Step and Wall Flashing Calculator
This step flashing calculator estimates how many step flashing pieces, counter flashing sections, headwall apron lengths, kickouts, nails and sealant tubes you need where a shingled roof meets a wall. Enter your joint length, shingle exposure and piece size to get a rounded-up shopping list.
Calculate Your Flashing
Pitch is only used to convert a horizontal measurement to slope length, and for safety checks.
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How to Use This Step and Wall Flashing Calculator
Enter the length of each roof-to-wall joint, tell the tool whether you measured along the slope or horizontally, and set your shingle exposure and flashing piece size. Add the number of identical sidewall runs and the width of any headwall where the roof slope ends against a wall. The calculator returns step flashing pieces, counter flashing and apron sections, kickouts, nails, sealant tubes, and a headlap check, all rounded up.
Why This Matters
Where a roof meets a wall is the most leak-prone spot on a house. Roofing contractors regularly trace ceiling stains to flashing that was skipped, undersized or installed backwards, and one failed joint can soak sheathing and framing for years before anyone notices. Getting the quantity right matters because step flashing has to be installed course by course while the shingles go on. Running out of pieces on the last day, or discovering you bought 8-inch pieces for a 6-inch exposure, means tearing off shingles to fix it.
Homeowners replacing a section of roof, siders re-flashing a dormer, and contractors pricing a re-roof all need a reliable count. A 20-foot sidewall on a 5-inch exposure takes about 49 pieces, and a house with a dormer, a chimney side and a porch tie-in can easily need 150 or more. Add counter flashing, kickouts at each eave termination, and sealant, and the list gets long. Buying 10 percent extra also covers bent pieces and cuts at corners, which is cheaper than a second trip to the supplier mid-job.
How It's Calculated
Step flashing goes in one piece per shingle course, so the count depends on how many courses fit along the joint. The calculator converts the joint to inches, divides by exposure, rounds up, and adds one piece as a start and finish allowance.
Pieces per run = ⌈ slope length × 12 ÷ exposure ⌉ + 1
Pieces to buy = ⌈ pieces per run × runs × (1 + waste%) ⌉
Headlap (in) = piece length − exposure
Counter flashing = ⌈ (sidewall ft × runs + headwall ft) × (1 + waste%) ÷ 9.67 ⌉ sections
Headwall apron = ⌈ (headwall ft + 1) × (1 + waste%) ÷ 9.67 ⌉ sections
Metal sections are assumed to be 10 feet long with a 4-inch end lap, so each adds 9.67 feet of coverage. Kickouts are one per sidewall run. Nails are one per step piece, one per foot of headwall apron, and two per kickout, rounded up to the next 10. Sealant assumes a conservative 20 feet of joint per 10-ounce tube. The method follows common practice reflected in the International Residential Code (R903.2 and R905.2.8): step flashing at each shingle course, at least 4 in × 4 in, with a typical minimum headlap of 2 inches. Local codes and manufacturer instructions vary and always take priority. These results are material estimates, not engineering, permit or inspection approval.
Worked Example
A sidewall measures 12 feet along the slope, with a 10-foot headwall at the top. The shingles have a 5-inch exposure, you are using 8-inch pieces, and you allow 10 percent waste. Enter those values with one sidewall run.
- Courses: 12 ft × 12 = 144 in ÷ 5 = 28.8, rounded up to 29. Add 1 for the start allowance: 30 pieces per run.
- Pieces to buy: 30 × 1.10 = 33 step flashing pieces.
- Headlap: 8 − 5 = 3 inches, which passes the 2-inch check.
- Counter flashing: (12 + 10) × 1.10 = 24.2 ft ÷ 9.67 = 2.5, so 3 sections.
- Headwall apron: (10 + 1) × 1.10 = 12.1 ft ÷ 9.67 = 1.25, so 2 sections.
- Kickout: 1. Nails: 33 + 11 + 2 = 46, rounded to 50. Sealant: 22 ft ÷ 20 = 1.1, so 2 tubes.
If you had measured 12 feet horizontally on a 6/12 roof, slope length would be 12 × √1.25 = 13.42 ft, which gives 33 courses plus 1, or 34 pieces per run.
Tips & Common Mistakes
- Measure along the slope. A plan-view measurement understates a 12/12 roof by about 41 percent. Use the horizontal option and set the pitch if you only have plan dimensions.
- Keep the headlap at 2 inches or more. On pitches below 4/12, aim for 3 inches or more so wind-driven water cannot travel behind the pieces.
- Nail the roof leg only. Never nail through the wall leg. Siding and counter flashing must be able to move independently of the roof.
- Don't forget kickout flashing. Without a kickout at the lower end, water runs behind the siding and into the wall. It is the most commonly omitted part.
- Calculate each different-length wall separately. The sidewall runs field assumes identical lengths, so run uneven walls one at a time and add the totals.
Frequently Asked Questions
How many step flashing pieces do I need per foot?
You need one piece per shingle course, so the count depends on exposure. At a 5-inch exposure, that is about 2.4 pieces per foot of slope length. At 5⅝ inches it is closer to 2.1 per foot.
What size step flashing should I use?
The most common size is 4 × 4 × 8 inches, which suits a 5-inch exposure with a 3-inch headlap. For metric or larger-exposure shingles, 10-inch pieces keep the headlap above 2 inches. Choose a piece at least 2 inches longer than your exposure.
Do I need counter flashing with step flashing?
Against masonry, yes. Counter flashing is set into a mortar joint or reglet and laps over the step pieces. Against siding, the siding and house wrap usually cover the wall leg, but a headwall still needs an apron plus a covering layer.
What is a kickout flashing and where does it go?
A kickout diverts water away from the wall at the lower end of a roof-to-wall joint, usually where a gutter or roof edge begins. Every sidewall that ends at an eave should have one, so this tool counts one per run.
Can I reuse old step flashing when re-roofing?
It is not recommended. Old pieces are often bent, corroded or sealed in place, and removing them damages the siding. New flashing is inexpensive compared to a leak, so replace it whenever the shingles come off.
Is this calculator accurate enough to order materials?
It is a conservative estimate that rounds every quantity up. Corners, valleys, dormers and chimneys add cut pieces, so confirm against your roof plan and the flashing manufacturer's instructions before ordering. Check your local code for any additional requirements.