Glue-joining of Cutoff Pieces for Lumber Recovery

[DLB]

Finger joints are in somewhat common use in construction for 2X lumber. IIUC the most common application is studs, and that the benefits are lower cost and the studs tend to start and stay straighter. My barn was built with them, and the upper half of the walls is unfinished on the inside so I can see them. The "fingers" are 5/8" long, about 2X the maximum capacity of the Freud 99-037 or -099. I'd conclude that the home workshop finger joint does not replicate the commercial version. Not saying it is better or worse, but I'm more confident that the commercial version is tested. The shorter fingers of the Freud version do allow for more fingers per inch compared to the commercial.

When my barn was built I had a lot of scrap I used for various things, including an upper support for a wall between two stalls in the barn. My horse loosened the joint quickly and eventually broke it cleanly at the glue joint from rubbing his neck on the board maybe 8 or 10 inches from the joint, lateral force primarily across its width. There is no possibility that he could have broken a standard 2X4 like that. So I'd stay away from this for horizontal applications such as joists and rafters.

- David

[BuckeyeDennis]

I'm curious, David. Did the glue in the finger joint fail, or did the fingers break off? It seems to me that the weak points would be at the ends of the fingers. Because the fingers don't taper to a sharp point, there's a fairly large percentage of the wood that's basically just in a butt-joint configuration, and doesn't contribute much to bending strength.

[chapmanruss]

Some good discussion already about different joints.

Steve in you last post you said.

As for projects, next year I expect to replace a T&G front porch deck, which will involve much lumber-milling and cutoff-generating, and I will have a lot of cutoff left over, enough for at least 10 8 foot 2x8's, which I can then use in other structural projects, but only if the joints are strong.

Something to consider is how these pieced together 2x8's would be used. Loads applied to ends will be different than loads applied to the face or edges of the boards. Another is how long each of the cutoffs are. How many pieces will be used to make that 8' long 2x8. What other structural components will be use connected to them.

If you are looking at using them as structural (load bearing) members in a building project, I would reconsider that thought and use full length (no splices) lumber. As per David's example. You may be able to find structural projects to use them in that are not load bearing.

[JPG]

I agree that any application involving tension load is not a good candidate for using spliced joints of any kind. Compression load applications are more suitable.

[DLB]

I'm curious, David. Did the glue in the finger joint fail, or did the fingers break off? It seems to me that the weak points would be at the ends of the fingers. Because the fingers don't taper to a sharp point, there's a fairly large percentage of the wood that's basically just in a butt-joint configuration, and doesn't contribute much to bending strength.

Some small pieces of the fingers broke off but I'd say 90% was glue failure. I suppose it's also possible that it was inadequately glued, but one would hope that they have process controls to prevent that. The application, evidently a horse neck-rubbing post, subjected the 2X4 to a lot more repetitive bending and rebounding than a typical wall stud experiences, so I'd theorize that was a factor.

- David

[rlkeeney]

The end-grain butt joint starts at 4:13, but you should watch it all if you haven't seen it.

https://youtu.be/Wmw5rayNIV4?si=iNGJ7neF1Q67tC0Z

I tested this with two small pieces of hardwood. I could not break the joint. I clamped one end in a vise and beat it with a 16oz hammer, and it didn't break. Finger joints should be stronger. I think it depends on what you are going to do with the wood.

[edflorence]

The end-grain butt joint starts at 4:13, but you should watch it all if you haven't seen it.

https://youtu.be/Wmw5rayNIV4?si=iNGJ7neF1Q67tC0Z

Interesting video...sort of a myth busters about the strength of end grain glue joints. This may be another one of those long held notions, sort of like the one about panels built of narrow strips being stronger than a single plank panel, that needs to be rethought. After watching the video I wondered if inserting a couple of dowels across a simple end grain butt joint, that is, in line with the long grain, would add to the shear strength...to my thinking the dowels might act like concealed finger joints.