope factor for an 8/12 pitch is about 1.202, giving an estimated roof area of roughly 1,803 square feet. With a 10 percent allowance, the estimate becomes about 1,983 square feet.
Dividing 1,983 by 1,000 gives 1.983 rolls. Since partial rolls generally cannot be ordered as a standard roll, the estimate is two rolls. If the roof has significant overhangs or complex features, the measured area and waste allowance may change the result.
Roofing Felt Roll Estimate at a Glance
| Roof area | Illustrative usable coverage per roll | Estimated rolls with 10% allowance |
|---|---|---|
| 1,000 sq. ft. | 400 sq. ft. | 3 |
| 1,500 sq. ft. | 400 sq. ft. | 5 |
| 2,000 sq. ft. | 400 sq. ft. | 6 |
| 2,000 sq. ft. | 200 sq. ft. | 11 |
| 2,000 sq. ft. | 1,000 sq. ft. | 3 |
These figures are examples based on the stated coverage assumptions, not product guarantees. They illustrate why two roofs of the same size can require different numbers of rolls when the underlayment changes.
What Can Change the Roll Count?
- Roof pitch: A steeper roof has more surface area than its horizontal footprint suggests.
- Roof shape: Hips, valleys, dormers, and multiple intersecting planes create more cuts and offcuts.
- Overhangs: Eaves and rakes can add area beyond the building’s footprint.
- Roll coverage: Width, length, and required overlaps differ by product.
- Installation details: Valleys, ridges, edges, and penetrations may require additional material or specific installation methods.
- Local rules: Code requirements can affect the underlayment type, placement, and installation details.
Felt, Synthetic Underlayment, and Product Instructions
“Roofing felt” is often used as a general term for roof underlayment, but traditional asphalt-saturated felt and synthetic underlayments are different products. They can have different roll sizes, handling characteristics, installation requirements, and coverage. The right product depends on the roofing material, roof design, manufacturer guidance, and applicable code.
Underlayment is part of a roofing system; it is not a substitute for properly installed shingles or another finished roof covering. It should be selected and installed according to the instructions for the specific roofing and underlayment products. Guidance from manufacturers such as GAF’s underlayment resources and Owens Corning’s underlayment information can help explain product options, but the selected product’s own installation documentation is the controlling reference for its coverage and application.
Before Buying Roofing Felt
- Measure each roof plane or calculate the sloped area from the footprint and pitch.
- Add the areas of all roof sections, including relevant overhangs.
- Find the selected roll’s usable coverage and installation instructions.
- Choose a reasonable waste allowance based on roof complexity.
- Divide adjusted roof area by usable coverage and round up to a whole roll.
- Check local code and confirm the order with a roofing professional if the roof is complex.
For safety, roof measurements should be taken from the ground, a roof plan, or by a qualified professional when accessing the roof would be hazardous. Accurate measurements and product-specific coverage data are the two most important inputs for estimating how many rolls of roofing felt are needed.