How to Cut Shiplap Siding

Select a rabbet bit that fits a router's collet size and the depth of the wood you are cutting. Use a 1/2-inch-shank, carbide-tipped rabbet bit if the router will take it; otherwise, use a 1/4-inch-shank, carbide-tipped rabbet bit. For a 1-by-6-inch board with a 1/2-inch shiplap, you need a minimum 3/8-inch rabbet, preferably on a 1/2-inch shank.

Attach the guide roller that allows you to cut a 1/2-inch rabbet. Unscrew the guide roller screw at the bottom of the blade, remove the screw and washer, slip off the guide, replace it with a 1/2-inch guide roller and reinsert and tighten the screw and washer. Some bits don't have interchangeable roller guides, in which case you will have to either buy a rabbet with a 1/2-inch guide roller or settle for whatever amount of overlap your bit creates.

Set the depth of the cut on the router, following manufacturer's instructions. Test the rabbet cut on a scrap piece of wood and measure the width and depth of the cut.

Run the router counterclockwise around the board. Since you are only cutting one side of the board at a time, start to the left and move the router to the right along the edge of the board, creating an L-shaped edge that is half the depth of the board you are cutting. Set the router aside.

Flip the top of the board toward you and route from left to right on the board.