Why the top course is the one that gives you away
Lap siding is laid from the bottom up, and almost nobody works out where the last course lands before starting. The bottom of the wall is easy: set the first board, set the second at whatever exposure the product allows, and keep going. Twenty courses later the remaining gap under the soffit is whatever it happens to be, and it can be anything from a full course down to half an inch.
The arithmetic is simple once you see the shape of it. The first board covers its own full width. Every course after that adds only its exposure. So the height you are covering equals one board width plus a number of exposure steps. Take the board width off the total, divide what is left by the exposure, and the fraction that comes out is the sliver waiting at the top.
A nine foot wall is 108 inches. An 8-1/4 inch lap board with 1-1/4 inches of headlap can show at most 7 inches. Take the first board off and 99.75 inches remain, which is 14.25 steps of 7 inches. You cannot lay a quarter of a course, so it becomes fifteen, and the fifteenth step is 1.75 inches instead of 7. Sixteen courses, and the top one is a quarter the height of the rest, directly under the soffit.
The fix is to move every course, not the last one
The instinct is to fix the top course by cheating the last two or three, spreading the shortfall over a small number of joints. That works arithmetically and shows badly, because the eye reads the change as a mistake in one place. Spread the same shortfall across all fifteen steps and each course changes by a third of an inch, which nothing but a tape will find.
Take the 99.75 inches and divide by fifteen rather than by seven. The answer is 6.65 inches. Fifteen equal steps, sixteen courses, top course exactly the same height as every other, and the headlap goes from 1.25 to 1.60 inches, which is more coverage rather than less. That is the whole trick, and it is why installers who care carry a story pole rather than a tape.
The one constraint is that the exposure has to stay inside the range the product allows. There is a maximum, set by the minimum headlap in the manufacturer instructions, and there is a practical minimum below which you are burning boards for no reason. This calculator holds the evened exposure inside the band you give it, and adds or removes a course when it cannot.
| Board | Common face width | Exposure band people work in |
|---|---|---|
| Nominal 1x6 lap | 5-1/2 in | roughly 4 to 4-1/2 in |
| Nominal 1x8 lap | 7-1/4 to 8-1/4 in | roughly 6 to 7 in |
| Nominal 1x10 lap | 9-1/4 in | roughly 7-1/2 to 8 in |
| Nominal 1x12 lap | 11-1/4 in | roughly 9-1/2 to 10 in |
Those are the ranges buyers tend to see rather than a specification. The minimum headlap for the product in your hands comes from that product literature, and it is the number that decides the top of the band.
Lining courses up with windows
The second thing that gives an amateur wall away is a course line that dies a half inch above a window head, or a butt joint that crosses a sill at an angle. If you have a run of windows all at the same head height, it is worth testing whether a slightly different exposure puts a course line at the head or the sill instead of just beside it.
Enter the sill or head height into the alignment field and the result names the nearest course line and the distance it misses by. Under a quarter inch reads as aligned from the ground. More than that reads as a near miss, which looks worse than a frank one. If the miss is large and the exposure band has room, nudge the target and look again, because a quarter inch of exposure across fifteen courses moves the top of the wall nearly four inches.
You cannot always win. A wall with windows at two different head heights, or a gable above a wall, will not let every line land where you want. Pick the elevation people see from the street and let the others follow it, because keeping a single exposure all the way round the house matters more than any one alignment.
What the layout does not decide
This is a geometry page. It works out where the lines fall and how many boards that takes. It says nothing about how the wall is fastened, how the joints are treated, or what goes behind the siding, and those are the parts that decide whether the wall lasts. Fastener type, length and pattern come from the manufacturer instructions and the code your building department has adopted, and both of those can be stricter than the other in either direction.
What keeps a wall dry is the water management layer behind the cladding and the flashings that get water back out to the face of it. Siding sheds most of the rain; the drainage plane handles what gets past. Those pieces have to lap in the right relationship to each other, and it is entirely possible to build a wall that looks finished, passes a glance and leaks, because a flashing laps the wrong way behind a course that looks perfect. Work out the water path before you work out the course lines. The house wrap calculator and the flashing and sealing calculator cover the material side of that; the sequence belongs with the instructions for the products you are actually using.
For total quantities across a whole house rather than one wall, the siding calculator takes perimeter, gables and openings and returns squares and boxes. For the boards and battens version of a cladding layout, see the board and batten calculator, and for the trim that frames all of it, the exterior trim and corner board calculator.
Questions people ask
What is exposure and how is it different from board width?
Exposure is the part of the board you can see once the course above it is on. Board width is the whole board, including the part that disappears under the next course. An 8-1/4 inch lap board with 1-1/4 inches of headlap shows 7 inches, so its exposure is 7 and its width is 8.25. Ordering by width and laying by exposure is how people end up short: a wall needs board width divided by exposure more running feet than its area suggests, which for those numbers is about 18 percent more.
Can I just make the bottom course shorter instead?
You can, and some installers do, because the bottom of the wall is often partly hidden by planting or shadow while the top sits in full view. It is a legitimate choice rather than a wrong one. The catch is that it only works if the shortfall is small enough to hide in a single course, and it fails completely on a wall where the bottom edge is visible across a lawn. Spreading the difference over every course is the version that works on any wall, which is why it is the default here.
Why does the calculator subtract one board width before dividing?
Because the first course is the only one that contributes its whole width to the covered height. Every course after it adds only its exposure, since the rest of the board is hidden. If you divide the full wall height by the exposure you get one course too many and an exposure that is slightly wrong, and the error compounds up a tall wall. Subtracting one board width first is what makes the story pole marks land where the boards actually go.
Does this work for shingles and shakes too?
The arithmetic is the same, but the constraints are not. Shingles and shakes are laid in courses with an exposure and a headlap in exactly this way, and evening out the exposure to avoid a short top course is the same fix. What differs is that the acceptable exposure band is tied to the length of the shingle and to how many layers cover any point of the wall, and those figures come from the product literature. Set the board width to the shingle length and the headlap to whatever the instructions require, and the course layout will be right.
Is the old siding a problem when I take it off?
It can be, and it is a testing question rather than a guessing one. Siding and the paint on it from before the late 1970s may contain asbestos in the cladding or lead in the finish, and cutting, sanding or breaking either one is what makes them a hazard. If the house is that old, find out what you are dealing with before anything is disturbed. Separately, most of this work happens on a ladder or a scaffold, and falls from height are the thing that actually kills people on residential jobs. Neither of those has a calculator answer.