ToolwickHow to Use This Drip Edge & Roof Trim Calculator
Choose your roof style, then enter the building length (the wall parallel to the ridge), the width between eave walls, the pitch as rise per 12 inches of run, and your eave and rake overhangs. Add any dormer or porch edges in the extra fields, pick a piece length and a waste allowance, and press Calculate. You get linear feet for eaves and rakes, the number of pieces to buy, nails, an optional cost, and a table comparing different waste levels.
Why This Matters
Drip edge is the thin, L-shaped metal flashing that sits along the eaves and rakes of a shingle roof. It pushes water away from the fascia and rake board, keeps shingles from curling over the edge, and blocks wind-driven rain and insects from getting under the sheathing. It is cheap compared with the rest of a re-roof, but running short on installation day costs a trip to the supply house and sometimes a mismatched profile or color.
Typical users are homeowners ordering materials for a tear-off, roofers writing quotes, and builders ordering for new construction. A 40 × 28 ft gable house with 12 inch overhangs and a 6/12 pitch has about 151 linear feet of edge. That is 16 pieces before waste and 18 after, or roughly $171 at $9.50 a piece. Add a dormer and a garage and the total can climb past 25 pieces. Rakes are longer than most people expect because they follow the slope, not the horizontal run. Measuring the ground footprint alone undercounts them by 10 to 40 percent on steep roofs.
How It's Calculated
The calculator converts your pitch to a slope factor, finds the sloped length of one rake edge, then adds up eave and rake runs for the chosen roof style.
Slope length = (run + eave overhang) × slope factor
Gable: eaves = 2 × (L + 2 × rake overhang); rakes = 4 × slope length
Hip: eaves = 2 × (L + 2 × o) + 2 × (W + 2 × o); rakes = 0
Shed: eaves = L + 2 × rake overhang; rakes = 2 × slope length
Pieces = ROUNDUP(length × (1 + waste) ÷ (piece length − 2 in lap))
Nails = ROUNDUP(length × 12 ÷ spacing) + 2 per piece
Run is half the width for gable and hip roofs and the full width for a shed roof. Eaves and rakes are rounded up separately, because they are often different profiles and offcuts cannot always be reused. Every result is rounded up, never down. The defaults follow the IRC R905.2.8.5 rule of thumb: drip edge at eaves and rakes of shingle roofs, at least 2 inch laps, and fasteners no more than 12 inches on center. We default to 10 inches, which is tighter than the code maximum. Codes vary by jurisdiction and edition, and this tool is a material estimate. It is not engineering, permit or inspection approval.
Worked Example
Gable roof, 40 ft long, 28 ft wide, 6/12 pitch, 12 inch overhangs on eaves and rakes, 10 ft pieces, 10% waste, nails at 10 inches.
- Slope factor = √(1 + 0.5²) = 1.1180
- Run = 28 ÷ 2 + 1 = 15 ft, so slope length = 15 × 1.1180 = 16.77 ft
- Eaves = 2 × (40 + 2) = 84.0 ft; rakes = 4 × 16.77 = 67.1 ft; total = 151.1 ft
- Usable piece length = 10 − 2/12 = 9.833 ft
- Eaves with waste: 84.0 × 1.10 = 92.4 ft ÷ 9.833 = 9.40, so 10 pieces
- Rakes with waste: 67.08 × 1.10 = 73.8 ft ÷ 9.833 = 7.50, so 8 pieces
- Total = 18 pieces. Nails = 182 + 2 × 18 = 218, so buy about 250.
- At $9.50 per piece the cost is 18 × $9.50 = $171.00.
Tips & Common Mistakes
- Measure rakes along the slope. Using the horizontal run underestimates rake length. A 12/12 roof has rakes 41% longer than its run.
- Count every edge. Dormers, porch roofs, bump-outs and garage wings each add eave and rake length. Use the extra fields for them.
- Match the underlayment order. A common practice is drip edge under the underlayment at eaves and over it at rakes. Check your shingle maker's instructions and local code.
- Match the metals. Use galvanized or aluminum nails with aluminum or galvanized drip edge to avoid corrosion at the fasteners.
- Don't cut waste too thin. Hips, valleys and awkward corners need mitered cuts. Ten percent is a sound default and 15% suits cut-up roofs.
Frequently Asked Questions
How much drip edge do I need for my roof?
Add the length of all eaves and all rake edges, then divide by the usable piece length and round up. A simple 40 × 28 ft gable roof with 12 inch overhangs needs about 151 linear feet, or 18 ten-foot pieces with 10% waste.
Is drip edge required by building code?
The IRC has required drip edge at the eaves and rakes of shingle roofs since the 2012 edition (R905.2.8.5), but local adoption differs. Check with your building department, and note that many shingle warranties also call for it.
Does drip edge go over or under the underlayment?
Common practice is to put drip edge directly on the deck at eaves, with underlayment over it, and over the underlayment at rakes. Water then flows over the metal at the eave and cannot get beneath it at the rake. Follow your manufacturer's installation instructions.
Do I need drip edge on a hip roof?
Yes, along all four eaves. A hip roof has no rake edges, so the calculator returns zero rake length for it. Hips and ridges are covered by cap shingles or ridge vent, not drip edge.
How much should drip edge pieces overlap?
Overlap adjoining pieces by at least 2 inches. The calculator deducts a 2 inch lap from every piece, so a 10 ft piece covers 9.83 ft of roof edge.
Should I round up or down when ordering?
Always round up. A short piece is hard to fix on installation day, while a spare piece is cheap and handy for repairs.