Mortise and Tenon

Mortise and Tenon 3D sketchThe mortise and tenon joint is central to timber framing. There are many variations of the basic mortise and tenon joint, and in this article we’ll be focusing on the shouldered mortise and tenon. You can think of the shoulder as a small ledge for the beam to rest on. The shoulder increases the bearing area between the post and beam, as opposed to a non-shouldered joint where only the tenon is bearing on the mortise. This increased bearing area dramatically increases the amount of shear force the beam can support, making it a strong and useful joint. The shouldered mortise and tenon is often used to join beams and girders to posts.

Step 1: Make a template

I like to use a template for laying out joinery. Templates allows you to be accurate and consistent, something you need to strive for in order to achieve tight joints. In this example we’re making a template for an 8″x8″ timber.

Template made out of plywood, masonite... something stable.

Template made out of plywood, masonite… something stable.

Here’s how you make a template for a shouldered mortise and tenon joint on an 8″ timber.

  1. Cut a piece of plywood that is 9″x8″. Use something like 3/4″ cabinet makers plywood. Don’t use OSB, the edges will fray and you’ll get inaccurate lines. Masonite seems to work nicely too. 
  2. Now we need to nibble off a triangle for our shoulder, so mark 1″ in from one of the sides that measures 9″.
  3. Mark a line from this point to the opposite corner creating a 1″x8″ triangle.
  4. Accurately cut along this line to remove the 1″x8″ triangle.

Step 2: Mark the location of the shoulder and mortise

Next we need to mark the location of the shoulder, and the location of the mortise. The location of the shoulder determines how tall the ceiling will be (assuming your beam supports a floor).

  1. Measure the height of the post, mark a line all the way across the timber. This line is the base of the housing.
  2. Align the bottom of the template to this line. Use the template to mark the location of the shoulder on both sides of the post. Use a carpenter’s square to ensure the template is held at a 90 degree angle. Later we’ll cut along this line to form the shelf that the beam will rest on.

    Using a template to layout the shoulder.

    Using a template to layout the shoulder.

  3. Mark the location of the mortise. In general, the location of mortises are 2″ from the outside of the frame. If the post is on the inside of the frame (not touching an outside wall), refer to your plans. The plans should specify if joints are to be centered on the post or laid out 2″ from the [fill in cardinal direction here] face.

    Layout line for shoulder and mortise.

    Layout line for shoulder and mortise.

Step 3: Cut the mortise, cut the shoulder

Use a heavy duty drill and an auger bit to drill a series of overlapping holes. The goal is to remove as much wood from the mortise as possible. If you were drilling a full through tenon, then you would mark and drill both sides of the timber. Drilling from one side only increases the chance for error. Drilling from both sides is more accurate and prevents splintering, which may be important if the post will be exposed to the interior. So drill one side, flip it over and drill the opposite side, meeting in the middle.

Use a chisel to remove the rest of the wood in the mortise. You want to work your way to the line. Use a heavy mortising or timber framing chisel. As you get close to the line, check the sides with a square. A corner chisel may be helpful cleaning up the corners. A slick may be helpful in finishing the sides.

A corner chisel is helpful at cleaning up the corners of mortises.

A corner chisel is helpful at cleaning up the corners of mortises.

Use a handsaw to make the 1″ shoulder cut. You may want to make more saw cuts along the face so it’s easy to knock out the chunks of wood with a chisel.

Then use a chisel and slick to shave down the face of the shoulder. Pay close attention to your layout lines, being careful not to over-cut. Use a square to check for high spots and get the shoulder nice and level.

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