Understanding your shiplap estimate
- The exposed coverage width is narrower than a shiplap board's full milled width because of the overlapping rabbet joint; always use the manufacturer's stated exposed or reveal coverage, not the full board width, for an accurate estimate.
- A 10% waste allowance is a common trade convention for straightforward rectangular walls; walls with many openings, angled cuts, or a diagonal/herringbone installation pattern typically need a higher allowance.
What is shiplap exposed coverage width?
Shiplap boards overlap at their edges through a rabbeted joint, so the exposed, visible width after installation is narrower than the board's full milled width. This calculator uses that exposed coverage width — a figure that should be read from the specific product's spec sheet rather than assumed from the board's nominal or full milled width.
A 10% waste allowance is a common trade convention for cutting waste, mis-cuts, and board selection on a straightforward rectangular wall. Walls with many windows, doors, or corners typically need a higher allowance, since offcuts around openings are harder to reuse elsewhere on the wall.
This is an estimating tool. Actual quantities can vary with installation pattern (horizontal, vertical, or diagonal/herringbone), door and window openings, and available board lengths; a professional installer's take-off is recommended for final material orders on complex walls.
How to use this shiplap calculator
- Measure and enter the total wall area to be covered.
- Enter the board's exposed coverage width, from the product's spec sheet, not its full milled width.
- Enter the board length.
- Enter a waste allowance percentage.
- Read the boards needed, total linear meters, and coverage per board.
The formula behind the shiplap estimate
Coverage per board is the exposed coverage width multiplied by board length. The number of boards needed is the wall area, inflated by the waste allowance, divided by that coverage per board, rounded up to a whole board.
Common mistakes
- Entering the board's full milled width instead of its exposed coverage width after overlap, which understates the number of boards needed.
- Using too low a waste allowance for a wall with many doors, windows, or corners, where offcuts are harder to reuse.
- Not subtracting door and window openings from the wall area first, which overstates material needed for walls with large openings.
- Assuming boards come in a single continuous length regardless of wall height — a wall taller than one board's length requires end-joints that this calculator doesn't place automatically.
Questions fréquentes
What's the difference between exposed width and full board width?
The full milled width is the board's actual physical width before installation. The exposed (or reveal) width is the visible portion after adjoining boards overlap at their rabbeted edges, which is always narrower than the full milled width and is the figure this calculator uses for coverage.
How much waste allowance should I use?
A 10% waste allowance is a common trade convention for a straightforward rectangular wall. Walls with many doors, windows, corners, or an angled installation pattern typically need a higher allowance, since offcuts are harder to reuse.
Does this calculator account for doors and windows?
Not automatically. Subtract the area of doors, windows, and other openings from your total wall measurement before entering the wall area, so the estimate reflects only the surface actually being covered.
What if my wall is taller than one board's length?
Boards come in fixed lengths, so a wall taller than a single board's length requires horizontal end-joints. This calculator estimates total board quantity and linear meters but does not place individual joints.
Is this calculator for horizontal or vertical installation?
Either. The calculation is area-based (wall area divided by coverage per board), so it applies the same way regardless of whether boards are installed horizontally or vertically.
Références
- APA – The Engineered Wood Association. General installation guidance for wood panel products. apawood.org.
- Manufacturer shiplap and tongue-and-groove installation technical data sheets (industry-standard exposed-coverage and waste-allowance conventions).
- Peterson AJ. Estimating in Building Construction. Pearson.