Shiplap Calculator

Article — Shiplap Calculator

Shiplap calculator: boards needed and coverage for accent walls

Shiplap is interlocking wood siding with a rabbeted edge that creates a small shadow line between boards. To count boards, divide wall area by coverage per board, then multiply by waste factor and round up. A 14 by 8 ft wall (112 sq ft) takes 28 boards of 1x8 shiplap at 8 ft length, accounting for 3/8 in overlap and 15% waste.

The math is straightforward; the choices are which board width, what species, and whether to paint or stain. All three drive cost and final look more than the calculator's count.

What is shiplap?

Shiplap is a flat wood board with an L-shaped rabbet cut along each long edge. When installed, the rabbet of each board sits behind the rabbet of the next, creating a small shadow line gap (the rabbet depth, usually 3/8 in) between visible board faces. The result is a uniform, slightly grooved wall surface with strong horizontal lines.

The name comes from shipbuilding. Norse boatbuilders used overlapping plank construction (clinker built) where each plank shipped over the lap of the one below. Modern shiplap inverts that idea — the planks sit flush and only the small rabbet creates the appearance of overlap. The construction technique migrated to barns in the American Midwest in the 1800s and to interior walls thanks to HGTV's Fixer Upper in the 2010s.

The shiplap board count formula

Three steps. First, compute net wall area as width times height minus opening area. Second, compute coverage per board as effective face width (nominal width minus overlap) times board length. Third, divide area by coverage, multiply by waste factor, and round up.

For a 14 by 8 ft wall: gross area is 112 sq ft. With a 20 sq ft window, net is 92 sq ft. Using 7.25 in face × 8 ft boards with 3/8 in overlap, effective face is 6.875 in (0.573 ft) and coverage per board is 4.58 sq ft. Boards needed at 15% waste: 92 / 4.58 × 1.15 = 23.1, rounded to 24 boards.

Shiplap sizes and dimensions

Shiplap is sold by nominal dimensions but cuts to smaller actual sizes. 1x6 shiplap nominally has a 6 inch width but actually measures 5.5 inches face width. 1x8 measures 7.25 inches. The difference is the wood lost in milling, planing, and rabbeting. Thicknesses run 1/2 inch for paneling, 5/8 to 3/4 inch for primary use.

Length options are 8, 10, 12, and 16 feet. 8 ft fits standard ceiling-height walls cleanly. 12 and 16 ft minimize end butt joints on long walls but are awkward to handle alone. Pick board length based on wall width: aim for one board across or one board plus a short cut, avoiding small offcuts.

• 1×4 = 3.5 in face, narrow accent
• 1×6 = 5.5 in face, traditional
• 1×8 = 7.25 in face, contemporary
• 1×10 = 9.25 in face, large modern
• 1×12 = 11.25 in face, plank look
• Thickness = 1/2 to 3/4 in typical

Rabbet overlap and shadow line

The rabbet is a 3/8 inch (0.375 in) notch cut along each long edge, at standard depth half the board thickness. When installed, the lower rabbet of one board sits on the upper rabbet of the next. The visible gap between board faces equals the rabbet depth — the famous shiplap shadow line.

Some manufacturers offer 1/4 inch overlap for a tighter look and 1/2 inch for a more dramatic shadow. The 3/8 in standard splits the difference and works for any room scale. Wider rabbets reduce effective face width and require more boards per wall, but the resulting visual reads as more rustic and farmhouse-style.

Shiplap vs tongue and groove

Tongue-and-groove (T&G) is the close relative of shiplap. Instead of a flat rabbet, T&G has a protruding tongue on one edge that fits into a groove on the adjacent board. The joint is tighter and the surface looks flatter — there is no visible gap between boards.

T&G performs slightly better at preventing air infiltration through the wall and gives a cleaner look for paneled ceilings. Shiplap is easier to install because the boards just lap; there is no groove to align and no tongue to break. T&G works best for tight, smooth installs where the shadow line would distract; shiplap works best for the deliberate farmhouse aesthetic where the shadow lines are the feature.

Installing shiplap step by step

Step one is acclimation: stack the boards loose in the room for 3-5 days so they reach the room's humidity before installation. Step two is wall prep: mark studs with a pencil line, install house wrap or vapor barrier if going on exterior wall, and check for level reference. Step three is the first row: start at the bottom, use a level to set the first board perfectly horizontal, nail through the upper rabbet only (so the lower rabbet of the next row hides the nails).

Step four is stacking: each subsequent row sits on the previous, rabbet to rabbet, with the nails driven into studs through the upper rabbet. Use 1.5 to 2 inch finish nails or 16 gauge brads from a pneumatic gun. Step five is end joints: stagger butt joints at least 16 inches between rows and never directly above one another. Step six is finishing: prime, paint, or stain after install. Caulk corners and top with quarter-round trim or stop.

Shiplap cost in 2026

Material costs vary widely by species and grade. Paint-grade pine runs $2-4 per square foot. MDF shiplap is cheapest at $1.50-3 per sq ft but cannot be used in wet areas. Cedar for natural finish or exterior use is $4-7. Primed engineered wood (LP Smartside) costs $2.50-4.50 and handles outdoor exposure well. Oak for stained installations is $5-10.

Installation by contractor adds $4-8 per sq ft, putting a typical 200 sq ft accent wall around $1,200-2,400 finished. DIY installation drops the all-in cost to $500-1,000 for the same wall. Tools needed: tape measure, level, pneumatic brad nailer ($150 plus compressor), miter saw ($200), table saw or circular saw for ripping. Tool rental drops the entry cost if you don't already own these.

Common shiplap mistakes

The first mistake is skipping acclimation. Boards installed at moisture content different from the room shrink or swell after install, opening gaps in winter or buckling in summer. The fix is letting the wood sit 3-5 days indoors before nailing. Costs nothing, prevents 50% of shiplap problems.

The second is ignoring stud locations. Shiplap held by drywall anchors alone falls off the wall under any sustained load (a child climbing, a heavy mirror, vibration from a slamming door). Hit every stud at every row. Use a stud finder; mark the studs with a pencil line top to bottom; nail at every crossing.

The third is forgetting the gap at floor and ceiling. Wood needs room to expand seasonally. Leave a 1/4 inch gap at the floor (hidden by base molding later) and 1/4 inch at the ceiling (hidden by crown molding or trim). Without the gap, summer humidity pushes the wall outward and pops boards off the studs.