Roofing felt, or tar paper, is a foundational underlayment used beneath shingles to provide extra weather resistance and a moisture barrier. Understanding how much coverage a roll provides helps builders and DIY homeowners estimate material needs, reduce waste, and budget accurately. This guide explains typical roll sizes, coverage in square feet and squares, and how to calculate coverage based on weight, overlap, and installation practices.
What Is Roofing Felt And Common Weights
Roofing felt comes in various weights and materials. Traditional organic felt rolls are rated by weight per 100 square feet, such as 15 lb or 30 lb. Synthetic underlayments, including polypropylene or polyethylene-based felt, offer higher tear resistance and lighter weight with similar coverage. The choice affects both the per-roll coverage and handling during installation.
Typical Roll Sizes And Coverage
Most roofing felt rolls are manufactured to standard dimensions. The most common specifications are:
- Width: 36 inches (3 feet)
- Length: 144 feet to 200 feet per roll
- Coverage: Roughly 400 to 430 square feet per roll, depending on width and length
Because coverage is expressed per roll, a roll at 36 inches by 144 feet yields about 432 square feet of material. However, practical coverage is slightly less due to overlaps, seams, and waste during installation. In terms of “squares” (one square equals 100 square feet), that roll provides about 4.32 squares of coverage before waste or overlaps.
How To Calculate Coverage For A Job
To estimate the number of rolls needed, use the formula:
- Roll Coverage (sq ft) x Number of Rolls ≈ Total Coverage
- Required Coverage (sq ft) ÷ Roll Coverage (sq ft) ≈ Rolls Needed
Steps to refine the estimate:
- Measure the roof area in square feet (length x width).
- Subtract areas not covered by roofing felt (chimneys, vents) if planning partial coverage calculations.
- Factor in overlaps (commonly 2-4 inches per seam) which reduces usable coverage per roll.
- Include a waste factor (typically 5-10% for cuts and mistakes).
Example: For a 1,200 sq ft roof, using a 432 sq ft roll, plan for 3 rolls (1,296 sq ft) plus an additional roll for waste and alignment, totaling 4 rolls.
Factors That Affect Actual Coverage
Several variables influence how much coverage you achieve per roll in practice:
- Overlap and Seams: Shingle installation requires overlapped seams, reducing net covered area by about 2-4 inches per seam, depending on orientation.
- Roof Pitch: Steeper pitches may require more felt to ensure full coverage without gaps, due to waste and alignment challenges.
- Waste and Cutting: Angles around chimneys, vents, and ridges generate waste; plan for extra material.
- Roll Quality and Type: Synthetic underlayment often provides similar coverage with less breakage, but rolls may have different lengths that affect total square footage per roll.
- Weather Considerations: Installing in cold or windy conditions can influence the amount of overlap and the number of cuts required.
Direct Coverage Versus Net Coverage: Practical Tips
When estimating, consider net coverage, which is the usable area after overlaps and waste. To be conservative, reduce the nominal roll coverage by 5-10% to account for seams and waste. For projects with unusual roof shapes, add an extra roll or two to avoid running short.
Installation best practices to maximize coverage:
- Lay felt with weather exposure direction aligned to shingles for consistent overlap.
- Overlap ahead of the drip edge and ridge lines as recommended by the manufacturer.
- Use nails or staples per manufacturer specifications and avoid tearing the felt during installation.
- Inspect for wrinkles and smooth out before applying shingles to prevent future leaks.
Choosing The Right Felt For Your Roof
Consider these criteria to select the appropriate felt type and roll size:
- : In areas with heavy rain or snow, high-quality synthetic or reinforced felt improves durability.
- Roof Type: Flat or low-slope roofs may benefit from heavier underlayment and more precise seam installation.
- Shingle Type: Certain shingles require specific underlayment compatibility; check manufacturer guidelines.
- Cost And Availability: Roll length and weight affect price; prefer suppliers that provide consistent dimensions for easier planning.
Waste, Ordering, And Budgeting
Accurate budgeting reduces surplus waste and ensures timely project completion. Consider ordering a little extra—often one additional roll per every 2-3 rolls used—to cover waste and unforeseen cuts. Keep track of manufacturer guidance for coverage per roll, as different brands may vary slightly in stated coverage.
Summary guidance for homeowners and contractors:
- Expect about 400-430 sq ft of coverage per 36″ x 144–200′ roll for standard felt; adjust for synthetic varieties as needed.
- Plan for 5-10% waste beyond exact coverage to accommodate overlaps and cuts.
- Convert between square feet and squares to align with material estimates (1 square = 100 sq ft).
- Choose underlayment type based on climate, roof design, and shingle compatibility to optimize performance and cost.