Shiplap Calculator

Shiplap boards are counted by exposure, not by width, and that distinction is the whole point of the calculation. A shiplap board carries a rabbet, a stepped recess milled along both edges, so each board overlaps its neighbor and only part of its face remains visible. Compounding this, softwood lumber is sold by nominal size while the actual dressed dimension is smaller: a 1x6 measures about three quarters by 5.5 inches, a 1x4 about 3.5 inches, and a 1x8 about 7.25 inches. Subtracting the rabbet overlap, roughly a quarter to a half inch depending on the mill profile, leaves a reveal near 5.25 inches for a nominal 1x6. Row count is wall height divided by that reveal, so an 8 foot wall takes 19 rows rather than the 16 that a nominal 6 inch width would suggest. Using nominal width instead of exposure is the single most common way to under-order by 10 percent or more. Waste and wood movement supply the remaining caveats. Ten percent covers an uninterrupted rectangular wall; 15 percent is realistic once windows, doors, and electrical boxes force cuts, and diagonal layouts justify 20 to 25 percent because every board is cut at both ends. Because boards run wall to wall, offcuts are only reusable if the wall is long enough for the remainder to start another row, so waste on short walls is effectively higher than the percentage implies. Wood expands and contracts across the grain with moisture, on the order of 1 percent of width across the seasonal indoor humidity swing, which is why boards should acclimate in the installation room for several days and why the shadow gap should not be closed tight during a humid spell. Fasteners go through the rabbet into framing at 16 inches on center where possible, and installations over drywall must locate studs rather than rely on the board face alone.

Overlapping board cladding is old boat and building craft. The term shiplap derives from wooden shipbuilding, where rabbeted planking laid edge to edge produced a comparatively flush and weather-resistant hull surface, in contrast to clinker or lapstrake construction where planks simply overlapped. The same joint moved ashore for barns, sheds, and exterior sheathing across northern Europe and North America, valued because the stepped lap sheds water and tolerates seasonal movement without opening a through gap. For most of its history it was structural sheathing or utilitarian exterior siding, frequently hidden beneath later finishes, not a decorative interior surface. The dimensional conventions the calculation depends on came out of an early twentieth century standardization fight. American lumber was sold under wildly inconsistent size and grading claims, and after conferences convened by the Department of Commerce under Secretary Herbert Hoover, the American Lumber Standard was adopted in 1924, fixing the relationship between nominal and actual dressed dimensions and formalizing grading. That framework survives as the federal Voluntary Product Standard PS 20 for American softwood lumber and is the reason a 1x6 is 5.5 inches wide today. Shiplap returned as an interior finish much later, largely through the American farmhouse revival of the 2010s, when renovation television popularized exposing or installing horizontal board walls, and mills responded with prefinished interior shiplap and with plywood strip products designed to imitate the reveal at lower cost.