# Make a board longer - scarf joint?



## greatview (Oct 3, 2009)

Without going into too much detail, I need to lengthen some boards by about 8 inches or so (currently about 6 feet long). Due to their use, I must add in the middle rather than either end. The boards are about 8 inches wide by about 1 inch thick and the material is probably mahogany. The appearance is not too important as they are not usually visible. I'm wondering what the best joint might be.

(If you really want the details, I'm converting a full size bed into a queen size and the rails and ends are covered by a skirt. I've made a new headboard and now need to lengthen the members that I'd like to keep. I don't really want to spend the money on new stock.)


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## farmerjohn (Feb 19, 2010)

Probably some type of bisquit or dowel in combination with dovetail or fingerjoint might be strongest. Trim is finger-jointed all the time, but not subject to same stresses. Best to withstand the weight in my opinion (as limited as it surely is) would be matching L-shaped overlaps, doweled, with a threaded rod vertically through the joint Have you considered metal frame rails instead? Might be free with the new mattress set. Jujst kidding! Another alternative would be to start with new stock for the complete side rails and recreate the pockets for the attaching hardware to be transferred.

Reminds me of a college friend who was student teaching shop and the student who came up to him after being instructed to re-cut a board that came up short for a project and said "I've cut this board twice more and it's still too short."


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## Cory (Jan 14, 2009)

Scarf joints have little, if any, structural support. They're great when you've got a long run of trim, but I wouldn't trust that type of joint for a bed rail. I agree with socalwood, a finger joint would be far superior. You'll need as much glue area as possible.

Good luck!


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## richgreer (Dec 25, 2009)

Many opinions here - I would drill holes, using a doweling jig, into the ends of the board. I would probably go about 3 inches deep in each end. Then insert oak dowels and glue. I would use the biggest dowel that will fit and I would probably put 4 in. I think this would give you more structural support and any other method suggested so far. Warning - Dowels are often not exactly the size they say they are - especially the larger dowels. Check this out before you have glue all over the place.


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## scrabby (Sep 4, 2009)

I would agree, as a civil-structural engineer, with cory and socal that fingerjoints are better than the proposed scarf joint.

There are tensile stresses incurred by the bottom third of the bed rail during typical loading (i.e. bending). I would recommend dowels, with at least six 3/8" dowels, at least 3" long, and four of them should be in the bottom half of the joint for the same reason that a concrete beam has more longitudinal reinforcing steel on the bottom where the tension occurs.


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## JJohnston (May 22, 2009)

No, all those boat builders don't have boards subject to simple bending.

<--Also a civil engineer


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## greatview (Oct 3, 2009)

I'm going to give a finger joint a try. Grizzly has a reasonably priced bit (C1682 $69.95) and I think I'll find a variety of other uses for such a bit.


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## scrabby (Sep 4, 2009)

Exactly…thanks JJ - extending a board for a boat or trim is a different loading condition. There are clearly different methods to solve this challenge, I was just offering an opinion based on basic principles.


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## Viktor (Jan 15, 2009)

I actually used scarf joint to extend a bed rail. It works just fine. The joint must be shallow as Dave suggested (at least 1:6 or more). 
There are tensile force and shear force normal and parallel to the scarf joint governed by two equations. Depending on the material and adhesive you could determine an angle that provides sufficient strength. At a shallow enough angle, strength of the joint continues to increase and (in wood) failure will occur outside of the joint. 
Finger joint (with triangular fingers) and scarf joint are IDENTICAL in strength if the glue area is equal (think of finger joint as a folded up scarf joint). Scarf joint though is easier to make, no special tools needed. Needless to say the faces have to be precisely flat. The down side is more waste (boards must overlap more).
Dowel joint would be greatly inferior to either of the above as effective cross section of the beam is diminished to the cross sections of the dowels.
Hope this helps.


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## Viktor (Jan 15, 2009)

Scrubby,
Wooden beam should NOT be compared with reinforced concrete beam. Concrete works on compression and steel on tension. Wooden dowels in a wooden beam create homogeneous(!) beam. Steel dowels in wooden beams is a better comparison. The beauty of wood is that it glues really well creating joint that is stronger than the surrounding material.
Just a note: Old English longbows sometimes were made of two pieces joint in the middle (at the handle) with a simple W-shaped finger joint and hide glue made of fish bones.


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## richgreer (Dec 25, 2009)

True story - - I was once building a deck. My wife told me she was going on to town and asked if I needed anything. I told her that some of my boards were coming up a little short and I asked if she could stop into the hardware to pick up a board stretcher. I told her that if she just asked a sales person, they would know what she met. She did it. There was a distinct chill in our household that evening.

True story 2 - - On another day, my wife was taking her car to the dealer for routine service. I told her to make sure they changed the air in the tires. She actually asked the service manager to make sure they changed the air in the tires. I'm a terrible person.

Yes - we are still married - 38 years.


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## scrabby (Sep 4, 2009)

I'm with you Viktor, and appreciate the input and positive discussion. I'm an engineer by training and career, and a budding woodworker at home with a great interest in learning and improving (which is why I read this site at work, I suppose).

Further, I agree with you that the scarf joint would be an elegant and strong joint, but it seems to me it would also take the highest (vs. fingerjointed or dowelled butt joint) level of skill to execute, given the need for jointed, well-fitted faces in order to create a thin, strong glue interface.


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## farmerjohn (Feb 19, 2010)

Rich, you need this http://kalecoauto.com/index.php?main_page=product_info&cPath=2&products_id=45


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## Viktor (Jan 15, 2009)

I agree, Scrubby. It is not easy, at least with hand tools, which is what I did. I clamped two boards together with an overlap and cut them together at shallow angle with a hand saw. I thought that the angles on two pieces would match. LOL!... And then there was sanding, and sanding, and more sanding. It is hard to maintain an even edge on a board cut at very shallow angle. At the end the joint was very much visible, but the bed survived kids jumping on it. Now, if you want to go with dowels, I would use steel dowels (more of them at the bottom half as you suggested). The problem is how you glue steel into wood. Epoxy?


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## scrabby (Sep 4, 2009)

Lesson on scarf joints learned here! Thanks guys.

Jim, aka Scrabby, aka Scrubby


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## JJohnston (May 22, 2009)

Got pictures of that scarf joint jig?


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## Gofor (Jan 12, 2008)

I always thought a scarf joint was strongest if the joint planes were perpendicular to the stress load (same with a finger joint, which is fine for a compression load, but usually not suitable for a lateral load.) The way most so far have described the scarf joint is with the load parallel to the joint faces. That said, splice plates in joists are usually 8 times the width of the board in length, so I guess a scarf joint over 1:8 ratio would be the same. I don't think a finger joint would hold up without some dowel support.

If a scarf joint is used, it would be better to use a "non-creeping" glue, such as a plastic resin.

JM2CW

Go


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## sphere (Feb 6, 2010)

If you can spare the WIDTH, cut the 1×8 from South west corner to North east corner ( diag the whole length)

Now slide the two triangles past each other till you have the length you want and reglue it back together…add biscuts if you wish.

Your glue joint is now long grain to long grain and after some trimming the board is as long as you want it ( within reason).


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## Magnum (Feb 5, 2010)

"Why don't more people do it?" Probably because we aren't all Engineers. We're *"Woodworkers"* and "Destructive Testing", whatever that is, is not our thing. We *BUILD *things. We don't Destroy things.


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## richgreer (Dec 25, 2009)

This conversation has been quite educational and beneficial for me. I've known what a scarf joint is and I always wondered how people cut them accurately. I never thought of the router approach mentioned above.

I wonder - would a large lap joint be just as strong as a scarf joint and probably easier to cut? I would cut the lap joint on my table saw with a dado.


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## stefang (Apr 9, 2009)

I'm kinda lazy. I think if there is room for it I would just fit a steel framers plate to the inside with some screws. What do you engineers say to that?


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## miles125 (Jun 8, 2007)

Woodworkers are asthetically conscious engineers. Give a bridge designer a bed frame and it'll last forever but look like crap


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## stefang (Apr 9, 2009)

Who cares if no one can see it? My last word on this issue.


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## Edziu (Jan 17, 2010)

I do not want to seem out of line here, but is lumber really that expensive that you really must lengthen this board, and not go buy two new, longer boards?

Let's think about this clearly.

*Choice A: Scarf Joint*.
1. make scarf joint jig.
2. prepare two extra short pieces.
3. Mill scarfs on both mating pieces.
4. fine tune the joint, glue, clamp.
5. Staining/finishing
5A. Spend countless hours matching stain and finish
5B. End up sanding and re-finishing all of both siderails.
6. Attach hardware

*Choice B: Finger Joints*

1. Purchase finger Joint Bit, minimum $50
2. Repeat steps 4-6 from Choice A
3. Think of another use for the finger joint bit you just bought.

*Choice C: Buy two new, full length boards. 
*
1. Go buy some new wood, Most domestics at Rockler are between $3 and $7 a board foot. 
(Two 8" wide boards, 84 inches long, its about 9 board feet.)
2. Mill, and prepare both boards for staining.
3. Stain and finish
4. Attach hardware.

I might be crazy, but I think Choice C is a winner.


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## KMJohnsonow (Feb 16, 2010)

Too funny.


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## Edziu (Jan 17, 2010)

13-14 board feet? Who makes you buy all that? If you ask nicely at Rockler, they cut the boards for you, they're so helpful there. And about the staining and finishing, yes I did omit some time, I was wrong; so lets take that out of the equation altogether, since they will be covered, lets skip sanding too. Back to my point, Do you really want to make things to make other things, or buy things to make other things? 
Yes, building and completing the scarf joint jig is a skill builder for a woodworker, and buying the router bit earns points for the "He who dies with the most tools, wins." guys.


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## a1Jim (Aug 9, 2008)

In my opinion scarf joints work but seldom use them ,I just get longer material .


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## AaronK (Nov 30, 2008)

shopguryl kind of has it, i think. whatever this post was originally about, it turned into a discussion of the best way to make a board longer and the question about how you might determine which was the best way.

The answer to the second question (i'm addressing Rick, mostly, here) is that you'd have to test the joints side by side. destructive testing is used all the time in woodworking - it simply means a type of testing where the sample isnt left in the same condition as when you started. it's not about "destroying" things for the hell of it. In this case it's about damaging in a controlled manner in order to learn something about the material or complete assembly. Destructive testing in woodworking (which many of the magazines do on a regular basis!) often takes the form of making different joints and comparing at what load they fail and very importantly: where they fail - in the wood or the glue. The information you get from that helps considerably in the design and construction.

Miles had it right with "Woodworkers are asthetically conscious engineers." and I'd add ever so slightly more towards the engineering side than architects, from what i understand about most architects.

anyway, don't knock destructive testing because of the name


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## AaronK (Nov 30, 2008)

rich:

I kind of like your large lap joint idea. You could always reinforce it with 2 or more screws or dowels as well. you could even mill some lumber to fit and glue in the two gaps on either side of the lap joint too, just for reinforcement. This method would require a much easier setup than the scarf joint, as well as preserve the exact width of the overall composite board/beam.


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## georgiaken (Sep 11, 2013)

I found this thread searching Google for scarf joint strength. While I know it is not a good practice to revive really old threads, it struck me that richgreer is a genius.

Richgreer, those two stories made me laugh almost to tears. Hilarious!


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## cjwillie (Sep 6, 2011)

I'm still looking for that "board stretcher" that we all need sometimes!


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