How to Cut the Slot in the Bottom of a Stair Rail

Lay your stair rail on your staircase so that it touches the tip of each step all the way down. One end of the rail should be touching the floor at the bottom of the staircase, and the other should be projecting into the air at the top landing.

Use your pencil to mark the stair rail at the upper end, where it touches the nose of the upper landing. Use your circular saw to cut the stair rail to size.

Measure the width of your balusters to determine the size of the groove you will need to cut on the underside of your stair rail. The width of the groove should match the width of the balusters, so that a tight and sturdy fit results.

Use your pencil and tape measure to mark the proper width on both ends of the stair rail. The ends of the rail are the only portion of the wood which will be visible when the wood is being passed over the router. It is essential that you mark the wood clearly in these locations so there is a guideline to work with.

Use your table router to cut a groove into the underside of your stair rail. Place the stair rail onto your router table so the bottom is facing downward. Run the rail over the router but as many times as necessary to obtain a groove of the proper width. The depth of your cut will be determined by the size of your router bit. Select a bit that will allow at least a 1/2-inch depth, so that the joints formed between your stair rail, and the balusters and end posts, will be solid.