Cutting a Tenon

I had a few beams to redo the joinery on because of warpage. As I was working on these, I decided to share the process for cutting a tenon. The beam upon which this tenon was cut was a girt along the front wall of the house. It is a 12' long 6"x8" oak beam. It's simple in that it has a 1-1/2" tenon on either end. Because of its location in the frame, I didn't need to cut mortises for braces. Additionally, I needed to make sure that the tenons were the proper height since the original timbers from which the girt was cut was not a true 6"x8". I had notes on the correct height, so matching that isn't a problem. Since this girt will help support the floor, it is important that the top of it is on a level plane with the floor; the bottom level doesn't matter. This post details the process of cutting one tenon. The tenon on the other end was cut the same way.

The first thing to do is to sight down the timber to determine the crown. That is the edge that bows upward (hopefully only slightly if at all). The crown side should be on the top when the beam is in the frame.

The second step is to square one end of the beam. I measured to make sure of the length and if there were any imperfections (knots, primarily) in the timber that would be good to work around or avoid. Then, using the framing square, I mark a square line all the way around the beam near the end. This line is the guide for cutting the beam square.



Then, using my circular saw, I cut along the line on the waste side of it. This saw doesn't complete the cut because it can only cut about 2-3/8" deep. So, I widened the kerf on one side by running the saw a second time. This allows my cross cut saw to fit into the kerf easily. I use this hand saw to complete the cut. This yields a squared end on the beam.

The tenon is to be 4" long. Because this girt is along the outside edge of the house, the tenon is set 2" from the outside. So, using the framing square, I lay out the location of the tenon, marking it clearly with a pencil.

Once it's laid out, it is time to begin making the necessary cuts to create the tenon. I use the circular saw and cut along each of the lines. It is only on the 2" deep cut along the outside edge that I need to adjust the depth of the saw's cut.

Once I've cut along all of the lines, it's generally easy to remove the chunk of wood on either side of the tenon. Sometimes, there is a knot hiding inside that makes this process more difficult, but when the grain is straight, it doesn't take but a couple of blows with the mallet on the end of the chisel to pop off the unneeded piece of the beam.

At that point, I'm left with the tenon with some areas that need to be cleaned up with the chisel. It doesn't take long to pare off the excess wood, leaving a smooth side on the tenon. I then need to check the thickness of the tenon to assure that it will fit in the 1.5" mortise that's already been cut in the post. I used the width of my 1.5" chisel to feel if the edges of the tenon are the right thickness. I also use a straight edge (like the edge of the chisel) to check for high or low spots along the sides of the tenon that need to be corrected.



Because the mortises are already cut for the girt that I'm replacing, I needed to make the tenons on the replacement the right size to correspond with the mortises. Generally, they need to be made smaller. I simply take the necessary amount, whether 1/2" or 3/4", off of the bottom of the tenon. This allows the tenon to fit while leaving the top of the girt on the same plane as the other beams/girts that support the floor.