Calculate lap siding exposure, overlap, and even course layout from board width and wall height, plus the total boards needed for your wall.
Lap Siding Exposure Formula
E = W - O
- E is the exposure, also called the reveal, in inches. This is the part of each board left visible after the course above laps over it.
- W is the actual board width in inches, not the nominal size. Actual cedar dimensions vary by product; measure or check the product specification.
- O is the overlap, also called the headlap, in inches.
- H is the wall height in inches.
- Etarget is the largest exposure you are willing to use, and N is the resulting number of courses, rounded up to a whole number.
- Eadj is the adjusted exposure that makes every course exactly equal so the top course is not a narrow sliver.
- L is the wall width in feet and LF is the total linear feet of siding needed for that wall.
The calculator uses these formulas in three modes. The first mode applies E = W – O directly: give it a board width and an overlap and it returns the exposure, or give it the exposure you want and it returns the overlap you must hold. The second mode handles even course layout: it divides the wall height by your target exposure, rounds up to a whole number of courses, then recalculates the exposure with Eadj = H / N and lists the story pole mark height for every course. The material mode uses LF = L × N for rectangles. For a symmetric triangular gable divided into equal-height courses it uses the width at each course bottom, giving LF = L × (N + 1) / 2. It then applies your waste allowance (blank means 10%) and rounds up to equivalent stock boards. This footage estimate assumes reusable offcuts and is not a cutting or joint plan.
Maximum Exposure and Minimum Lap by Siding Type
Exposure is limited by how much lap the material needs to shed water. The table below illustrates width-minus-overlap arithmetic for the listed assumed dimensions. It does not establish a product’s permitted exposure. Enter the actual width and minimum overlap from its current installation guide.
| Board | Actual width | Typical lap | Max exposure |
|---|---|---|---|
| 4″ cedar bevel | 3.5″ | 1.5″ | 2″ |
| 6″ cedar bevel | 5.5″ | 1.5″ | 4″ |
| 8″ cedar bevel | 7.5″ | 1.5″ | 6″ |
| 10″ cedar bevel | 9.5″ | 1.5″ | 8″ |
| 5.25″ fiber cement | 5.25″ | 1.25″ | 4″ |
| 6.25″ fiber cement | 6.25″ | 1.25″ | 5″ |
| 8.25″ fiber cement | 8.25″ | 1.25″ | 7″ |
| 9.25″ fiber cement | 9.25″ | 1.25″ | 8″ |
| 12″ fiber cement | 12″ | 1.25″ | 10.75″ |
Use the current product instructions and locally applicable requirements to set the permitted exposure and minimum lap. The calculator compares overlap with the minimum you enter; it does not establish code or warranty compliance.
| Material | Minimum lap | Where the requirement comes from |
|---|---|---|
| Wood bevel / clapboard | 1″ | Illustrative reference only; verify the specific profile, board width and installation guide |
| Fiber cement example | 1.25″ | Illustrative 1.25-inch reference; verify the specific product and current guide |
| Engineered wood example | 1″ | Manufacturer installation instructions; check your product’s current install guide |
| Vinyl and Dutch lap profiles | Fixed by profile | Interlocking edges set the exposure automatically, so no exposure calculation is needed |
Lap Siding Exposure Example Problems
Example 1: even course layout. You are siding a 9 ft tall wall with 8.25″ fiber cement planks and want the standard 7″ maximum exposure. The wall height is 9 x 12 = 108″. Dividing 108 by 7 gives 15.43 courses, which rounds up to 16. The adjusted exposure is 108 / 16 = 6.75″, and the resulting overlap is 8.25 – 6.75 = 1.5″. That clears the 1.25″ minimum lap, so 16 equal courses at 6.75″ is the layout. Mark a story pole every 6.75″ and every course lands level, with no narrow strip at the top.
Example 2: material estimate. A rectangular wall is 24 ft wide and 10 ft tall. For this example, its measured boards are 5.5″ wide and its specified lap is 1.5″. The exposure is 5.5 – 1.5 = 4″. The wall height is 120″, so 120 / 4 = 30 courses. Linear footage is 24 x 30 = 720 lf. Adding 10% waste gives 792 lf, and dividing by 12 ft boards gives 66 boards.
Lap Siding Exposure Frequently Asked Questions
Is exposure the same as reveal?
Yes. Exposure and reveal both describe the portion of each board left visible to the weather, and installers use the terms interchangeably. Overlap and headlap are also interchangeable and describe the hidden portion. The three quantities are locked together: exposure plus overlap always equals the actual board width.
Why shrink the exposure instead of just cutting the top course?
A ripped-down top course reads as a visible mistake, especially under a soffit or frieze board where the eye follows the last line of siding. Shrinking every course by a small fraction of an inch is invisible, keeps the lap slightly stronger than minimum, and lets courses land cleanly at the tops of windows and doors. Professional installers lay out the whole wall on a story pole before nailing the first board for exactly this reason.
Can I use a different exposure on different walls?
You can, and it is common when wall heights differ, but keep the exposure identical on any walls that share a corner, since the courses must line up where they meet. Calculate each connected run of walls as one layout using the tallest section, and only change exposure on walls that are visually separated, such as a gable above a roofline.
