How to Mortise a 4X4 Post

Gracie Sprouse

Creating a mortise in a 4x4 post can be done at many different angles and depths. Woodworkers and contractors must mortise joints in many projects where two pieces of wood must be joined at a 90-degree angle. Mortise joints are also used for installing locks in doors and door jambs, as well as in landscaping to lock lumber together. The most popular use is in building fences or setting gateposts.

Fence posts typically need to be mortised.
  1. Put on the safety glasses to shade your eyes from wood chips. Determine the size of the mortise you need for the project.

  2. Use the pencil or marker to make an outline of the mortise on the post.

  3. Insert the 1/2-inch drill bit in the drill and place the tip of the bit in the center of the outlined area. Drill a hole to the desired depth. Drill several holes inside the outline following the outline marked on the post.

  4. Place the chisel on the outline, with the beveled edge to the inside of the outline facing the mortise mark center. Tap the chisel with the rubber mallet and chisel out the mortise to the desired depth and width. Be sure to work only inside the outline you marked.

  5. Use the chisel to clean out the mortise hole so it is fairly smooth and will fit well.