How strong is finger jointed lumber

Many years ago I worked in a lumber mill that made finger jointed 2x4 studs cut to standard lengths for 8ft and 9ft walls. I actually did quality control for the factory and the studs we made were only to be used vertically (structurally) They would fail tests in horizontal orientation where you put lateral stress on the glue joints. But under compression they were stronger than solid pieces. Mostly because we'd cut the knots and other weak points out of the material that we made the finger jointed studs out of. The finger joints and glue didn't make them stronger, the lack of knots and other inclusions did.

He salvaged stuff to break it in a scientific fashion. The world needs more people like this.

I've watched a number of videos on gluing endgrain and the important point that was made--and backed up here--was that end grain tends to soak up the glue into the fibres, which makes them much stronger, but tends to starve the joint, itself. I believe that you can see this in some of the samples, where there is barely any glue between the two pieces of wood. I suggest you first try adding a generous amount of thinned glue to the finger joints, leaving it for 5-10(?) minutes to soak in, then wipe away the excess and add the proper amount of glue, followed by clamping/etc. I suspect that you will find the joint to be MUCH stronger than just gluing the joint as you would with edge grain. Cheers!

These tests are always interesting. This machine has been very useful. This videos make me think more about my glue joints.

My wood tech professor in university invented that joint. I observed the testing. You are see a production quality issue. The joints, prepped and glued and cured with the designed equipment do not fail that way. Prof Talbot also invented Masonite, particleboard and truss joists.

Thanks for the video. Made round chair bases using veneer in a RF press for fast cure time for leather chairs. They where put together using finger joints. We did a test using the standard glue, and Lepage construction adhesive ( this would be 15 ago so different then you could buy today ) . When the test came back the standard glue we used failed at the joint. The Lepage glue tested had to stop machine limits where reached, ie the joint did not fail the machine was just destroying the part as a whole. For stupid reason the company did not use the Lepage glue, even tho Lepage wanted to give them a free system to dispense the glue from a 50 gallon drum.

99.9% of YouTube videos aren’t presented this well…. 😂

You did a great job in your testing. Very eye opening

Something I've seen quite a bit of IRL is that the finger jointed material uses an adhesive that is not outdoor rated and so old pieces will fall apart very easily right at the joints.

The joints you were testing with the Radiatta pine and rubber wood samples were 1/4” or 6mm joint lengths, these joints were designed to ‘stretch’ board length i.e. get longer pieces out of short scraps. They are designed to be strong enough but not structural by any means. Having said that I’ve handled and used pine molding up to 16’ length made from 4” and longer blocks using 1/4” joints that was plenty strong for the job. 18 years in the finger jointing business taught me that it is THE most exacting and difficult job in a molding or lam beam plant. Thanks for showing a very creative test machine and very interesting video.

Very interesting tests and analysis. Thank you for sharing !

For gluing end grain, I saw a vid by a long time cabinet maker who advised applying glue to end grain twice. He applied end grain glue to all end grain butt joint faces in a face frame first, then started at one corner of the face frame & applied glue to all joint surfaces hitting the end grain faces a 2nd time. Theory being it takes a few moments for glue to wick into end grain pores so 1st application won't do as well if that's all you have for the end grain face. Seems logical, but it needs to be tested to see if the theory matches practice.

When I was a kid I loved to take molding (finger jointed like this) and stomp on it to see the joint fail. My dad was not amused!

In schip building there is a rule that a scarf joint 8:1 (length to thickness) in a kiel and 12:1 in spars, is as strong as the original wood.

love seeing you test joints \ glues ect,, i've read so many claims from books and magazines that suggest one is better then the other, and even though it published,,, i believe what i can see over a written claim trying too sell a publication or manufactures brand , thank you for these videos, seeing is believing, and i was always weary of these joints,even on trim work ,i seen them fail while handling during a install .

Its a good day when Mathias uploads. I get an indepth video on something i was probably wondering about! Thank you for making!

Maybe it's because it's closer to end-grain-to-end-grain rather than long-grain-to-long-grain? Would be interesting to see if the strength changes with the angle of the fingers.

We use fingerjointed construction lumber for rafters, floor joists etc. Same ratings as regular-non jointed timber. A bit strange to work with as usual wood has one bow direction but finger jointed can bend like a snake, so varying direction, but the bow is not as large as regular lumber though. Finger jointed can also be ordered in any length which is nice. We've ordered over 7 meter lengths.

very high quality content, thanks a lot!!

My stair hand rail broke at the finger joint glue line. I re-glued it and it's noticeably weaker at the joint. I need to add a wall support at the joint before it breaks again. This was good information Matthias, to give relative strength % values to what we intuitively knew. Also great comparison info on hardwood vs softwood.