How Much Roofing Felt Do You Need for Your Roof
Determining the correct amount of roofing felt ensures adequate underlayment protection and helps prevent leaks. Roofing felt, also known as tar paper, is installed beneath shingles to provide a water-resistant barrier and to improve wind resistance. This guide explains how to calculate felt coverage, account for roof geometry, and choose appropriate roll sizes. Readers will learn practical steps, typical roll dimensions, and how to factor in waste and overlaps for a precise estimate.
Understanding Roofing Felt Coverage
Roofing felt coverage is typically measured in square feet, with one roofing square equal to 100 square feet. The actual coverage per roll depends on the roll width and the length. Common felt products come in roll widths such as 36 inches or 54 inches, with varying lengths. Overlaps between courses are required to ensure complete protection. A typical overlap is 19 inches for standard felt installations, but always check the manufacturer’s recommendations.
Calculating Your Felt Needs
Begin by determining the total roof area in square feet, then translate that into the number of rolls needed after accounting for roll width and waste. The steps below provide a straightforward method you can apply to most residential roofs.
- Measure the roof’s horizontal length and vertical width for each plane (or use existing blueprints). Multiply to get the roof area in square feet.
- Convert the area to “roofing squares” by dividing by 100 (one square equals 100 square feet).
- Identify the roll width available (commonly 36″ or 54″). Compute coverage per roll: roll width in feet times roll length in feet, minus waste for overlaps and irregular edges.
- Divide the total roof area by the coverage per roll to estimate the number of rolls. Increase the count by a waste factor (typically 5–15%) to accommodate waste and cut losses.
Roll Sizes And Waste Factors
Roll size varies by manufacturer. The most common options are 36-inch by 144-foot and 54-inch by 84-foot rolls, but lengths can differ. The following table shows typical coverage before waste since precise results depend on your chosen product.
| Roll Width | Roll Length | Approximate Coverage | Notes |
|---|---|---|---|
| 36 inches | 144 feet | 432 square feet | Common residential option |
| 54 inches | 84 feet | 378 square feet | Wider roll reduces seams |
Waste factors should be added to account for field cuts, penetrations, valleys, and wasteful trimming. For most standard roofs without complex features, a waste factor of 5–10% is typical. For roofs with many penetrations or irregular shapes, use 10–15% or more.
Step-By-Step Calculation Example
Consider a simple gable roof with two equal slopes, each 30 feet long and 40 feet wide at the eave, totaling 2 x (30 x 40) = 2,400 square feet. Convert to rolls using a common 36″ x 144′ roll (432 sq ft per roll).
- Total roof area = 2,400 sq ft.
- Roll coverage = 432 sq ft per roll.
- Baseline rolls needed = 2,400 / 432 ≈ 5.56 rolls.
- Apply waste factor 10%: 5.56 x 1.10 ≈ 6.12 rolls.
- Round up to 7 rolls to ensure full coverage and account for cuts near edges and penetrations.
In this example, purchasing seven rolls of 36″ x 144′ felt would provide sufficient coverage, with room for adjustments during installation.
Tips For Accurate Measurements
- Draw a quick scale diagram of the roof to capture all planes, hips, ridges, and valleys for precise area calculation.
- Use a tape measure to verify each plane’s length and width, especially on roofs with multiple sections.
- Account for waste in corners, around vents, chimneys, and skylights by adding a dedicated waste allowance per opening.
- Confirm the chosen felt’s recommended overlaps and installation pattern from the manufacturer to avoid shortages.
- Consider wind uplift and local building codes when selecting underlayment type and coverage expectations.
Common Mistakes To Avoid
- Underestimating roof area due to not including dormers or overhangs.
- Ignoring valleys and penetrations that require additional material and careful cutting.
- Using the wrong roll width for particularly narrow roof sections, leading to increased seams.
- Failing to apply the manufacturer’s stated overlaps, which can compromise water resistance.
Practical Quick Reference
- Measure total roof area in square feet; divide by 100 to estimate roofing squares.
- Choose a roll width (36″ or 54″), multiply by roll length to determine coverage per roll.
- Apply a 5–15% waste factor depending on roof complexity and penetrations.
- Round up to whole rolls to ensure complete coverage and accommodate future repairs.