Drip Edge and Flashing Calculator
Last reviewed How we calculate
Most roof leaks start at an edge or a junction rather than in the middle of the roof. Drip edge protects the eaves and rakes, valley flashing lines the valleys, and step flashing seals the line where a roof slope meets a wall. This counts the pieces of each.
Drip edge, valley and step flashing
Drip edge, valley flashing and step flashing pieces from your eave, rake, valley and wall lengths.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Drip edge pieces
- Drip edge length
- Valley flashing pieces
- Step flashing pieces One per shingle course.
Drip edge goes under the underlayment along the eaves and over it along the rakes. Chimneys also need counter-flashing and a back pan, and skylights usually come with a flashing kit from their maker; neither is counted here. Old step flashing is rarely worth reusing, so plan to replace it.
How the calculation works
Drip edge and valley flashing come in lengths that overlap where they join, so each piece covers its length less the lap. Divide the edge length by that and round up. Step flashing is different: one bent piece goes in with every course of shingles along the wall, so the count is the length of the wall along the slope divided by the shingle exposure.
A worked example
A roof with 80 ft of eaves and 64 ft of rakes has 144 ft of edge. With 10 ft pieces lapped 2 in, that takes 15 pieces of drip edge. Where the roof runs 12 ft up the side of a wall, with shingles at a 5 in exposure, it takes 29 pieces of step flashing.
Getting the order of layers right
Along the eaves the drip edge goes on first and the underlayment laps over it; along the rakes the underlayment goes on first and the drip edge laps over it. That way water always runs from one layer onto the one below. Chimneys need counter-flashing and a back pan as well as step flashing, and skylights usually come with a flashing kit made for them. Old step flashing is seldom worth reusing on a new roof.
Frequently asked questions
Is drip edge required on a shingle roof?
The International Residential Code requires drip edge along the eaves and rakes of asphalt shingle roofs, and most shingle manufacturers require it too. It directs water into the gutter and away from the fascia.
What is step flashing?
Small L-shaped pieces of metal woven in with each course of shingles where a roof slope meets a wall or the side of a chimney. Each piece overlaps the one below, so water is passed down the roof and never reaches the wall.
Can old flashing be reused on a new roof?
Rarely. Step flashing is usually bent, nailed through or corroded by the time the old roof comes off, and new flashing costs little compared with a leak.