How do you calculate drip edge for a roof?
Short answer. Add the length of every eave and rake in feet, divide by about 9.8 feet of usable coverage per 10-foot piece to allow for 2-inch overlaps, and round up with a couple of spares. If you have a roof measurement report, use its eave and rake totals rather than the total perimeter.
What counts and what does not
- Eaves: the level bottom edges where gutters usually hang. Always counted.
- Rakes: the sloped edges at gable ends. Always counted.
- Hips, ridges and valleys: not counted. Those are interior lines covered by caps or valley flashing.
- Roof-to-wall edges: not drip edge. Those get step flashing or headwall flashing instead.
Measuring rakes correctly
A rake is longer than the horizontal distance it covers because it runs up the slope. Measure it on the roof edge itself, or multiply the horizontal run by the pitch factor: about 1.118 for a 6 in 12 roof and 1.202 for 8 in 12. Measurement reports list rake length directly.
Worked example
A two-story gable house has two 50-foot eaves and four rake edges of 18 feet each. That is 100 plus 72, or 172 feet. Divided by 9.8, you get 17.6, so 18 pieces, plus two spares for the corners and any bad cuts, for 20 total.
Other things to plan for
- If you are using gutter apron on the eaves and standard drip edge on the rakes, count them separately.
- Fasteners go no more than 12 inches apart, so expect roughly ten nails per piece.
- Pick a color that matches the fascia or gutters, since it shows from the street.
Learn how footage ends up on estimates in how roofers measure a roof, and why it matters in drip edge explained.