Board count comes from coverage, not nominal width
The trap is dividing by the board's nominal width. Shiplap boards overlap, so a 6-inch board only covers about 5.625 inches once the rabbet laps under the next board. Coverage per board is that effective width (in feet) times the board length. Using the nominal 6 inches undercounts your order by 10 to 15 percent.
Coverage by board size
Effective width is the nominal width minus a standard 3/8-inch overlap. These figures assume 8-foot boards.
| Nominal board | Effective width | Coverage each | Boards per 100 sq ft |
|---|---|---|---|
| 5.5″ × 8′ | 5.125″ | ~3.4 sq ft | ~30 |
| 6″ × 8′ | 5.625″ | ~3.75 sq ft | ~27 |
| 7.25″ × 8′ | 6.875″ | ~4.6 sq ft | ~22 |
| 8″ × 8′ | 7.625″ | ~5.1 sq ft | ~20 |
Wider boards cover more per piece, so a wall in 7.25-inch shiplap needs about a quarter fewer boards than the same wall in 5.5-inch.
Getting the count right
- Always round up. A wall that works out to 24.3 boards needs 25, then the waste factor on top. You cannot buy a third of a board.
- Subtract openings. Take windows, doors, and fireplaces out of the wall area first, or you overbuy.
- Vertical runs cost more. Vertical installation forces height-fitting cuts on nearly every board, so use the higher 15 to 20 percent waste range.
Common questions
How many shiplap boards do I need for a 10 by 12 wall?
A wall 10 ft wide and 12 ft tall is 120 square feet. With common 5.5-inch boards that cover about 5 square feet each after overlap, that is roughly 24 boards. Add 12 percent for waste and you buy about 27, so round up to a full carton.
What waste factor should I add for shiplap?
Ten to fifteen percent for a plain wall with one or two openings. Go to 15 to 20 percent for a wall with many windows, angled cuts, or vertical installation, where more offcuts are unusable.
Should I subtract windows and doors from the area?
Yes. Measure each opening and subtract it from the wall area before you divide by the coverage per board. Skipping this on a wall with a large window or fireplace can have you buying 15 to 20 percent more material than you use.
