Understanding your result
The headline is the total number of step flashing pieces. The stats split them between sidewalls and the chimney and show the counter flashing length. The breakdown lists every part: the step pieces per run, the kick-outs, the chimney apron, the back pan or cricket decision, the counter flashing, the boxes, the nails and the sealant.
The number to remember is the pieces per course: one. Every course of shingles that meets a wall gets its own piece of step flashing, woven in as the course is laid, so the count is the wall length divided by the exposure, the same exposure the shingle calculator assumes for the shingle type. If the exposure changes, both change.
The cricket decision matters for the budget: a cricket is framed in plywood and covered in metal or membrane, a few hours of work, while a back pan is a single bent sheet. The roof repair cost calculator prices flashing repairs when the problem is an existing leak rather than a new roof.
How we calculate this
The one conversion that is easy to miss is the chimney depth. On a plan the chimney is, say, 2 feet deep up the slope, but the roof it sits on is tilted, so the flashing runs 2 × 1.118 = 2.24 feet along a 6/12 roof. That is one more course on a steep roof, which is one more piece on each side. The pitch calculator gives the factor for any pitch.
Counter flashing is counted as the chimney perimeter with 10 percent for laps, because it is cut into pieces that overlap at the corners and along the stepped sides. On masonry sidewalls the whole sidewall length needs counter flashing too; on siding, none.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Step flashing size | 4 × 4 × 8 in | IRC R905.2.8.3 (minimum 4 in up the wall and 4 in on the roof); 8 in is the standard length sold |
| Shingle exposure | 5⅝ in | The exposure of most architectural shingles; three-tab is 5 in |
| Lap between pieces | piece length − exposure (2⅜ in at 5⅝) | Follows from one piece per course; IRC requires at least 2 in |
| Cricket threshold | 30 in chimney width | IRC R1003.20 (cricket required where the chimney is more than 30 in wide perpendicular to the slope) |
| Counter flashing on masonry | let into a raked mortar joint, 3 in laps | IRC R903.2 and NRCA details |
| Siding stand-off | 1 to 2 in above the roof | Siding manufacturers’ installation instructions |
| Pieces per box | 100 | Common packaging; also sold singly and in bundles of 25 |
| Sealant | 1 tube per 12 ft of counter flashing | Estimating practice (a 10 oz tube runs about 12 ft of ¼-in bead) |
Assumptions last reviewed October 7, 2026.
The calculator does not count pipe boots, skylight flashing kits or headwall flashing where a roof meets a wall at the top of the slope (one continuous piece, the wall length plus 8 inches), and it does not size the cricket’s framing. It reports the cricket’s approximate metal area so you can buy sheet or membrane. The guide to drip edge, step flashing and valleys shows each flashing in installation order, and the valley flashing calculator covers the valleys that often start at the sides of a dormer.
Two worked examples
A dormer and a wide chimney on a 6/12 roof
A dormer with two 12-foot sidewalls (24 feet in two runs), architectural shingles at 5⅝-inch exposure, a 3-foot-wide by 2-foot-deep chimney, siding on the walls, boxes of 100.
- Sidewall pieces: 24 ft ÷ 5.625 in = 51.2 courses, so 52 + 2 starters = 54
- Kick-outs: 2
- Chimney sides: 2 ft × 1.118 = 2.24 ft up the slope; 2.24 ÷ 0.469 = 4.8, so 5 + 1 = 6 pieces each side, 12 in total
- Chimney width 36 in is over 30 in: cricket required, about 6.3 sq ft of metal
- Counter flashing: 2 × (3 + 2) × 1.1 = 11 ft, 1 tube of sealant
- Total: 54 + 12 = 66 pieces, 1 box of 100, 132 nails
One box covers it with a third spare. The cricket is the expensive line: a framed saddle covered in metal rather than one bent sheet.
Four sidewalls and a narrow chimney against brick
A second-storey wall meets the roof in four runs totalling 40 feet, three-tab shingles at 5-inch exposure, a 2 by 2 foot chimney, brick walls, step flashing in bundles of 25.
- Sidewall pieces: 40 ft ÷ 5 in = 96 courses, so 96 + 4 starters = 100
- Kick-outs: 4
- Chimney sides: 2.24 ft ÷ 0.417 = 5.4, so 6 + 1 = 7 each side, 14 pieces
- Chimney width 24 in: back pan, no cricket
- Counter flashing: chimney 2 × 4 × 1.1 = 8.8 ft plus the 40 ft of brick sidewall = 48.8 ft, 5 tubes of sealant
- Total: 100 + 14 = 114 pieces, 5 bundles of 25, 228 nails
The brick walls move the cost from the step flashing to the counter flashing: nearly 50 feet of metal to let into mortar joints, which is grinder work, not roofing.
Where to find your inputs
Sidewall length along the roof. Where a roof plane runs alongside a wall, measured along the slope. For a dormer, the side of the dormer from its front wall to where the dormer roof meets the main roof, times two. From the ground, measure the horizontal length and multiply by the pitch factor.
Number of runs. Each separate stretch of sidewall. A dormer has two; a two-storey section with roof on both sides has two; an L-shaped upper storey can have four.
Exposure. On the shingle wrapper: 5 inches for three-tab, 5⅝ inches for most architectural, 5 to 8 inches for designer products.
Chimney width and depth. Width is measured across the slope (parallel to the ridge), which is the dimension that decides the cricket; depth is measured up the slope on the plan. Both from the ground with a tape at the base of the chimney.
Common mistakes
- Counting pieces by the foot. Step flashing is one piece per course, so it depends on the exposure, not on a rule of thumb per foot.
- Face-nailing step flashing to the wall. It is nailed to the roof deck only, at the top corner, so the wall can move and the siding can cover it. A nail in the wall leg is a leak.
- No kick-out. The bottom piece must throw water into the gutter. Without it, water runs down inside the siding and rots the sheathing at the corner.
- Surface-mounted counter flashing on brick. Caulked-on counter flashing peels within a few seasons. Cut the reglet into the mortar joint.
- A back pan on a wide chimney. Over 30 inches the code wants a cricket. Snow and leaves pile behind a wide chimney and the back pan overflows.
- Reusing old flashing. Old step flashing has nail holes in the wrong places for the new exposure. New shingles, new flashing.
- Siding touching the roof. Siding wicks water. Hold it 1 to 2 inches off the shingles so the step flashing can drain.
Questions people ask
- How many pieces of step flashing do I need?
- One piece per course of shingles along the wall, plus a starter piece at the bottom of each wall run. Divide the length of the wall along the roof slope by the shingle exposure: 24 feet of sidewall at a 5⅝-inch exposure is 51.2 courses, so 52 pieces plus 2 starters for two runs, 54 pieces. Chimney sides are counted the same way, on both sides.
- What size is step flashing?
- The standard piece is 4 by 4 by 8 inches: an 8-inch-long sheet bent at a right angle so 4 inches lies on the roof and 4 inches goes up the wall. With a 5⅝-inch exposure each piece laps the one below by 2⅜ inches. The International Residential Code (R905.2.8.3) requires at least 4 inches up the wall and 4 inches on the roof. Heavier 5 by 5 by 10 inch pieces are used with thick shingles and in snow country.
- When does a chimney need a cricket?
- When it is more than 30 inches wide measured across the slope (IRC R1003.20). A cricket, or saddle, is a small gable built behind the chimney to split the water and snow that would otherwise pile against its back. Narrower chimneys get a back pan, a single bent sheet turned up the chimney and under the shingles above. The calculator applies the rule from the width you enter.
- What is a kick-out diverter and do I need one?
- A kick-out is the first step flashing piece at the bottom of a sidewall, bent to throw water outward into the gutter instead of letting it run down the wall behind the siding. Every sidewall that ends at an eave needs one. Missing kick-outs are one of the commonest causes of rotted sheathing and framing at the corner of a wall below a roof.
- Does step flashing need counter flashing?
- Against siding, no: the housewrap laps over the top of the step flashing and the siding covers it, held 1 to 2 inches off the roof. Against brick or stone, yes: counter flashing is let ¾ to 1½ inches into a raked mortar joint, bent down over the step flashing and sealed. Surface-mounted counter flashing held on with caulk alone fails within a few years.
- How is a chimney flashed?
- In four parts, installed with the shingles: an apron across the front (the downhill side), step flashing up both sides one piece per course, a back pan or cricket behind, and counter flashing around all four sides let into the mortar. A 3 by 2 foot chimney on a 6/12 roof takes about 12 step pieces, one apron, a cricket and about 11 feet of counter flashing.