Roofing Felt Calculator
This roofing felt calculator divides roof area by the 400 sq ft one roll covers: a 600 sq ft roof needs 2 rolls, about $50 at $25 a roll.
Roofing felt calculator: rolls from area
A roofing felt calculator divides the roof’s surface area by what one roll covers. The calculator uses 400 square feet for a roll of 15 lb felt and rounds up to whole rolls. A 30 × 20 ft roof section is 600 sq ft, which is 1.5 rolls, so you buy 2.
Roof area must be the sloped surface. If you only have a footprint, convert it first.
The formula in plain words
Rolls = roof area ÷ 400, rounded up. Cost = rolls × the roll price. With 10% waste ticked, the area is multiplied by 1.1 before the division, then rounded up.
Worked example, a shed roof
A roof plane measures 30 ft by 20 ft. Felt is $25 a roll.
- Area: 30 × 20 = 600 sq ft
- Rolls: 600 ÷ 400 = 1.50, rounded up to 2
- Cost: 2 × $25 = $50
- With 10% extra: 1.65, still 2 rolls
You end up with half a roll left, which is useful for flashing and patches.
Worked example, a larger home
A house has 1,500 sq ft of roof surface, measured across all planes.
- Rolls: 1,500 ÷ 400 = 3.75, rounded up to 4, or $100
- With 10% extra: 4.13, rounded up to 5, or $125
The waste box adds a full roll here. That is one more roll for a roof with hips and valleys, which is what you want.
Rolls by roof area
| Roof surface | Rolls | With 10% extra |
|---|---|---|
| 150 sq ft | 1 | 1 |
| 600 sq ft | 2 | 2 |
| 1,000 sq ft | 3 | 3 |
| 1,500 sq ft | 4 | 5 |
| 2,400 sq ft | 6 | 7 |
Roofers talk in squares. One square is 100 sq ft, so a roll of 400 sq ft is 4 squares.
How to measure the roof
- Measure each plane’s length and width on the slope, or measure the footprint and multiply by a pitch factor.
- Add all planes together.
- Include dormers, porches and attached sheds if they get felt.
- Write the total in squares as well as square feet, because some suppliers quote that way. 600 sq ft is 6 squares.
- Do not subtract skylights and chimneys unless they are large, since the cuts around them waste felt anyway.
- Check the roll’s printed coverage if you buy 30 lb felt or a different brand.
Common mistakes
- Measuring the footprint instead of the roof surface. A steeper roof has much more area than the floor below.
- Forgetting that 30 lb felt covers less per roll.
- Buying too little. A partial roll left on the roof at the end is better than a trip to the store.
- Leaving felt exposed too long. Sun and wind damage it, so cover it with shingles soon after.
- Ignoring the shingle manufacturer’s underlayment instructions. They can affect the warranty, and local codes may require ice barrier in cold areas.
Cost per square foot
At $25 a roll, felt costs $25 ÷ 400 = $0.0625 per square foot of roof, a little over 6 cents. The 1,500 sq ft house at 4 rolls costs $100, which is about $0.067 per square foot because of the rounding. Compared with the cost of shingles and labor, felt is a small line, which is why adding the waste roll is usually worth it.
Start laying at the eave and work up the roof, so each higher course overlaps the one below and sheds water. Staple or nail it just enough to hold it until shingles go on. Do not leave it exposed for weeks in sun or wind, since it wrinkles and tears.
Related pages
A modern alternative is covered by the roofing underlayment calculator. To find the area first, use the roof area calculator, and for the shingles on top, use the roofing shingle calculator.
Frequently asked questions
How much does a roll of 15 lb felt cover?
This calculator uses 400 square feet per roll, which is 4 roofing squares, with the usual overlaps already counted. Check the label on your roll, since brands and weights differ.
How does 30 lb felt compare?
A roll of 30 lb felt is heavier and usually covers less, often about half as much, so you need more rolls for the same roof. Check the roll's label and enter its coverage, or double the roll count as a rough check.
Is felt still used under shingles?
Yes, though synthetic underlayment has taken a lot of the market. Felt is cheaper, while synthetic is lighter and more tear resistant. Many manufacturers and codes accept either, so check your shingle maker's instructions and local code.
How much overlap does felt need?
It depends on roof pitch and the product. Typically each course overlaps the one below by a few inches, and the ends overlap more. The roll's printed lines show the overlap. The 400 sq ft coverage already assumes it.
How much waste should I add?
Tick the 10% waste box for roofs with valleys, hips, or lots of cutting. A plain gable roof needs less, but the box is cheap insurance because felt tears and gets blown around on installation day.
Do I measure the footprint or the roof surface?
The surface. A sloped roof is larger than the floor plan under it. The roof area calculator converts a footprint and pitch into surface area.
Estimates only. Verify quantities with your supplier before purchasing.