Shiplap
Is shiplap the same as tongue-and-groove?
No. Shiplap uses overlapping rabbets on board edges, while tongue-and-groove uses a projecting tongue and matching groove that interlock more tightly.
What geometry defines true shiplap?
The defining feature is a rabbet on opposite sides of adjacent edges, allowing boards to lap over each other and form a flush joint with a visible reveal line.
What matters most for CNC production tolerance?
Edge consistency and repeatable rabbet depth matter most, because small variations change how the lap seats and whether the face reads flat across multiple courses.
Shiplap is a rabbeted board profile in which each board is milled with opposing rabbets on its two long edges so adjacent boards overlap to form a flush, weather-shedding joint. In millwork, it commonly uses 3/4-inch boards with 3/8- to 1/2-inch rabbets, creating a visible reveal line. In interiors, the term also refers to decorative horizontal wall cladding that mimics this traditional overlapping look.
On the shop floor, shiplap is produced by profiling both long edges of each board so one edge laps over the next while holding a consistent face plane and reveal. A CNC router, moulder, or shaper line must hold rabbet depth, edge straightness, and board thickness within tight limits; any variation alters the overlap, changes the shadow gap, and can produce a lip at the joint. Moisture content also matters, because wood movement after profiling can open seams or cause the laps to bind. In assembly, boards run horizontally across studs, each course engaging the previous one, which creates a self-spacing effect that speeds installation and holds alignment. The same geometry that sheds water in exterior siding now serves interior aesthetics, hiding minor substrate irregularities. Shiplap should be treated as a profile specification rather than a style label, with exact rabbet dimensions and species documented.
Rabbet depth mismatch: A deep cut on one edge and a shallow cut on the mating edge stops the boards from seating flush, leaving lip lines and uneven reveals that read as a step-off across the wall.
Moisture drift before install: Boards that move after profiling open up the laps or bind at the joint, because the fit depends on stable thickness and predictable expansion; this also telegraphs fasteners once installed.
Interior-grade substitution: Using decorative interior shiplap on an exterior envelope fails to shed water the way true rabbeted siding does, leading to water intrusion and premature failure at the lap.