Floor joists: step by step

In a previous post I discussed my ideas about how to cut a dovetail tenon and the arc I planned for the floor joists. At that point in time, all I had done was to try out my ideas with the portable band saw. Last Wednesday, August 15, Mark the Intern and I began putting my ideas into action on the floor joists. The frame has 33 first floor joists, 26 second floor joists, and 37 purlins. All of these are cut from 12 foot oak 4"x6"s. We started with the first floor joists since these wouldn't be seen. This gave us 33 to practice on. As I've mentioned earlier, we are only going to have one end completed before the raising. Once the frame is up, we can measure to make sure that the floor joists and purlins are the correct length. Some beams and rafters have a little bow to them and all the timbers' actual measurements don't correspond exactly with their nominal measurements.

Here are the steps we took for cutting the dovetail tenons and decorative arches on the joists and purlins:

First, we have the 4"x6"s in the garage on the saw horses. We worked with four at a time because this amount gave us enough room to move them around and work on them.

For the second-floor joists and the purlins, we planed the visible surfaces. We were able to run the two vertical sides through my 12" Powermatic planer, but because it can plane a 5" maximum thickness, we had to plane the bottom of each by hand. We didn't plane the first-floor joists because they won't be visible from inside the house. (Yes, that's me in the picture.)

After planing the joists/purlins, we marked and cut one end square. The other end will be cut square at the time it is finished. The next step was to mark the dovetail and the arc on the end using the templates I had previously made.

Once the end was marked, we cut the bottom of the tenon with the circular saw. We also cut across the bottom of the joist on the line next to the arc.

After making these two cuts with the circular saw, we knocked out the square at the bottom of the joist along the underside of the tenon and in front of the arc. The portable band saw doesn't have a deep enough throat to cut the arc without removing this block of wood.

Using the portable band saw, we next cut the arc. It's quite easy to cut this crooked until you get a feel for the saw as one side or the other cuts higher, lower, faster, or slower than the other side. It wasn't critical on the first-floor joists since they won't be seen. So, that gave us 33 to practice on.

After cutting the arc, we used the circular saw to cut the shoulders of the dovetail to the correct depth, and then we used the band saw to cut the angle from the corner to the shoulder.

After completing all these steps several times, we had a stack of floor joists ready for raising day.

Figuring out how to cut a dove tail tenon for the floor joists

As I worked on the braces, I gave some thought to one of the next steps in the project: cutting the dove tailed tenons on the ends of the floor joists and purlins. Cutting the mortises for these tenons wasn't too difficult, but I hadn't given much thought to how to cut the tenons. I just knew that I would do so at some point.

The floor joists and the purlins will all be 4"x6" oak. One of the features I want to incorporate is an arc from the tenon into and toward the bottom of the joist/purlin. It will provide a nice aesthetic touch and should help mitigate some of the potential weakening caused by checking as the wood dries. As I considered my intentions, it became clear that I didn't have the right tools for this job. It seemed that a band saw would be a good choice, but a stationary one would present some challenges when working with 12 foot long pieces of oak that weigh 100 pounds or more (depending on how much they've dried while in storage).

Several companies make a portable band saw. Generally, these are used for cutting metal. Apparently, they do a pretty good job of that when equipped with the right band saw blade. I thought that if I had the right blade, this would be a good tool for my purposes. So, I searched on Ebay for one. I didn't want to spend the $260 or more that a new one costs. I eventually won an auction for a used Porter Cable portable band saw. I ordered some 6 tooth per inch blades from MK Morse. After receiving the saw and the blades, I had to try it out.

I marked the dovetail and a shallow arc on the end of a short piece of 4"x6" oak that I had in the shop. I then tried to cut it out. When I was done, there was a dove tail tenon on the end of the timber, but the cuts were a bit crooked and wavy. I marked the other end and tried again. Then, I chopped off the first end and tried a third time. By paying careful attention to the orientation of the saw and making sure I stay on the lines on both sides, I was able to cut out a decent dove tail tenon. I'll practice a bit more before I begin on the 100 joists and purlins that I'll need for the frame. But, at least it looks like my idea will work.