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Discussion Starter · #1 ·
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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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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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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Discussion Starter · #5 ·
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?
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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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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Discussion Starter · #7 ·
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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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)?
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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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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Discussion Starter · #10 ·
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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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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He needs a lot more than those small steel rods supporting all this weight..a rod is weak...
 

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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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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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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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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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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)? View attachment 434514 View attachment 434513
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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