Calculator
Step Flashing Calculator
Step flashing does not get bought by the linear foot of wall it protects, because it is not installed as a continuous strip — it is a stack of individual L-shaped pieces, one woven in with every single course of shingles as the roof climbs past the wall. A wall's length sets how many shingle courses run past it, and the shingle course count, not the wall's footage, is what actually decides how many pieces of flashing the job needs.
This calculator counts it that way: wall length converted into inches, divided by the shingle exposure the roof is actually laid to, plus one piece to start the run. It also totals the other roof-to-wall and roof-to-roof metal a job typically needs alongside step flashing — counterflashing, valley metal, apron flashing and kick-outs — since a roof-wall intersection is rarely just one of these on its own.
Calculate your quantity
5-5/8 in is standard for most architectural shingles.
Step flashing pieces
36 pieces
- Wall line length
- 16 lin ft
- Shingle exposure
- 5.625 in
- Boxes to buy
- 1 boxes of 100
- Kick-out flashings
- 1 pieces
- Counterflashing
- 16 lin ft
- Valley metal
- 0 pieces
- Apron flashing
- 0 pieces
- At 5.625 in exposure the courses repeat 2.13 times per foot, so the step count follows the shingle course, never the wall length divided by the flashing width.
Shopping summary
- • 1 box(es) of step flashing (100 per box)
- • 1 kick-out flashing(s)
- • 16 lin ft of counterflashing
- • 1 tube(s) of roof sealant
This is an estimate — confirm structural work with a professional.
How this calculation works
Every course of shingles that runs past a sidewall needs its own piece of step flashing tucked behind the shingle above it and over the shingle below, which is why the count follows shingle exposure rather than the flashing's own width. A roof laid to a tighter exposure has more courses over the same wall length, and more courses means more flashing pieces, even though the wall itself hasn't changed length at all.
The other materials in this calculation follow different logic entirely. Counterflashing runs the same length as the wall line it protects, cut and set into a reglet or lapped over the step flashing rather than counted per course. Valley metal and apron flashing are ordered by length divided by a stock piece size, the same way drip edge is, since they run continuously rather than stepping with individual shingles. Kick-outs are a fixed count — one at the bottom of essentially every step-flashed run, where it directs water clear of the wall below rather than letting it run down behind the siding.
- Shingle exposure, not shingle size, is the number that sets step count — a 5-5/8 in exposure architectural shingle and a 5 in exposure three-tab shingle produce different step counts on the identical wall length.
- A kick-out flashing at the bottom of a step-flashed wall run is not optional trim — its job is directing roof runoff away from the wall face rather than letting it wash down behind the siding at exactly the point most likely to rot.
- Valley metal pieces lose length to their own overlap the same way drip edge does, which is why the piece length used in the count is slightly less than the stock length.
- This page counts step flashing, counterflashing, valley metal, apron flashing and kick-outs. It does not size chimney flashing kits or pipe boot counts, which are sold as their own products against a penetration count rather than a length.
The formula
steps = ceil((wallLineFt × 12) ÷ shingleExposure) + 1; valleyPieces = ceil(valleyLinFt ÷ (valleyPieceFt − 0.5)); apronPieces = ceil(apronLinFt ÷ 9.5)
- (wallLineFt × 12) ÷ shingleExposure
- How many shingle courses run past the wall — wall length converted to inches, divided by the vertical distance each course advances, which is the shingle's exposure, not its full length.
- + 1
- One additional piece to start the run at the bottom of the wall, before the first full course above it needs its own step.
- valleyLinFt ÷ (valleyPieceFt − 0.5)
- Valley metal pieces, each one losing half a foot of its stock length to the overlap with the piece below it along the valley, the same overlap-loss pattern drip edge uses.
- apronLinFt ÷ 9.5
- Apron flashing pieces at a fixed 9.5 ft effective length per stock piece, covering a roof-to-wall transition where the roof plane meets a wall from below rather than alongside it, as at a porch roof.
- kickouts
- A direct count, not a calculated one — entered as however many step-flashed runs terminate at a wall below, each needing its own kick-out at the bottom.
Where these numbers come from
- 5-5/8 in default shingle exposure
- Trade convention for standard architectural laminate shingles at their typical exposure. Three-tab shingles commonly run a 5 in exposure instead, and the step count moves with it — exactly why exposure is a field here rather than a number buried in the formula.
- One step flashing piece per shingle course, plus one to start the run
- Derived on this page from how step flashing is actually woven into a roof — a new piece goes in with every course that meets the wall, which is a count driven by the roofing pattern rather than a length divided by the flashing's own width.
- 9.5 ft effective apron flashing length per stock piece
- Trade convention reflecting typical stock length for apron flashing minus a standard overlap, similar in principle to the overlap loss on valley metal and drip edge.
- Roof sealant at roughly one tube per 40 linear feet of combined wall, valley and apron flashing
- Trade convention as a rough sealant allowance for fastener heads and terminations along flashing runs — actual consumption depends heavily on how many penetrations, corners and terminations the specific roof has.
Worked examples
A 16 ft dormer sidewall, standard architectural shingle exposure
| Roof-to-wall length | 16 ft |
|---|---|
| Shingle exposure | 5.625 in |
| Kick-out flashings | 1 |
| Valley length | 0 ft |
| Valley metal piece length | 10 ft |
| Apron flashing length | 0 ft |
| Pieces per box | 100 |
| Step flashing pieces | 36 pieces |
|---|---|
| Wall line length | 16 lin ft |
| Shingle exposure | 5.625 in |
| Boxes to buy | 1 boxes of 100 |
| Kick-out flashings | 1 pieces |
| Counterflashing | 16 lin ft |
| Valley metal | 0 pieces |
| Apron flashing | 0 pieces |
A 16 ft dormer sidewall at a 5-5/8 in shingle exposure runs 35 courses past it, plus the starting piece, for 36 step flashing pieces total — comfortably inside a single box of 100. With no valley or apron in this run, the only other metal needed is 16 linear feet of counterflashing and the single kick-out at the base of the run.
Read the next example against this one for what a wider exposure and a longer wall do together, since both move in the same direction here — fewer courses per foot of wall, but a longer wall to cover.
A 24 ft two-story sidewall with a valley and a porch apron, three-tab shingles
| Roof-to-wall length | 24 ft |
|---|---|
| Shingle exposure | 5 in |
| Kick-out flashings | 2 |
| Valley length | 18 ft |
| Valley metal piece length | 8 ft |
| Apron flashing length | 30 ft |
| Pieces per box | 50 |
| Step flashing pieces | 59 pieces |
|---|---|
| Wall line length | 24 lin ft |
| Shingle exposure | 5 in |
| Boxes to buy | 2 boxes of 50 |
| Kick-out flashings | 2 pieces |
| Counterflashing | 24 lin ft |
| Valley metal | 3 pieces |
| Apron flashing | 4 pieces |
This wall is half again as long as the first example's and uses a tighter 5 in exposure rather than 5-5/8 in, and both push the same direction — 59 step flashing pieces, more than the first example's ratio of wall length alone would suggest, because more courses fit over the same length at the narrower exposure. At a 50-count box size, that needs two boxes rather than one.
The valley and apron here are genuinely separate materials from step flashing, not more of the same thing: eighteen feet of valley at 8 ft stock, each piece losing half a foot to its overlap, needs three pieces, while thirty feet of apron flashing at a 9.5 ft effective length needs four. Two kick-outs, not one, reflects two separate step-flashed runs meeting the wall below at different points rather than a single continuous run.
Common mistakes
- Dividing wall length by the flashing piece's own width instead of counting by shingle course. Step flashing is not a continuous strip, and a length-divided-by-width calculation produces a number with no relationship to how the material is actually installed.
- Using a generic shingle exposure figure instead of the roof's actual one. A three-tab roof at 5 in exposure and an architectural roof at 5-5/8 in exposure produce different step counts on the identical wall, and using the wrong figure under- or over-orders by a predictable margin.
- Skipping the kick-out flashing at the bottom of a step-flashed run to save a small material cost. It is a small, cheap piece whose absence is one of the more common causes of hidden water damage at a roof-to-wall termination, well out of proportion to what it costs to include.
- Treating valley metal and step flashing as interchangeable materials counted the same way. Valley metal runs continuously along a length with its own overlap loss; step flashing steps with individual courses. They are ordered by entirely different arithmetic.
- Forgetting counterflashing at a masonry chimney or a wall the roof meets, assuming step flashing alone seals the joint. Step flashing handles the roof side of the connection; counterflashing, set into the wall or chimney above it, covers the top edge of the step flashing so water can't get behind it.
Shopping summary
Order step flashing by the box at the piece count this page reports, plus counterflashing by length, valley metal and apron flashing by piece count where the roof has either, and kick-outs and sealant tubes at the counts in the breakdown above. Underlayment goes down before any of this metal — the roof underlayment calculator covers that layer separately, since its lap-loss math has nothing to do with the course-by-course counting this page runs.
Step flashing goes in as the roofing itself is shingled, woven course by course rather than installed as a separate pass — so it needs to be on site and staged before the roofing calculator's shingle order reaches the wall in question, not ordered as an afterthought once the roof is already covered.
A cold-climate eave below this same wall may also need ice barrier membrane, which has nothing to do with shingle courses at all — that quantity runs on a fixed horizontal code distance instead, covered on the ice and water shield calculator rather than anything this page counts.
FAQ
Why does step flashing count follow shingle exposure instead of the flashing's own size?
Because step flashing is woven in one piece per course as the shingles go up, not laid end to end like a continuous strip. A tighter shingle exposure means more courses fit over the same wall length, which means more flashing pieces — the flashing's own dimensions barely factor into the count at all.
What does a kick-out flashing actually do, and do I always need one?
It sits at the bottom of a step-flashed run and redirects water coming down the wall's step flashing outward, away from the wall, instead of letting it continue down behind the siding below the roofline. Essentially every step-flashed sidewall termination needs one where the roof edge meets a wall that continues down past it.
Is counterflashing the same thing as step flashing?
No — they work together but solve different halves of the same joint. Step flashing is woven into the shingle courses on the roof side of the connection; counterflashing covers the top edge of the step flashing from the wall or chimney side, set into a reglet or lapped over it, so water running down the wall can't get behind the step flashing's top edge.
How is valley metal different from step flashing in this calculation?
Valley metal runs continuously down a valley's length and is ordered by dividing that length by a stock piece size minus its overlap, the same logic as drip edge. Step flashing has no continuous length of its own — it steps with individual shingle courses along a sidewall, which is a fundamentally different count.
Does apron flashing apply to every roof-to-wall condition?
No — apron flashing specifically covers where a roof plane meets a wall from below, such as where a porch roof runs into the house wall above it. A sidewall that a roof runs alongside, rather than into, uses step flashing and counterflashing instead, which is why this page tracks the two as separate inputs rather than one combined figure.
Why did my step flashing box count come out to two boxes instead of one for a fairly small wall?
Box sizes vary by supplier — 50-count and 100-count boxes are both common, and a piece count that comfortably fits inside a 100-count box can still need two of a 50-count box. Check the box size your supplier actually stocks before assuming a given piece count needs a specific number of boxes.
Where to go next
The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.