Shiplap Calculator: Plank Face Width, Gap Type, Rows Per Wall, Board Count

The most frequently asked question from US first-time shiplap installers is whether to caulk the gaps between planks. The answer is always no. The shadow gap between shiplap planks is the defining visual element of the installation. It creates the linear texture, the depth, and the characteristic look that makes shiplap unmistakably shiplap. Caulking the gap fills in this shadow line and flattens the wall surface back to the appearance of ordinary painted drywall. Caulk does have a place in shiplap installation, but only in two specific locations: at the very top edge where the shiplap meets the ceiling or crown molding (a thin bead of paintable caulk seals the gap between the top edge of the top plank and the ceiling surface), and at the very bottom edge where the shiplap meets the baseboard or floor (same thin bead to prevent visible gap at the termination point). At these two edges, you are caulking the transition to another surface, not the inter-plank gaps. Never caulk between the planks. If a gap is too wide or uneven due to a warped board, the solution is to replace the board, not to fill the gap with caulk.

Real wood shiplap (solid pine, cedar, or poplar) absorbs and releases moisture from the air, expanding and contracting in width as the ambient humidity changes between seasons. In most US climate zones, the difference between summer humidity (often 60 to 80 percent indoors without air conditioning running) and winter heating season humidity (often 20 to 35 percent with forced air) is significant enough to cause visible gap variation and, in extreme cases, board cupping or buckling if the wood was not acclimated before installation. The correct preparation: bring the shiplap boards into the room where they will be installed at least 48 hours before starting work. Stack the boards loosely so air circulates between them (use small spacer blocks between layers) and allow the wood to reach equilibrium with the room’s ambient temperature and humidity. This acclimation step is especially important in climates with large seasonal swings: the Pacific Northwest wet season, cold dry Upper Midwest winters, humid Gulf Coast summers, and high-altitude Colorado homes with very low winter humidity all create conditions where unacclimated shiplap can shift noticeably after installation. MDF shiplap (which does not acclimate in the same way as real wood) does not require this step but should be primed on all sides before installation to prevent moisture absorption through unprotected surfaces.

Shiplap boards are typically 3/4 inch thick (nominal 1-inch lumber) and need to be fastened securely to wall studs (at 16 or 24-inch spacing) or ceiling joists. The correct tool is a 15-gauge or 16-gauge pneumatic finish nailer loaded with 2-inch or 2-1/2-inch nails. The nail needs to penetrate through the shiplap board (3/4 inch), through the drywall or sheathing behind it (1/2 inch), and into the stud or joist by at least 1 inch, requiring a minimum total nail length of about 2-1/4 inches. The 18-gauge brad nailer commonly used for baseboard and trim is too light for shiplap. The small 18-gauge nail does not have enough holding power to support a 3/4-inch solid wood board against a wall over time, and on ceiling installations, an under-fastened shiplap board can sag or even release completely as the wood cycles through seasonal expansion and contraction. For ceiling shiplap especially, use 2-inch or 2-1/2-inch 15-gauge or 16-gauge nails and locate every joist before starting. Drive two nails per stud or joist crossing, separated by approximately 1 inch vertically, to resist the natural tendency of wood to cup at the edges. For MDF shiplap on walls (not ceilings): construction adhesive on the back of the board in addition to nails is acceptable and reduces the number of nails needed, but do not skip the nails entirely.