Understanding your result
The headline is the rolls. The stats show the squares of roof, the net coverage of one roll and the fasteners. The breakdown walks from the gross roll size to the net coverage after the side lap and end lap, then to the rolls with waste.
The row to remember is net coverage per roll: it is what a roll actually covers, and it is noticeably less than the nominal squares on the wrapper. The difference is the lap, which is not waste but a requirement, and it is largest with double coverage, where a roll covers half of its nominal area.
The roof area is the same sloped area the shingle calculator uses, not the floor plan. If you only have the house dimensions, the roof area calculator gives the sloped area from the footprint and the pitch.
How we calculate this
The step that catches people out is the double-coverage lap. A 36-inch roll laid with a 19-inch headlap exposes only 17 inches per course, so every point on the roof is covered by two layers and the roll covers about 47% of its nominal area. That is what the code asks for on pitches between 2/12 and 4/12, where wind-driven rain can travel up under the shingles. The same geometry applies to a 48-inch synthetic roll: a 25-inch lap leaves 23 inches exposed.
The end lap is where one roll runs out and the next starts, 6 inches by convention; it costs half a foot per roll and matters little except on small rolls. The coverage table lists the net coverage of the common rolls at the common laps.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Roll sizes | #15 felt 36 in × 144 ft; #30 felt 36 in × 72 ft; synthetic 48 in × 250 ft and 48 in × 100 ft | ASTM D226 Type I and II roll dimensions; common synthetic roll sizes from manufacturers’ data sheets |
| Side lap, single coverage | 4 in (synthetic), 2 in for felt | ASTM D226 and IRC R905.1.1 (2 in minimum for felt); synthetic makers print lap lines at 3 to 4 in |
| Double coverage lap | Half the width plus 1 in (19 in for a 36-in roll) | IRC R905.1.1 for pitches from 2/12 to below 4/12 |
| End lap | 6 in | IRC R905.1.1 and manufacturers’ instructions (4 to 6 in) |
| Fasteners per square | 50 | Synthetic manufacturers’ patterns: 12 in on laps, two rows at 24 in in the field |
| Waste | 10% | Estimating practice; 15% for roofs with many hips and valleys |
Assumptions last reviewed October 7, 2026.
The calculator does not model the ice barrier (see the ice and water shield calculator), the extra layer some manufacturers want under tile or metal, or high-wind fastening patterns, which can double the fasteners. The guide to felt versus synthetic underlayment compares the two in weight, cost, exposure rating and grip, and explains the ASTM ratings to look for on the wrapper.
Two worked examples
A 20-square roof with synthetic underlayment
A roof of 2,000 square feet (20 squares) at a pitch above 4/12, 48 in × 250 ft synthetic rolls, 4-inch side lap, 6-inch end lap, 10% waste, 50 fasteners per square.
- Area to cover: 2,000 × 1.10 = 2,200 sq ft
- Net width: 48 − 4 = 44 in
- Net per roll: 44 in × (250 − 0.5) ft = 915 sq ft
- Rolls: 2,200 ÷ 915 = 2.40, rounded up to 3 rolls
- Fasteners: 20 × 50 = 1,000 cap nails
Three rolls leave more than half a roll over; two rolls would be 370 square feet short. There is no in-between, which is why the 4-square roll exists: two 10-square rolls plus one 4-square roll would be the tighter order.
A low-slope porch roof in double-coverage #30 felt
A 1,200 square foot roof at 3/12, #30 felt (36 in × 72 ft), double coverage, 6-inch end lap, 15% waste for a roof with several hips.
- Area to cover: 1,200 × 1.15 = 1,380 sq ft
- Lap: 36 ÷ 2 + 1 = 19 in, so 17 in exposed
- Net per roll: 17 in × 71.5 ft = 101 sq ft, about one square
- Rolls: 1,380 ÷ 101 = 13.62, rounded up to 14 rolls
- Fasteners: 12 × 50 × 1.5 = 900
Fourteen rolls for twelve squares: the double coverage is what the code wants below 4/12, and it doubles the material. On a slope this low, many roofers use a self-adhering membrane over the whole deck instead.
Where to find your inputs
Roof area. The sloped area, from the roof area calculator or the shingle calculator, in square feet or square metres. Not the floor area.
Roll size. On the wrapper. Synthetic rolls vary: 48 by 250 feet (10 squares) is the common large roll, 42 by 286 feet (also 10 squares) and 48 by 100 feet (4 squares) exist. Enter the real dimensions under “Other roll size” if yours differ.
Pitch. Decides single or double coverage. Measure it with a level and a ruler on a rake board, or use the pitch calculator to convert degrees. Below 4/12 is double coverage; below 2/12 asphalt shingles are not permitted at all.
Fasteners. From the underlayment’s installation sheet, which gives the lap and field spacing for normal and high-wind conditions.
Common mistakes
- Using the nominal squares on the wrapper. A “10 square” roll covers about 9 after laps. Count net coverage, or the last roll will run out a course from the ridge.
- Single coverage on a low slope. Between 2/12 and 4/12 the code requires two layers. It is the difference between a roof that leaks in wind-driven rain and one that does not.
- Running felt horizontally on a steep roof and leaving it exposed. Felt wrinkles when wet, and wrinkles telegraph through the shingles. Cover it the same day.
- Fastening synthetic with roofing nails. Plain nail heads tear through synthetic in wind; use plastic-cap nails or cap staples as the manufacturer directs.
- Running the underlayment under the drip edge at the rakes. At the eaves the drip edge goes under the underlayment; at the rakes it goes over. Reversing them sends water behind the fascia.
- Forgetting the end laps add up. Fourteen small rolls lose 7 feet to end laps. On small rolls, count them.
Questions people ask
- How many rolls of underlayment do I need per square?
- A 48 in × 250 ft synthetic roll is 10 squares gross and about 9.1 squares net after a 4-inch side lap and 6-inch end lap, so a roll covers about 9 squares of roof. A #15 felt roll (36 in × 144 ft) covers about 4 squares net at a 2-inch lap and a #30 roll (36 in × 72 ft) about 2 squares. Add 10% for waste, so a 20-square roof needs 3 synthetic rolls, 6 of #15 felt or 11 of #30.
- What is double coverage underlayment and when is it required?
- On pitches between 2/12 and 4/12 the International Residential Code (R905.1.1) and shingle manufacturers require two layers of underlayment, laid so each course overlaps the one below by half its width plus about an inch. A 36-inch roll laid with a 19-inch lap covers 17 inches per course, so its coverage is roughly halved: a #30 roll covers about one square.
- Is synthetic underlayment better than felt?
- Synthetic is lighter (a 10-square roll weighs about as much as a 2-square roll of #30 felt), does not wrinkle when wet, tears less, and is rated for weeks or months of exposure before shingles go on. Felt is cheaper per square and some installers prefer it for its grip underfoot and because it breathes. Both meet the code if they carry the right ASTM rating.
- How many cap nails for synthetic underlayment?
- Manufacturers typically call for fasteners every 12 inches along the laps and in two rows at 24 inches across the field in normal wind, which comes to about 50 cap nails or plastic-cap staples per square. Double coverage adds about half as many again. The calculator's default roof of 20 squares needs about 1,000 fasteners in single coverage.
- Can underlayment be left exposed before shingling?
- Felt should be covered the same day; it wrinkles when wet and tears in wind. Synthetic products are rated for 60 to 180 days of UV exposure depending on the brand, which is why reroofers dry a roof in with synthetic when the shingles cannot go on at once. Check the wrapper for the rating.
- Does ice and water shield replace underlayment?
- Where it is installed, yes: the self-adhering membrane goes on the deck at the eaves and in the valleys in place of the underlayment, which laps over its upper edge. On a small roof the overlap is within the waste allowance; on a roof with long eaves and many valleys you can subtract the membrane area from the roof area here and count the membrane with the ice and water shield calculator.