# Best way to join two large planks of walnut



## Smidarnar (Jan 6, 2022)

I have to join two pieces of 2”x12” (45x300 mm) walnut boards.
It will become a floating shelf fastened to the wall with steel rods.

Joining them with a reinforced butt joint such as dowels feels like something that will just crack open sooner than later.

The shelf will be holding mostly heavy books so this has to be very sturdy.

Should I make a japanese scarf joint or do you have other ideas?
Is it wise to attempt making a scarf joint with boards as wide as these (45x300 mm)?


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## Dave McCann (Jun 21, 2020)

What is your reason for choosing this particular style of scarf joint? Is it your intent to disassemble the joint at a later date?
If you have no need or intent to disassemble, a simple 8 to 1 ratio glued scarf joint would more likely take less skill to execute and have a greater margin of error, without loss of joint strength.

Epoxy glued, 8 to 1 ratio, scarf joints are used to join timbers, planks and also sheet goods, all time in the wood boat building environment. Joints as wide as 1,219mm are called for when joining sheet goods.

I have no resource to compare the actual load test between a mechanical scarf joint versus an 8 to 1 glued scarf joint. I do have confidence that the 8 to 1 epoxy glued scarf joint will carry as much load as the wood it is made from.

How many support rods do you intend to use? Would it be possible to use additional support rods to relieve some of the stress on whatever joint is used?


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## GeorgeC (Jul 30, 2008)

What is wrong with a simple glue joint. We are always saying on this forum that a glue joint is as strong as the original wood. 

If the support rod is longer than a single board, then there will be minimal stress on the joint.

gmc


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## Dave McCann (Jun 21, 2020)

GeorgeC said:


> What is wrong with a simple glue joint.


George,
Do you understand the application for a japanese scarf joint?

It is designed for joining pieces end to end.


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## Smidarnar (Jan 6, 2022)

Dave McCann said:


> What is your reason for choosing this particular style of scarf joint? Is it your intent to disassemble the joint at a later date?
> If you have no need or intent to disassemble, a simple 8 to 1 ratio glued scarf joint would more likely take less skill to execute and have a greater margin of error, without loss of joint strength.
> 
> Epoxy glued, 8 to 1 ratio, scarf joints are used to join timbers, planks and also sheet goods, all time in the wood boat building environment. Joints as wide as 1,219mm are called for when joining sheet goods.
> ...


Thanks for the tip!
I just assumed I needed a japanese scarf joint to be absolutely certain and in my mind a 8:1 scarf joint just wasn’t good enough.
After reading your comment I looked into the strength of a 8:1 (or similar ratio) scarf joint and got pretty confident it was more than enough.

Since it’s visible and it’s supposed to be a fine looking shelf the 8:1 scarf joint will also blend more in I think, which is good.

The shelf will be around 4m long and I intend on using M12 threaded rods 60cm apart or so.

Would it be best to make a router jig to cut the joint?


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## KI5AAI (Nov 4, 2021)

What would be the problem with glue and dowels or biscuits? How much weight will be on the leading edge of the shelf?


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## Smidarnar (Jan 6, 2022)

GeorgeC said:


> What is wrong with a simple glue joint. We are always saying on this forum that a glue joint is as strong as the original wood.
> 
> If the support rod is longer than a single board, then there will be minimal stress on the joint.
> 
> gmc


I think you’re talking about joining them on the long side.
I’m talking about making the shelf longer by joining end to end.


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## woodnthings (Jan 24, 2009)

Smidarnar said:


> *I have to join two pieces of 2”x12” (45x300 mm) walnut boards.*
> It will become a floating shelf fastened to the wall with steel rods.
> 
> Joining them with a reinforced butt joint such as dowels feels like something that will just crack open sooner than later.
> ...


Are you needing to make the shelf *longer* than 300 mm or *wider* than 45 mm?

Joining them boards edge to edge, making them wider is no problem. Straighten and square the edges and clamp them up.
Joining them end to end requires a scarf joint or dowels, the scarf joint being many times stronger.
The answer determines the method!


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## Rebelwork (Jan 15, 2012)

Smidarnar said:


> I think you’re talking about joining them on the long side.
> I’m talking about making the shelf longer by joining end to end.


You can do the same on the ends.


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## Smidarnar (Jan 6, 2022)

woodnthings said:


> Are you needing to make the shelf *longer* than 300 mm or *wider* than 12 mm?


The depth of the shelf is 300 mm and the thickness around 45 mm. The plank I’m using exceeds those measurements so that’s all good.
The problem is the length.
The shelf needs to be around 4000 mm long but the plank is only 2500 mm or so.
Now I have two of those planks and need to join them end to end.


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## Smidarnar (Jan 6, 2022)

Rebelwork said:


> You can do the same on the ends.


So joining two pieces of 2”x12” planks, each around 2000 mm long, with a simple glue joint?


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## Rebelwork (Jan 15, 2012)

Dowelling , Dominos, etc..


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## woodnthings (Jan 24, 2009)

Smidarnar said:


> So joining two pieces of 2”x12” planks, each around 2000 mm long, with a simple glue joint?


I would use some additional means of support in the joint, probably dowels. OR, if there is a shelf support directly under the joint, that may eliminate the need for dowels?
A long board joined with only a butt joint where the ends are glued together will not only sag, but may fail at the joint. 
This will depend a lot on the amount of weight placed on the shelf. Stuffed animals would be fine, heavy books and glassware would not.


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## Rebelwork (Jan 15, 2012)

He needs a lot more than those small steel rods supporting all this weight..a rod is weak...


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## FrankC (Aug 24, 2012)

The rods are supporting the boards, you need a joint that will keep the two ends lined up in case the boards tend to twist, a couple dowels will keep them lined up. If the dowels are glued the long grain to long grain will make a solid joint and depending on the grain of the wood be almost invisible from the edge. Only use a scarf joint if you have the tools to make a perfectly fitted joint.


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## Rebelwork (Jan 15, 2012)

Those little rods aren't going to support those heavy books. Looks like rods drilled into studs to me..a 2x12 isn't a 1x6...


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## FrankC (Aug 24, 2012)

I personally would use solid rod slightly oversize to the hole, threaded rod will dig into the wood and shelf will eventually be sloping down.


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## DrRobert (Apr 27, 2015)

I agree with something more than those rods for books. Most floating shelves are for nick knacks, pictures and such.

I think you are looking for trouble unless you go with 3/4” thick rods.


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## TominToledo (Sep 29, 2019)

I used a lap joint for something like that years ago. I pinned it vertically in a few places with a one inch dowel. 
It's still intact as far as I know.


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## B Coll (Nov 2, 2019)

Smidarnar said:


> I have to join two pieces of 2”x12” (45x300 mm) walnut boards.
> It will become a floating shelf fastened to the wall with steel rods.
> 
> Joining them with a reinforced butt joint such as dowels feels like something that will just crack open sooner than later.
> ...


Another option if you do not want to lose material is a blind spline tenon. Cut a groove from the back stopping short of the front edge on both edges. With stock that wide I would likely cut the grooves 1/2" by 1 to 1 1/2" deep. Insert a tenon the width and depth of the groove you cut. Make sure the tenon is cross grained to the joint/ same grain direction as the boards. If you fit the tenons/splines well you can not glue. glue one side, or glue both as you wish, depending on how ight you want the joint.


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## Jim Frye (Aug 24, 2016)

If this were my project, I'd probably use #20 biscuits spaced 4" apart. Been doing biscuit joins for decades and never had one come apart. As an alternative, the blind spline join B Coll described would work. I've done those using plywood as the spline also.


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## Smidarnar (Jan 6, 2022)

Rebelwork said:


> Those little rods aren't going to support those heavy books. Looks like rods drilled into studs to me..a 2x12 isn't a 1x6...


I intend to drill all the rods into studs and then sink them into the shelf about 3/4 of the shelf’s depth. Those steel rods are 1/2 inch in diameter.
Do you think it won’t be enough?


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## Smidarnar (Jan 6, 2022)

DrRobert said:


> I agree with something more than those rods for books. Most floating shelves are for nick knacks, pictures and such.
> 
> I think you are looking for trouble unless you go with 3/4” thick rods.


Yes maybe it’ll be wiser to use 3/4” rods instead of 1/2” to be safe.


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## woodnthings (Jan 24, 2009)

Smidarnar said:


> I intend to drill all the rods into studs and then sink them into the shelf about 3/4 of the shelf’s depth. Those steel rods are 1/2 inch in diameter.
> Do you think it won’t be enough?


Yes, 1/2" diameter rods will not bend. This is why all the details must be given to avoid the "what if' types of questions. How much weigh? How many books? Did you weight them? How far apart are the rods, 16 on center like normal studs? We are just normal humans who try to use common sense and reason to figure out these issues. There may be some "geniuses" among us, but they choose not to brag about it, and are waiting to be discovered!

I have bent a lot of metal including rods, even rerods which are the toughest steel I've ever encountered. I had to heat it red hot to bend it.
Make a test using the 1/2" rod, drill your 1/2" hole 3" into the edge and slip a pipe over the rod trying to bend it across the board.
I'll wager the wood will split before the rod bends.


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## Smidarnar (Jan 6, 2022)

FrankC said:


> Only use a scarf joint if you have the tools to make a perfectly fitted joint.


Ok which tools are those?


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## BigCountry79 (Jun 2, 2021)

This is just a bad idea. Choose a different design or build a bookshelf.


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## Jim Frye (Aug 24, 2016)

DrRobert said:


> I agree with something more than those rods for books. Most floating shelves are for nick knacks, pictures and such.
> 
> I think you are looking for trouble unless you go with 3/4” thick rods.


Been thinking some about this project and I'm thinking like DrRobert. This design may not be sufficient for the intended use. If I were doing a floating shelf to hold heavy loads, I would do a torsion box design and attach it with a ledger board on the wall. I did a fireplace mantle that floats. The design is a bit thicker than what you have in mind, but it could be done thinner. The torsion box is strong enough to hold a lot of weight. Maybe something to ponder.


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## yomanbill (Jul 7, 2020)

Confusing thread. It seems like there are two pertinent points, how to join the two shelf pieces and strength of the supporting rods. The simple answer to the shelf joinery problem is to not join them. Just cut the ends square, sand them smooth, butt them together (or leave a bit of a gap so it looks intentional) and let the steel rod supports hold them in place. 2x12 planks are not going to flex under a load of books. If you really want them joined, then just cut a groove in the end of each piece, insert a spline, and glue them. With the proposed mounting system, there will be little stress on the joint.
For the steel rod supports, I don't know the answer, but you can find it by either finding someone to do some calculations or do a test; bore a hole in the edge of a 2x4 stud, tap a rod in, and start loading it incrementally and take notes. When it bends, make note of the weight and then remove the weight and see if the rod returns to straight. When it bends and does not return your done. Hopefully it will support the weight of the books that each rod will need to support. If your support rods will be located in each stud at 16" on center, then that is the number of books it will need to support; what ever fits within 16". That's not much.


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## woodnthings (Jan 24, 2009)

There's floating shelves all over the contemporary world. 
In this case, the wood is 45 mm thick or 1.77" and 300 mm or 11.87" wide.
So far we have had dowels, rod, biscuits, Dominos, and butt glue suggested.
A good 3" half lap joint will have plenty of long grain surface for glue strength to bond with as well as 7/8" of end to end grain all 11.7"wide.
I'll wager it will hold all the books you can stack within a 16" span, having 1/2 the load on 8" of span, and the other 1/2 on the remaining 8". As the load gets closer to the support rod it will have a shorter lever arm (distance to the support) so the downforce will be less. This is a calculus problem and it is over my paygrade.


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## TimPa (Jan 27, 2010)

i will throw finger joint into the suggestion pool. if you lay the boards where you can go along and find similar grain and color where they will mate up, the joint will almost disappear. and it should be adequately strong for that application. 
realize that no matter the joint, you will need min 2 people to handle the install, maybe 3. i would tilt the rods up slightly (5 degrees), to compensate for future sag...


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## Lamar (Feb 3, 2021)

Just finished reading this thread and thought I'd post my initial thoughts. First off, for joining the two boards, if the look of one continuous shelf board is what you like, I would go with the scarf joint for best appearance and should be plenty strong.
The one thing to me that hasn't been mentioned is the use of 2" X 12" boards for the shelf. I'm guessing you want that thick look, but if I were making a shelf of that length and width I would be going with 1" thick stock. If your shelf supports are 16" on center, 1" thick should be plenty thick for support with out any sag. The weight of a 2" X 12" X 13 ft long should be about 86lb. A 1" thick shelf would be half as much. Less weight on your supports.


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## Smidarnar (Jan 6, 2022)

Ok thank you all for posting your advise and thoughts! I’m new here (probably the only Icelander as well) and this seems to be the platform I’ve been needing for a while now.

I suddenly took a different turn and decided to go with a 45 mm thick wawa/obeche plank.
It’s super light so good for the support rods. I’ve done a few stain tests and with Rustin’s dark teak I get close to the walnut look.










I still haven’t decided which way to go regarding the joint but I’ll put pictures on this thread when it’s done.


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## panofish (Sep 24, 2009)

Festool Domino


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