Key takeaways
- Starter bundles = eaves and rakes ÷ 120 linear feet, and cap bundles = ridge and hips ÷ 25 linear feet, using GAF product figures.
- Where the 2021 IRC calls for an ice barrier, it runs from the eave edge to a point at least 24 inches inside the exterior wall line.
- On a 6/12 roof with a 12-inch overhang, that's about 40 inches along the slope, so a 36-inch roll takes two courses.
Shingles are only part of a roof order. Enter your roof area, edge lengths, overhang and pitch, and whether your area needs an ice barrier. The calculator turns them into bundles, rolls and pieces, and every coverage figure can be changed to match your product.
Get the area and edge lengths from the roof area calculator or the hip roof calculator. Shingle bundles come from the shingle calculator.
How the calculator works
Each line is rounded up, because these items are sold whole:
- Starter bundles = (eaves + rakes) ÷ starter coverage
- Cap bundles = (ridge + hips) ÷ cap coverage
- Underlayment rolls = roof area × 1.10 ÷ roll coverage
- Drip edge pieces = (eaves + rakes) ÷ piece length
- Ice barrier courses = (overhang + 24 in) × slope multiplier ÷ roll width
- Ice barrier linear feet = eaves × courses
The slope multiplier is √(1 + (pitch ÷ 12)²), as in the roof pitch calculator. On 8/12 or steeper, the ice barrier line uses the larger of the converted figure and 36 inches along the slope.
The default coverages
| Item | Default | Where it comes from |
|---|---|---|
| Starter strip | 120 lin ft per bundle | GAF: Pro-Start covers about 120.33 lin ft |
| Hip and ridge cap | 25 lin ft per bundle | A retailer listing for GAF Seal-A-Ridge |
| Underlayment | 1,000 sq ft per roll | Ours; use your roll's label |
| Lap allowance | 10% | Our rule of thumb |
| Drip edge | 10 ft per piece | Ours; use your supplier's length |
| Ice barrier | 36 in wide | Ours; use your roll's width |
The starter and cap figures are GAF manufacturer figures, not code figures. Other products differ, so use your wrapper's figure.
What each layer does
Underlayment is the sheet over the deck, under the shingles. The 2021 IRC (R905.2.2) allows asphalt shingles only on 2/12 or steeper, with double underlayment from 2/12 up to 4/12. Our roll count is for one layer, so in that band read minimum roof pitch for shingles.
Ice barrier protects the eaves where ice dams are a known problem. Section R905.1.2 calls for "not fewer than two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet", extending "from the lowest edges of all roof surfaces to a point not less than 24 inches (610 mm) inside the exterior wall line". Detached accessory buildings without conditioned floor area are an exception.
Drip edge is the bent metal along the roof's edges. Section R905.2.8.5 requires it "at eaves and rake edges of shingle roofs", with pieces overlapped "not less than 2 inches" and fastened "at not more than 12 inches o.c." At the eaves the underlayment goes over the drip edge, and at the rakes it goes under.
Starter strip goes under the first row of shingles. GAF says Pro-Start helps resist blow-offs "at the eaves and rakes", so the calculator counts both edges.
Hip and ridge cap covers the peaks where two planes meet. Its coverage depends on the product's exposure.
The ice barrier, converted to roof inches
The code measures 24 inches inside the wall line, but you lay the roll along the slope from the eave edge. So it must cover the overhang plus 24 inches, stretched by the pitch. This conversion is our own geometry:
Slope distance = (overhang + 24 in) × slope multiplier
On slopes of 8:12 or steeper, the 2021 edition of R905.1.2 adds "not less than 36 inches (914 mm) measured along the roof slope from the eave edge". That sentence is from the 2021 edition, so check which edition your area uses.
| Pitch | Overhang | Slope distance | 36 in courses |
|---|---|---|---|
| 6/12 | 0 in | 26.8 in | 1 |
| 6/12 | 6 in | 33.5 in | 1 |
| 6/12 | 12 in | 40.2 in | 2 |
| 6/12 | 24 in | 53.7 in | 2 |
| 8/12 | 0 in | 28.8 in, so 36 in governs | 1 |
| 12/12 | 12 in | 50.9 in | 2 |
The 8/12 row with no overhang uses a 36-inch roll exactly, with nothing to spare. Courses also lap by the amount in the roll's instructions.
Whether you need an ice barrier at all is local. R905.1.2 applies where your jurisdiction's Table R301.2 shows a history of ice damming, so ask your building department.
Worked example
A 40 × 30 foot gable house with a 6/12 roof, 12-inch overhangs all round, in an area that requires an ice barrier:
- Roof area: 42 × 32 = 1,344 sq ft level, × 1.118 = 1,503 sq ft
- Eaves: 2 × 42 = 84 ft
- Rakes: four of 16 × 1.118 = 17.9 ft each, so 71.6 ft
- Ridge: 42 ft, and no hips
| Item | Arithmetic | Order |
|---|---|---|
| Starter | (84 + 71.6) ÷ 120 = 1.30 | 2 bundles |
| Cap | 42 ÷ 25 = 1.68 | 2 bundles |
| Underlayment | 1,503 × 1.10 ÷ 1,000 = 1.65 | 2 rolls |
| Drip edge | (84 + 71.6) ÷ 10 = 15.6 | 16 pieces |
| Ice barrier | (12 + 24) × 1.118 = 40.2 in ÷ 36 = 1.12 | 2 courses |
| Ice barrier length | 84 × 2 | 168 lin ft |
If your shingle's instructions call for starter at the eaves only, that line drops to 84 ÷ 120 = 0.7, or 1 bundle. The drip edge's 2-inch overlaps leave about 9.83 feet of cover per piece, and 155.6 ÷ 9.83 = 15.8 still rounds to 16. For ice barrier rolls, divide 168 feet by the roll length on the label.
What to ask your roofer
- Does our area require an ice barrier, and how far up the roof will it go?
- Which underlayment, and is it one layer or two at our pitch?
- Is starter going on the rakes as well as the eaves?
- Which cap shingle, and what's its coverage per bundle?
- Is drip edge included at both eaves and rakes, and how is it lapped?
- Is the quote's quantity measured area, or area plus waste?
A roofer's measured takeoff should replace this estimate, especially with valleys and dormers.
Common mistakes
- Using floor area. Underlayment follows roof area, which is larger than the footprint.
- Measuring rakes level. Rakes run along the slope, so apply the slope multiplier.
- Starting the 24 inches at the eave edge. The code counts from inside the wall line, so add the overhang.
- Ignoring laps. Overlapping courses and pieces cover less than their full size.
- Borrowing another product's coverage. Use your wrapper's figure.
Frequently asked questions
How many rolls of underlayment do I need?
Divide roof area by the roll's coverage, add an allowance for laps, and round up. A 1,503-square-foot roof with 1,000-square-foot rolls and 10% for laps needs 2 rolls, for one layer.
How much starter strip do I need?
Add the length of your eaves and rakes, then divide by the bundle's coverage. With GAF Pro-Start at about 120 linear feet per bundle, 156 feet of edge takes 2 bundles.
How many bundles of ridge cap do I need?
Add your ridge and hip lengths, then divide by the cap's coverage per bundle. At 25 linear feet per bundle, a 42-foot ridge takes 2 bundles.
How far up the roof does an ice barrier go?
Under the 2021 IRC, from the eave edge to at least 24 inches inside the exterior wall line, and on 8/12 or steeper slopes at least 36 inches along the slope. Convert it with (overhang + 24) × the slope multiplier.
Is drip edge required on a shingle roof?
The 2021 IRC requires it at eaves and rake edges of shingle roofs. Check which edition your area uses.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- 2021 International Residential Code, Chapter 9, Roof Assemblies (R905.1.2, R905.2.2, R905.2.8.5)
- GAF: Pro-Start starter strip shingles
- Lowe's: GAF Seal-A-Ridge listing, 25 lin ft per bundle (retailer listing)