Calculate how many step flashing pieces you need for a roof-to-wall run from its sloped length and shingle exposure, with waste and pieces per foot.
Step Flashing Formula
For an ordinary sloped sidewall takeoff, estimate courses from the length of each run divided by the installed shingle exposure. The default is one flashing piece per course; change this if the selected manufacturer detail uses more. Round each unequal run separately. For identical runs:
P = A * ceil(L / E) + S Total = ceil(P * R * (1 + W))
To size each piece so it laps the one below correctly:
Overlap baseline = E + H Compared length = max(E + H, M)
Variables:
- P is the pieces in one run; A is pieces per course (default 1), and S is the selected extra starter allowance
- L is the length of each identical sidewall or chimney-side run measured along the roof slope beside the wall, in inches
- E is the shingle exposure, the exposed height of one course, in inches
- S is the starter piece, 1 if you add one at the bottom of the run, otherwise 0
- R is the number of identical-length sloped sidewall runs; headwall and apron details are outside this model
- W is the waste allowance as a decimal, so 10 percent is 0.10
- H is the assumed overlap along the slope (default 2 inches). M is the entered manufacturer minimum length (blank means not supplied). Exposure plus overlap is a baseline, not proof of a sufficient product size
In piece-count mode, enter the length of each identical sloped run and the installed shingle exposure. Defaults are one run, one piece per course, one extra starter piece per run and 10% waste; adjust them in the assumptions section. The optional price gives piece cost only. The nominal counterflashing output is bare junction length before laps, corners or terminations. In size mode, enter exposure, assumed lap, manufacturer minimum length if known, and wall/roof legs. The tool compares lengths with an illustrative 7, 8, 10, 12 and 14 inch list; if none is long enough it requests a longer/custom piece.
Exposure, Piece Count, and Size Reference
The exposure of your shingle drives both how many pieces you buy and how long each piece must be. This table pairs the two so you can order the right size and the right quantity together, something a flat “2.4 per foot” rule leaves out.
| Shingle exposure | Pieces per linear foot | Overlap baseline (E + 2 in) | First illustrative listed length |
|---|---|---|---|
| 4 in | 3.0 | 6 in | 7 in |
| 5 in | 2.4 | 7 in | 8 in |
| 5 5/8 in | 2.1 | 7 5/8 in | 8 in |
| 6 in | 2.0 | 8 in | 8 in |
Flashing dimensions depend on the selected product and installation detail. GAF TAB-R-173 specifies 5-inch wall and roof legs as standard; its 4-inch alternative requires the stated leak-barrier conditions. The GAF Pro Field Guide also gives product examples longer than exposure plus 2 inches. Owens Corning Oakridge instructions specify a product-specific 4 by 4 by 8 inch minimum. The calculator’s 4-inch legs are adjustable planning defaults. Confirm the complete manufacturer detail and local requirements; this estimate does not certify compliance or size starter, kick-out, apron or counterflashing details.
Example Problems
Example 1: Pieces for a wall run.
A dormer wall meets the roof over a 20 ft run and the shingles have a 5 in exposure. The courses come to 240 / 5 = 48, plus one starter piece is 49. With one run and a 10 percent waste allowance, the total is 49 * 1.10 = 53.9, rounded up to 54 pieces. The nominal counterflashing junction length is 20 ft before laps and end details.
Example 2: Sizing for a wider exposure.
For an installed exposure of 5 5/8 in and a 2 in assumed overlap, the baseline is 5.625 + 2 = 7.625 in. If the selected product requires at least 8 in, enter that minimum; 8 in is then the first listed length meeting the length comparison. Other dimensions and installation conditions still need checking. With one piece per course, the rate is 12 / 5.625 ≈ 2.13 pieces per sloped foot before starter and waste.
Frequently Asked Questions
How many pieces of step flashing do I need per foot?
For one piece per course, divide 12 by the installed exposure in inches to get a rate per sloped foot before starter and waste. A 5 in exposure gives 2.4; 5 5/8 in gives about 2.13. For a takeoff, round courses upward for each run, then apply the selected pieces-per-course, starter and waste assumptions.
What size step flashing should I use?
Exposure plus the assumed overlap gives only a length baseline. Use the larger of that value and the entered manufacturer minimum, then confirm wall/roof legs, metal, coverage, end conditions and the complete installation detail. A 4 by 4 inch bend is not universally suitable. A required length above the largest 14-inch illustrative option returns no suitable listed length.
Do I still need counter flashing with step flashing?
Yes at a masonry wall or chimney. Step flashing carries water down the roof, and counter flashing set into a mortar joint or reglet covers the top of the vertical legs so water cannot run behind them. Against wood or fiber-cement siding the siding itself acts as the counter flashing, so you leave a small gap and slide the step flashing up behind it.
