Exposure and courses
Siding calculator
Lap siding covers less wall than its width, because every board laps the one below it. Enter the wall and the exposure and this works out the courses, the lineal feet and the squares.
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Exposure, not width, is what covers the wall
A 6 inch lap board does not cover 6 inches of wall. It laps the course below it, and what is left showing — the exposure, or reveal — is what actually covers. At a 5 inch exposure that 6 inch board covers a fifth less wall than its width suggests, and ordering off the width is the standard way to come up short.
Everything here works from the exposure. Change it and the lineal feet, the courses and the squares all move together.
What an exposure does to a 300 sq ft wall
| Exposure | Each course covers | Lineal feet needed | Courses on a 10 ft wall |
|---|---|---|---|
| 4 in | 0.333 ft | 900 ft | 30 |
| 5 in | 0.417 ft | 720 ft | 24 |
| 6 in | 0.500 ft | 600 ft | 20 |
| 7 in | 0.583 ft | 514 ft | 18 |
Set the courses out before you nail the first one
Divide the wall height by the exposure before you start. If it leaves a two inch sliver at the top, adjust the exposure slightly across all the courses instead — losing a quarter inch per course over twenty courses absorbs five inches, and nobody will ever see it. A sliver under the soffit is visible forever.
The same applies around windows. Setting the courses so a lap lands at the head or sill of a window is worth adjusting the whole wall for.
What is not on the ticket
Corner posts, J-channel, trim, house wrap and flashing are all counted by the linear foot off your own elevations, and they vary so much by system that a number here would be noise. The starter strip is the one edge run this does give you, because it is always the bottom of every wall.
Weather-resistive barrier and flashing details are what keep water out of the wall, and they are governed by code and by the manufacturer's instructions. Getting the quantity right is the easy half.
Common questions
How do I calculate siding?
Divide the wall area by the exposure to get lineal feet, and the wall height by the exposure to get courses. Siding is then sold by the square, which is 100 square feet, so the same wall area converts straight across.
What is exposure on lap siding?
The part of each board left visible after the board above laps over it. It is always less than the board width, and it is the figure every siding quantity depends on.
How many squares of siding do I need?
Wall area divided by 100, plus waste. A square of siding is 100 square feet, the same unit roofing uses.
How much waste should I allow for siding?
Ten percent on straightforward walls. Fifteen where there are gables, dormers or a lot of openings, because every angled cut at a gable end wastes the offcut.
Should I adjust the exposure to make the courses fit?
Yes, and it is standard practice. Spreading a small adjustment across every course is invisible; leaving a narrow sliver course at the top is not.
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