How strong is finger jointed lumber

I always wondered how strong various finger jointed lumber is that you can buy, so took a few samples and broke them on my joint strength tester to find out.

[@kronographer]

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.

[@csours]

Yup! You have to match the product to the use case - ESPECIALLY for engineered wood products.

[@MS-ig7ku]

The main problem is it always delaminates, look at the problems in windows and doors.

[@lastcent5140]

Nice, thanks for the insight. Love reading comments from people in a related industry

[@KeithOlson]

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!

[@andreachinaglia5804]

What you say is true, but in my experience of restoration of ancient pieces and repairing old ones I noticed that if the glue joint is strong when is new the things change with time. To make it simple a table top made with recently glued boards nailed to the frame so it can not expand and contract with the seasonal change of humidity will develop a crack in the wood, the glue is stronger then the wood itself, but if you bring it to a dry environment after some years has passed is very likely that the glue joint and not the wood will fail. This is even more true when gluing endgrain to long grain or to other end grain as edge grain, end grain parallel to the growth rings and perpendicular to them have 3 different coefficents of expansion. Finger joints can be used to joint the heads of two pieces to make a longer one, but then that longer piece should never used in a place where loads are involved, even if precautions to don't starve the glue line are taken. Tests on freshly glued wood can be somehow useful, but can also very misleading if we build something that has to last, the more the time passes the more the glue cristallizes and becomes brittle and with time and seasonal movement micro cracks will begin, we can allow some movement fixing a table top to the frame to avoid problems, but we can not allow movement inside a glued joint between end and edge grain, it can seem to be strong if the glue has not starved, but it will surely fail with time, and with time I mean some years, not centuries.

[@mannurse7421]

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

[@cassis1018]

Is that what is happening to us. Cheers.

[@almarkowbender]

Not really

[@NormanRamsey]

There is only one Matthias.

[@MikehMike01]

I need a nap

[@Stethacanthus]

I teach experimental design and I love doing experiments like this. No one tends to care about the results though and I am not a video maker.

[@brianbarney1885]

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.

[@Mia-bz]

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

[@EngineerMikeF]

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.

[@mm9773]

It’s been tested in practice for hundreds of years, and nobody advises gluing end grain to anything. Maybe it helps, in addition to the long grain surfaces, but the general consensus is that it’s the long grain surfaces that count when you glue wood together. Was that guy perhaps talking about mitre joints? they’re surprisingly strong, and maybe that’s a technique that makes them even stronger.

[@P_RO_]

It also could be compensating for the glue being used. Different glues and different woods have different rates of absorption, and glue which gets absorbed by the wood too quickly leaves the joint dryer and weaker. You can prove this by gluing two pieces of MDF together as it rapidly absorbs glue. On one joint apply and wipe the glue on both sides, leaving it to sit a bit until it's beginning to tack up. Apply another light coat then clamp. On the other test joint slobber it with glue and clamp. Once both have cured fully, the latter joint will separate cleanly at the glue joint with little provocation, while the first joint will be tearing apart the MDF itself from both sides and will take more force to break it. Softer woods will act more like the MDF with harder woods not so much, so harder woods will join better with one coat of glue.

[@rafezetter8003]

@@mm9773 no - it's to reduce glue starvation from the glue being absorbed too much into end grain.

[@Mrcaffinebean]

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

[@leovanzantvoort]

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.

[@P_RO_]

This is good to know. It was several decades ago when I learned, but IIRC in normal jointery I think the ratio called for a minimum or 6:1, preferably with a locking feature involved.

[@dutchhank]

Sorry, but that doesn't make sense. The joint doesn't know where it ends up. So if it's just as strong, then location would not matter. It's probably considered at least strong enough for the intended purpose.

[@RichardTapp1]

I had similar instructions in a wooden glider repair course. 8:1 generally, 16:1 on spars. We scarfed 1/64 plywood RC glider skins by chamfering their thickness over 1/4" (8:1) and never had a joint failure.

[@scottcates]

I believe airplane spar repairs use a similar ratio.

[@keithklassen5320]

​@@dutchhankI don't know the details, but perhaps it has to do with the traditional/normal shapes of those pieces. A different width/thickness profile might need a different scarf ratio.

[@awldune]

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!

[@LeesChannel]

You must have gotten quite a whoopin'!

[@awldune]

@@LeesChannel They were mostly scraps, but yeah I think spanking was involved

[@ninomaiorano6697]

I wonder if you would replicate the same type of finger joint as the manufactured ones and then glue them together with with tight bond three ultimate glue, which is a quality glue. I would bet that they would show better bonding results equal to solid wood.

[@felixcosty]

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.

[@alexevenly]

Marvelous video! No surprise you have such strong fan base :) Nothing excess or redundant talks - just business plus fascinating experiment - pure joy!

[@BleuCollarFndryMTL]

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

[@declankell597]

Glad to see still creating content

[@MatchaMakesThings]

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

[@prototype3a]

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.

[@cofa4011]

Very interesting tests and analysis. Thank you for sharing !

[@andrewgalbreath2101]

Good thing to keep in mind for dumpster diving woodworkers. About 70% of my projects have one of these finger joints somewhere in them just because the material I start with

[@Hazzard2theworld911]

Fantastic, thorough science. Thank you!

[@chriswilliams8607]

very high quality content, thanks a lot!!

[@christopherleblanc9599]

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 .

[@weldabar]

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.

[@robertfraser9551]

Love the test setup !! Timber joints were tested extensively back in the 40's and 50's when timber was used extensively in aircraft. Your roughly 50% value reproduces those results for simple joints with good shear area. The only way to get much higher values is by using the more complex stepped lap joints that manage the bond load and peel stresses at each step to get cohesive failure i.e half the adhesive is evident on each side of the laps after failure. In the 70' programs became available to analyse the joints in detail and versions of these are still used today for bonded joints in metal and composites.

[@owendodman3037]

To see a tool Mathias hasn’t mastered is not a once in a blue moon opportunity, it’s more like once in a life time. Good show🎉

[@kidheadcase]

Excellent upload!

[@artswri]

Great presentation as always, thanks lots

[@bmich281]

It's apparent that the finger joints for this type of use is only for utilizing short scraps from other manufacturers. When being sold to other businesses to make longer boards. Its very apparent that these joints have no structural bearing other than the structure needed to make a flat board.

[@mm9773]

I’ve never seen them as single sticks of wood, I only know them as laminated boards, table tops and stuff like that. Those are more than strong enough - although I really don’t like the scrap wood look.

[@brothyr]

@@mm9773 I believe Lowe's hardware stores sell them as 2x4 but I don't recall if they're labeled as studs.

[@jameskelly1680]

@@brothyr It would be interesting to test these. I would also like to see a structural test of a regular wood beam vs. a Glu-lam beam (which is like a stack of finger jointed 2x4s or 2x6s stacked and glued together).

[@P_RO_]

@@jameskelly1680 Include a LVL beam in that testing and we'll be all set. Matthias will first need to upgrade to a much stronger test rig though!

[@PSUQDPICHQIEIWC]

They sell finger-jointed hollow posts. They ostensibly do support a load ... for a while. If there's any moisture, they start peeling open and splaying out like an upside-down banana. Very economical.

[@MRrwmac]

Matthias, interesting that your V joints did as well. Thanks for the demonstrations!

[@yoelai]

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.

[@AllenBrosowsky]

That's exactly what I was thinking.

[@generessler6282]

Same thought here. I wonder how a box joint would work laid out flat.

[@greenjom]

If you have a very long finger (small angle) you are basically gluing endgrain to endgrain. But you "waste" a longer piece of wood. Having more smaller steep fingers is probably harder to manufacture, so I guess the given pattern is the best compromise

[@johngaltline9933]

@@greenjom Seems the opposite to me. with longer fingers there is more edgegrain surface to glue, where as with shorter ones it is more end grain. Up to the point where you use squared off box joints and the amount of end grain stops changing and the finger length just affects the edgegrain surfaces.

[@P_RO_]

@@johngaltline9933 You are correct here. If we remember that wood's strength is in it's overlapping and intertwined lignins and fibers, then it's clear that having a longer glue joint parallel to them will bind them together better. This will also give a more gradual failure mode which is always desirable. The finger-joints we buy are mainly designed to maximize production and minimize waste of both wood and glue.

[@Fekillix]

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.

[@ManyouRisms]

You know, these days it's so easy to throw an opinion out there, and misinform people. It's nice to have superheroes like you actually testing this stuff. Thank you!

[@alberttreado3713]

Nice analysis…

[@rogerolsson7303]

Matthias Wandels woodworking clips are among the best there is. Thank you!

[@matthiaswandel]

Please include link to video of you doing it better.

[@rogerolsson7303]

@@matthiaswandel No, I was wrong, the test is fine. I have replaced my comment with the praise you deserve!

[@vlord47]

nicely demonstrated

[@thejebbiest6706]

Great video

[@debluetailfly]

I have a bookcase my parents got at some finish it yourself furniture store c1960. The middle shelf is adjustable and rests on four metal pins that fit into holes in the sides. The shelf is made of boards joined lengthwise right in the middle of the shelf! I never realized that until I removed an entire set of old World Book Encyclopedias off the shelf. Those things are heavy, but the shelf is still straight as a board! No sign of failure.

[@fuzzybuns]

Cool set up for testing.

[@MinaBassilious]

Come on Matthias you know we need to see the screw advance box joint jig compared to these joints now! As always, a perfectly paced thorough video.

[@hassleoffa]

You can check out the Canadian SPS 1, Fingerjointed Structural Lumber - interchangeable with standard lumber. There is also SPS 3 (Vertical Stud use only) and SPS 4 - Machine Graded

[@jankoodziej877]

Is it just one piece of wood glued end to end with another piece, it is more interlocking with multiple boards of different lengths glued together, where the finger joints are not in the same place. That's how I've seen it used in construction lumber and the seller claimed it has higher strength than regular solid wood. Which I believed, because different grain structure would mean it's less prone to cracking from humidity changes.

[@ericberman4193]

Good video.

[@dansknettv5147]

I have been building boats, and we always said that a skarf had to be the thickness of the board times 12 - T x 12 - to be sure that it was strong enough. Both on the hulls and on masts.

[@paulkline515]

The mystery panel shown at 2:42 looks like rubberwood. Harvested from rubber plantations as trees age out of production. I pick up discarded tables just for the panels which make excellent project wood.

[@MrDowntemp0]

The sound it makes is so satisfying! POP!

[@matthiaswandel]

yes, those all mostly went with a nice bang!

[@stewanish]

Hi Matthias I look forward your next video making finger joints. Something I ponder upon a long time :)

[@haydensmith3341]

It would be interesting to see a comparison of a section where the finger joints are in line with a solid piece Vs the two solid pieces glued together. As in the boards the finger joints are usually staggered.

[@eta.tauri32]

Yes !

[@JimmyKip]

I was thinking the same thing; most of the places I see finger jointed wood are in trim where strength isn't really a big factor compared to stability I suppose - and then in wider boards where the joins are staggered in strips so each join has solid wood on either side.

[@MazeFrame]

I remember years ago reading a paper where they did similar tests, except on I-Beams for steel-frame skyscraper construction.

[@HenkjanDeKaasboer]

Wow. This is one of the very few video's I watched from start to finish without my distraction-seeking mind wandering or even considering to look away. The ending came out of left field entirely. Amazing

[@steveggca]

A practical application for Mattias's experiment. I have several paint grade finger jointed interior doors in my house (french style with 15 glass panels which makes them quite heavy) so far i've had to repair 3 of them because the door handles fell almost prefectly on the finger joint glue line. in evey case the failure was in the glue. In fairness more than 50% of the joint was removed for the mounting holes and it took almost 20 years to fail. I would by the same door again but would only accept ones were the door handle set falls on solid wood. P.S. there are several Manufacturing finger joints videos here on youtube.

[@KnowArt]

love it!

[@allwaysareup]

My cousin recently bought a post hole digger and one of the handles had a finger joint about 8 in up from the bottom. You can imagine how long that lasted.

[@alberttreado3713]

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

[@fookingsog]

"This Old Tony” does a Great Job!!! 😂

[@matthiaswandel]

But the 0.1% of videos that most people end up watching ARE presented as well or better.

[@gorak9000]

@@fookingsog When he uploads - his videos are so few and far between now, but his back catalog is pretty impressive, especially for someone just dipping their toe into machining

[@rupe53]

I'll bet this guy is a fam of Project Farm. Good testing with repeatable results!

[@madtitan9639]

I suspect those joints suck up a lot of glue, and manufacturers don't want that kind of squeeze out. Very interesting the results you got from a V. I further suspect you're gluing technique is superior to the manufacturer, but maybe letting the glue soak in a bit longer would increase the strength of the joint. I'm looking forward to further testing with the finger joint router bit! And if you're taking suggestions, I'd be curious to see the difference in results from that V vs scarf joint with a similar glue area.

[@Tarkov.]

I think it's super interesting that the V joints are so strong, considering how much faster they are to actually cut and glue.

[@refsvik]

Surprising but interesting results. I was under the impression that modern glue was better than this. I wonder how different types of joints or glue would impact the results… Thank you for doing the test🙏🏻

[@woodduck]

I work in a sawmill and my dad works as an inspector for a lot of the sawmills and finger joint mills in the region. He said they do batch tests and will test until they fail. Sometimes they'll fail at a super high pressure and will slip out of the rig and fly up and hit the roof 30ft up lol

[@jonathany1240]

This is making me dig out my old fracture mechanics readings but a lot of the models aren’t making it easy to predict a 50% stress loss or a stress intensity factor of ~2…i haven’t had coffee yet though

[@YouPlague]

I'd be super interested in a box joint test on these samples

[@Matt-kl1pg]

So really, the answer is that the finger joints are more than strong enough for application they are used for, especially when glued in a staggered pattern like on the large panel. Surprising that the partially seated joint was as strong as the fully seated ones. That doesn't make much sense to me.

[@MCsCreations]

Really interesting indeed, Matthias! Awesome testing! 😃 But yeah, it's better than an endgrain glue up I guess. Anyway, stay safe there with your family! 🖖😊

[@igorfilin8342]

That's an attitude!

[@t7063z]

Would be interesting to see the same test on strength rated construction wood. Would need a bigger test rig though.

[@LuizDahoraavida]

Adorable ascii plotting

[@nasonguy]

Drawing stuff in ascii is my jam. Linux devs have been doing it for decades. Always makes me happy when I see something as simple as a little time taken to make plain old ascii have a bit of fun and flair.

[@sevenismy]

Glue is stonger then lignin, but glue is not stronger that the fibers. It would be interesting to see how strong end grain to grain is for the same wood. My Ikea replaces the broken slats without asking for a receipt, but I rather glue them and then add some long bamboo 6mm dowels from the side.

[@jdkemsley7628]

Wood be interesting to see test cohorts of your in-shop joints, ranging in surface area from butt joints to finger joints. Or even more interesting, an emphasis on finger-length vs total glue surface area as factors

[@smartbuildengineering]

You might want to look into 4 point bending tests, it gives a uniform bending force over a larger distance, negating to some extent localised stress effects. Also as you surely know, the dimensions of the fingers, quality of glue up etc. signifiantly affect the strength of the finger joint. Design Codes for structural glulam beam staves cover this sort of thing.

[@P_RO_]

MicroLam or GlueLam beams use resorcinol glue which is far stronger, but it neither takes paint well and it's hardness makes for accelerated wear on shapers and cutters.

[@monophoto1]

Very interesting. I suspect that part of the reason is that finger joints are closer to end-grain than face grain. But I wonder if joint strength is really part of the rationale for commercial finger-jointed wood. Most of the material I have seen has been intended for decorative rather than structural applications (molding, etc) so strength isn't as critical. I really like the rigor that Mathias puts into these testing videos.

[@eitantal726]

Enlightening. I expected finger-joints to be as good as no-joint. I was wrong. Reasoning: grains are about 3x strong as glue, but Finger-joints have roughly 3 times the glue, so it cancels out

[@eherlitz]

Nordic carpentry rule of thumb when joining softer woods is a 7:1 ratio on the finger length the reach near solid wood strength, assuming that you use proper glue.

[@xfire301]

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.

[@jankoodziej877]

Interesting, then where did the name come from if not from William H. Mason, as the Wikipedia claims? Also when did you have classes with that professor, since all of the technology you mention is now very old. Wikipedia says the first masonite-like product was already made in 1898.

[@paulbalegend]

Would love to see a direct comparison with box joints (end-to-end box joints, i mean -- not the normal 90-degree corner), of course created on the box joint jig. And with standard- and thin-kerf blades :)

[@Convolutedtubules]

Recently had a finger joint break. I thought it was just poorly glued, so I prepared, glued, and clamped it. The joint still split with surprisingly little effort.

[@matthiaswandel]

Did you carve off any remains of the old glue? Need to do that for new glue to bond properly.

[@Convolutedtubules]

@@matthiaswandel Not exactly. Good point, thank you.

[@KeithOlson]

@@matthiaswandel 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 regular 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!

[@markoshun]

There’s no structural wood sold around here with finger joints. All you find are 2x2s and 1x2s meant for strapping, etc. I like the idea of using up scraps in this way, like with LVL, OSB, and MDF, shelving/counters, etc.

[@Deckzwabber]

Thanks for testing and sharing! If you're going to run more tests, would you consider cutting up the board into slats that are half solid and half jointed?

[@Peter-House-Jr]

I would be interested in a comparison of the angle joints VS. a box type joint. I believe the angle joint puts the glue more in tension and the box joint would put the glue more in shear resulting in a stronger joint closer to the strength of the wood.

[@jacque6184]

I agree. I suspect straight cut finger joints would be stronger, as the glue surface would be a higher proportion of long grain to long grain, as opposed to a semi endgrain to semi ingrain joint with the V finger joints.

[@aborntexan]

When you think about it, since the fingers are cut at an angle you are getting a glue up of partial end grain. Of course end grain glue ups are extremely weak. So a weaker joint from this sort of joining process is to be expected.

[@supersonicgamerguru]

The strength of wood comes not from local hardness but instead from long contiguous stands of the grain that can spread the stress out, so it makes sense that no matter what joinery you use it caps or at around the same strength. Said another way, the joint will never be able to bend as far as unbroken wood and still be able to return to its neutral state. The harder woods get more out of joinery because they do less bending to begin with.

[@darrellgibson]

I'd love to see how your homemade finger joints will fare. I've always wondered if they use cheaper, less effective glue than what a regular carpenter would chose.

[@bt410382]

I think the main reason to use finger joint is to use as much of wood as possible for parts that don't require much strength like flat boards. and it's practical because it's quick and it doesn't require precise alignment. so, even it's half the strength of original blank, finger joint is here to stay. btw I love the test apparatus.

[@CerealKiller.]

This is why I follow Mattias channel. It look like I know HIM for years hahaha. Best health to you! I love seeing you around. 2024

[@luisaparecido5885]

Parabéns.... ótimo trabalho 👏👏

[@paulfrizzell31]

👍great video

[@MatthewMcKee]

the difference between science and screwing around is writing it down - Adam Savage. I like these kind of videos.

[@Leandro1983123]

Goodnight. I really like your work and have been following your channel for years. I already completed a project and it turned out very well. I wanted to know if you have a wooden stroller project. (I'm Brazilian, from Soledade-PB) Thank you.

[@mwganson]

Good stuff. This has me wondering what types of glue might be better for standing the test of time even if initially the wood glue might be stronger.

[@ColMcWillis]

You should test rectangular finger joints too. The flat surface slightly decreases the glue surface due to it not being an angle, but flat-grain glues Much cleaner to other flat grain

[@RNMSC]

Was a bit surprised that you didn't test doing box joints for splice. I would think that might affect things, but I don't know.

[@mwganson]

Have you tested these tapered finger joints versus square box joints? With the square joints you have more end grain to end grain maybe, but you also have more long grain connections along the sides of the teeth. The tapered finger joints will also have end grain exposed all along the length of the taper, but the connection is at an angle, so that could improve the bond greatly. I would expect the tapered finger joints to be stronger, but by how much I don't know.

[@yodaiam1000]

The code actually has two different ratings for finger jointing. They look the same but are very different. The finger joints used in glulam are rated to meet the minimum strength of the wood(including tension load). You can also special order dimensional lumber with the stronger finger joints. Other finger joints are only rated for compression gravity load or short term bending/tension loads. The strength in the code is based on the 5th percentile capacity. The 5th percentile strength of the joint has to be more than the 5th percentile strength of the unjointed wood. So even though the mean strength is less for the joint, you can still assume a full strength for the joint. I would also be curious what would happen to the strength of the v-notch if you also clamped perpendicular to the joint during glue curing. I think your glue joint would be stronger.

[@wizrom3046]

Excellent video! So much for the "glued joints are stronger than the original wood" myth...

[@cdoublejj]

glue them up and then test them again in 35 to 45 years. i've noticed after a good long while the glue to turns to powder but, the wood can often still be fine.

[@14Mechatronics]

Always love your videos Matthias, hopefully caught you early enough to get your eyes on a Q: I've always heard that the glue is stronger than the wood itself, and that seems opposed to your results. What am I missing?

[@gorak9000]

It depends - these are mostly glued end grain to end grain - probably the worst case scenario. If you glue side grain to side grain, then I think it's stronger, but go back and watch the other strength tester videos to be sure - he's done a lot of them now, and they each show you something interesting (just that I don't remember them all)

[@devinfisher5171]

PVA (and really any glue) is stronger than the lignum in wood. Lignum is what holds the wood grain fibers together. Think of a handful of straws being the wood grain fibers that are held together with a rubber band. Lignum is the rubber band. Glue is never stronger than the wood grain fibers. A typical panel glue up is gluing the long boards together along the grain. So the glue is replacing the lignum that was cut when ripping the boards down to size but the wood grain fibers run end to end. In that joint, the glue is stronger than what the wood would naturally be without the joint. But that is not the case for the finger joint show in this video. Think about a scarf joint, the finger joint is like a bunch of really short scarf joint. The longer a scarf joint is the closer it becomes to being like long grain joint (like a panel glue up). So these finger joints are on a continuum between a long scarf joint and a butt joint. They will be strong than a butt joint but will never be as strong as a continuous piece of wood.

[@P_RO_]

@@devinfisher5171 I think that glue can be stronger than wood, however it lacks one quality of wood: It is solid, while wood fibers can and do actually move and stretch under load. Ideally would be a glue which exactly matches that quality of each type of wood it's being used on.

[@regularguy9264]

I hear another well known you tuber suggests polyurethane construction adhesive would make all the difference!

[@refisherdesktop]

All joking aside I think the main ingredient in that stuff is the will of god, 'cause once it's set that's it. It's not going anywhere.

[@P_RO_]

Not always. If you read the instructions and recommendations of the manufacturer, you'll see where they recommend that with dense woods you dampen the glue joint first, which helps the wood draw the poly glue in deeper for a stronger joint. I've seen several failures of poly adhesives where the glue didn't penetrate well enough. With your usual wood glues this is not a problem, and if you use them properly you can get a joint about equal to the strength of polyurethane where the wood fails with or before the joint does. Where poly excels is bridging gaps and regular wood glues do poorly with that.

[@nobodydoesithalfasgoodasyou]

​@@P_RO_ for a moment there I thought you were comparing two things that started with the prefix poly- and distinguishing one from the other by shortening it to poly

[@djtecthreat]

I'm redoing some rooms in my house. They used trim that is made up of pieces that were attached with finger joints. I was breaking this trim down to dispose of it, and it ALWAYS breaks at the finger joints no matter where I bend it.

[@Finding_K_Factor]

Love your home built “Instron”😀

[@matthiaswandel]

I got a lot of mileage out of that so far -- been about 3 years since I built it.

[@fxm5715]

I wonder how finger joints that use rectangular fingers instead of triangular fingers compare. The triangular fingers don't really have true long grain to long grain glue surfaces. Maybe a narrow kerf blade on your boxjoint jig would put up a better fight. Thanks for doing the hard work of empirical evidence gathering.

[@AndrewLatham]

I would be interested in how the moisture in the wood impacts the glue strength, both during application and during testing. I can imagine a large number of tests would be needed for that.

[@seephor]

Curious to see you do this using box joints since I think the weakness of the finger joints is due to the angle of the grain

[@harkbelial]

if you have it in Canadistan you should try the beams that are connected with finger joints that are used in construction.

[@geoffdrew5207]

I would like to see the same test with high strength epoxy glue.

[@bowang5556]

I ALWAYS thumb up before even watching the vid