Why step flashing is counted in pieces
Where a slope runs alongside a wall, the flashing is not a length of metal. It is a stack of L-shaped pieces, one per shingle course, each lapped over the one below and covered by the next course of shingles. So the count is the number of courses, and the number of courses is the wall length divided by the shingle exposure — the distance from the bottom edge of one course to the bottom edge of the next.
That makes the exposure the governing number. A 24 foot sidewall at 5-5/8 inches of exposure has 52 courses running into it. The same wall at 5 inches has 58. The wall did not change; the shingle did. Anyone ordering by the foot has skipped the only calculation that matters, and the error runs one way — you are short, on a roof, with the wall open.
The extra piece per run is for the bottom of the wall where the first piece has nothing below it to lap, and in practice most crews want a spare or two per run anyway because the first and last pieces on any run usually get cut.
Boots are sized to the pipe
A pipe boot is specified by the pipe it seals, not by the hole in the deck. A three inch boot fits a three inch pipe, and the hole you cut is the pipe diameter plus enough clearance to get it through without fighting it. The clearance field here is total rather than per side, so half an inch on a three inch pipe gives a three and a half inch hole.
Grouping the list by size is the point of the boot section. It is common to have three sizes on one house — a large soil stack, a couple of smaller vents and a flue — and it is equally common to buy four of the wrong one because the list said seven pipes. The boot itself is a wear item: the collar is the part that fails, usually well before the roof does, and a re-roof is the moment to replace every one rather than reusing any.
Curbs, and the thing above them
A curb is a box standing up off the deck, and its flashing is a perimeter: apron at the low side, step or pan up the sides, and a head detail at the top. The perimeter is what this page gives you, which is twice the width plus twice the length.
The problem with a curb is not its perimeter, it is what is uphill of it. Water coming down the slope arrives at the high side and has to go somewhere, and past a certain width across the slope it will not reliably part around the box on its own. That is what a cricket is for. This page does not tell you the width at which one becomes required, because that is set by your adopted code and by the instructions for whatever the curb carries — it counts how many of your units exceed the threshold you entered, and leaves the threshold to you.
What a take-off cannot do
Every count on this page is the easy half. The hard half is the sequence and the laps: what goes under what, where the underlayment turns up the wall, whether the counter flashing is let into a reglet or hangs on the surface, and how the step flashing interleaves with the courses rather than sitting on top of them. Those are manufacturer instructions and code requirements, they vary by product and jurisdiction, and getting the count right while getting the lap backwards produces a roof that leaks in exactly the same place it would have anyway.
For the rest of the roof, the underlayment, drip edge and ice barrier are on the underlayment and drip edge calculator, and the shingles, starter and ridge cap that go over all of this are on the roofing squares calculator — which is also where you get the exposure figure this page depends on. Where a skylight or a boxed opening is going in new rather than being reflashed, the shaft below it is on the skylight shaft and daylight calculator. Wall openings, which use a different family of materials entirely, are on the window and door flashing calculator.
One more thing worth saying without hedging: this is roof work. Falls from roofs are among the leading causes of death in construction, and replacing one boot puts you in the same place as replacing all of them. Steep, wet, high or brittle is not a do-it-yourself situation, and a material list keeps its value if someone else installs it.
Questions people ask
Where do I get the shingle exposure figure?
Off the shingle wrapper or the installation instructions, where it is given as the exposure to the weather. On a roof that is already shingled you can measure it directly: put a tape on the bottom edge of one course and read to the bottom edge of the next. Common figures cluster around five and five and a half inches for three-tab and architectural shingles respectively, but they are product specific and the whole step count depends on getting it right, so measure or read rather than assume.
Should I reuse the existing step flashing on a re-roof?
It is done, and it is the decision that produces the most call-backs. Reusing means the new shingles interleave with metal that has been bent, has old nail holes in it, and has been sitting in a wall for twenty years, and it means not being able to see what is behind it. The metal is the cheapest component in the assembly and the wall is open exactly once. What is actually being decided is whether the siding comes off far enough to do it properly, which is the expensive part, not the flashing.
Why does the boot count use the pipe diameter and not the hole?
Because the boot has to seal against the pipe, and its collar is sized to the pipe outside diameter. The deck hole is a separate dimension, sized so the pipe passes through with a little room, and it is under the boot flange where nothing depends on it being exact. The calculator reports both so you can cut with the right hole saw and buy with the right collar, and those are two different numbers on the same line.
Does this count the sealant and the fasteners?
No, and that is deliberate rather than an oversight. Which fasteners, how many, and whether sealant appears at a given interface at all are set by the flashing detail you are using and by the shingle manufacturer, and they vary enough between products that a generic figure would be misleading. Sealant is also frequently used to patch details that should have been made watertight by geometry, which is worth avoiding rather than budgeting for. Sealant quantity for openings, where the bead cross-section is well defined, is on the window and door flashing calculator.
My roof has a valley as well. Is that in here?
No. A valley is a drainage question rather than a penetration one — two planes concentrating their water into a single line, with the flow at the bottom depending on the area of both planes and the rainfall intensity. That belongs on the roof valley and drainage concentration calculator, which sizes the problem rather than the metal. The valley metal itself is a linear quantity you can take off directly from the valley length, and the trim piece counts on the metal roof panel calculator handle it for a panel roof.